research paper for bilingualism

SYSTEMATIC REVIEW article

The impact of bilingualism on executive functions in children and adolescents: a systematic review based on the prisma method.

• 1 Dipartimento di Psicologia, Sapienza Università di Roma, Roma, Italy
• 2 Instituto de Estudios Sociales y Humanísticos, Universidad Autónoma de Chile, Santiago de Chile, Chile
• 3 Dipartimento di Psicologia e dei Processi di Sviluppo e Socializzazione, Sapienza Università di Roma, Roma, Italy
• 4 Dipartimento di Psicologia Dinamica e Clinica, Sapienza Università di Roma, Roma, Italy

Approximately half of the world's population is bilingual or multilingual. The bilingual advantage theory claims that the constant need to control both known languages, that are always active in the brain, to use the one suitable for each specific context improves cognitive functions and specifically executive functions. However, some authors do not agree on the bilingual effect, given the controversial results of studies on this topic. This systematic review aims to summarize the results of studies on the relationship between bilingualism and executive functions. The review was conducted according to PRISMA-statement through searches in the scientific database PsychINFO, PsycARTICLES, MEDLINE, and PUBMED. Studies included in this review had at least one bilingual and monolingual group, participants aged between 5 and 17 years, and at least one executive function measure. Studies on second language learners, multilingual people, and the clinical population were excluded. Fifty-three studies were included in the systematic review. Evidence supporting the bilingual effect seems to appear when assessing inhibition and cognitive flexibility, but to disappear when working memory is considered. The inconsistent results of the studies do not allow drawing definite conclusions on the bilingual effect. Further studies are needed; they should consider the role of some modulators (e.g., language history and context, methodological differences) on the observed results.

Introduction

Approximately half of the world population is bilingual or multilingual ( Ansaldo et al., 2008 ). In 2016, 64.6% of the European population aged 25–64 declared they knew one or more foreign languages. When considering only 25–34-year-olds, this percentage rises to 73.3% ( Eurostat, 2016 ). Moreover, the number of immigrant children worldwide who do not speak the majority language of their place of residence has increased ( OECD, 2010 ). Despite that, there is no single definition of bilingualism. Among the definitions of bilinguals, the most inclusive is the one by Edwards (2004) , who states that “everyone is bilingual” because there are no (adult) people in the world who do not know at least some words in a language different from their native language. According to other definitions ( Abdelgafar and Moawad, 2015 ), only people who know two languages with a level of competence equal to that of a native speaker can be considered bilingual. The more common definition is “someone who can function in both languages in conversational interaction” ( Wei, 2020 ). The age of acquisition (AoA) of the second language is another factor that characterizes bilinguals, allowing to classify them in simultaneous bilinguals, when both languages are learned during infancy, and sequential bilinguals, when they are exposed to the second language after infancy, usually at school entry ( Gross et al., 2014 ). Other authors also include learning a second language as they define bilinguals who can correctly produce sentences in a language other than their native language ( Hakuta, 1986 ). The absence of standard guidelines has led to heterogeneity in the populations considered by studies on bilingualism, often including people with different language histories and competencies (for a list of terms used to describe bilinguals, see Wei, 2020 ).

The first studies on bilingualism date back to the early 1900s. Initially, several researchers supported the hypothesis that bilingual children had lower mental abilities than monolinguals because the knowledge of several languages would generate a mental confusion with deleterious consequences on every cognitive aspect ( Hakuta, 1986 ). Peal and Lambert (1962) were the first to contradict this negative view about the bilingualism effect. Because of the positive results of subsequent studies, a new theory advanced the view of bilingualism advantage. The positive effect of bilingualism would depend on the constant need to control both known languages to use the one suitable for each specific context, and this process would generate more significant neurological development ( Bialystok, 1999 , 2001 ). According to the Joint Activation Model of Green (1998) , both languages would always be active in the brain of a bilingual person regardless of the language used at the given moment; for this reason, it would be necessary to use a general suppression mechanism to inhibit the activation of the non-target language. Green and Abutalebi (2013) highlighted the importance of the context in which language exchanges take place. They proposed the Adaptive control hypothesis and identified three possible contexts of interaction: single-language, dual-language, and dense code-switching contexts. Depending on the communicative context in which bilinguals are immersed, the languages may cooperate or compete. For this reason, each context is characterized by a different use of processes that are the basis of communication. The use of multiple languages would seem to modify both the language network and the control network ( Green and Kroll, 2019 ).

Some of the cognitive functions that would seem to benefit from the knowledge of several languages are the metalinguistic and metacognitive awareness, the ability to represent abstract and symbolic concepts (for a review see Adesope et al., 2010 ), and specifically, the bilingualism should improve the executive functioning.

According to the model of Miyake et al. (2000) , executive functions refer to cognitive flexibility (e.g., the ability to switch between tasks), inhibition (e.g., the ability to suppress dominant responses) and monitoring (e.g., the ability to update information in the working memory).

According to Bialystok (2011) , bilinguals have an advantage in executive functions because they would continuously train them to carry on a conversation that must be based on the context and require constant access to the information contained in the working memory. Furthermore, it is necessary to select the appropriate language for the specific communicative situation (inhibiting the other language) and to monitor what happens during the interaction (cognitive flexibility).

It has been shown that executive functions can be improved through training ( Karbach and Kray, 2009 ; Moreno et al., 2011 ). The study of the “bilingual advantage” is not only one of the main topics discussed in bilingualism research, but it is also the most controversial one. After the publication of positive evidence on the bilingual advantage, the difficulty in replicating previous results and the publication of several studies with null findings led to questioning this theory. Recently, the use of the term “bilingual advantage” has been questioned because its presence or absence could depend on the interpretation or perspective of the observer. Leivada et al. (2020) suggested adopting the more neutral term “bilingual effect.” Paap et al. (2015) stated that “bilingual advantages in executive functioning either do not exist or are restricted to very specific and undetermined circumstances” and pointed out that 80% of the tests carried out after 2011 failed to obtain results in support of the bilingual effect.

Paap et al. (2015) hypothesized that the results of previous studies on this topic could be due to the lack of control of several external factors, the experimental tasks chosen to evaluate it, and the limited number of participants included in the studies. Other factors that play a role in determining these results are socioeconomic status (SES) and the participants' cultural and linguistic background. For example, the tests used for the assessment of bilinguals are usually the same as those used and validated for monolinguals. The condition of bilingualism can influence the performance in various domains (positively or negatively). In that case, it follows that some of the standardized tests currently in use are not always suitable for the assessment of bilinguals and that the normative data currently available do not reflect the real abilities of bilinguals (e.g., assessment of linguistic abilities in bilingually developing children, see for example Core et al., 2013 ; Bailey et al., 2020 ). One of the characteristics of the experimental tasks that seem to influence the performance of people who know several languages is the use of verbal stimuli ( Duñabeitia et al., 2014 ).

Many studies have shown that bilinguals perform more poorly than monolinguals on linguistic tasks (e.g., Bialystok, 2009a ), have a smaller vocabulary than monolinguals ( Bialystok et al., 2010 ) and produce fewer words in verbal fluency tasks ( Zeng et al., 2019 ). These findings could be due to the lower use and the specificity of each language. The characteristics of the two languages could depend on how they were learned and used ( Blom et al., 2014 ). When the vocabulary size is assessed considering both known languages, this deficit disappears, and bilinguals show a more extensive vocabulary size than monolinguals ( Bialystok, 2009b ).

The use of verbal stimuli implies the activation in the brain of bilinguals of two different linguistic forms per stimulus and difficulty in coding when the presented word is known in the other language than the one used for the assessment. Other factors related to language skills seem to affect the performance of bilinguals. In tasks using verbal stimuli, both the similarity of the languages known and the native language would seem to affect the results. Unfortunately, however, for many of the aspects of the linguistic experience, there is still no agreed conclusion between the different researchers. For instance, what is the degree of balance that must exist between the two languages to generate the bilingual effect? Some studies argue that the bilingual effect emerges when bilinguals have complete mastery of the two languages ( Filippi et al., 2015 ). Therefore, the advantage should be due to the higher cognitive effort needed to reduce interference between the two languages ( Blom et al., 2014 ); other researches asserted that the potential cognitive effects are proportionate to the degree of balance between languages ( Carlson and Meltzoff, 2008 ; Ladas et al., 2015 ).

Other authors argue that the degree of control that bilinguals must apply is higher when they are not equally fluent in the two languages; therefore, the absence of significant differences in the studies could be due to the inclusion of participants with a balanced competence in the two languages for whom the process of switching has become automatic ( Gathercole et al., 2014 ). A factor that does not seem to affect the degree of advantage in executive functioning is the knowledge of more than two languages ( Poarch and van Hell, 2012 ; Poarch and Bialystok, 2015 ). The type of language known and the degree of similarity between them is also an aspect to be considered. Several authors have pointed out that the similarity between languages is a decisive factor in determining the bilingual effect (e.g., Bialystok et al., 2003 ), while phonological and orthographic differences can negatively affect performance, generating interference during the evaluation ( Jalali-Moghadam and Kormi-Nouri, 2015 ).

There are also specific characteristics of the experimental tasks that seem to affect the performance of bilinguals. Several studies agree that the bilingual effect would emerge in more complex experimental tasks where there is a higher demand for control (e.g., Engel de Abreu et al., 2012 ; Barac et al., 2016 ). Further, the tendency to use experimental tasks that empirically isolate executive functions seems to contribute to unclear results ( Barac et al., 2016 ). Most experimental tasks inevitably engage other cognitive processes while evaluating a specific domain (task impurity problem; Miyake and Friedman, 2012 ). Isolating the executive functions experimentally also does not allow the evaluation of real conditions since, in daily life, rarely exist tasks involving a single component of cognitive functions. Another aspect to consider is test-retest reliability. Several experimental tasks used to evaluate executive functions are characterized by low test-retest reliability, and this factor should lead to a more cautious interpretation ( Karalunas et al., 2016 ; Leivada et al., 2020 ). Additionally, bilingualism seems to have a more significant impact when it is required to coordinate multiple functions simultaneously ( Bialystok, 2011 ).

Other factors, such as socioeconomic status, cultural aspects, or immigrant status, would seem to have a role in determining the results achieved by bilingual participants. In several American countries, the condition of bilingualism is a consequence of migratory phenomena, and it is associated with low socioeconomic status ( Calvo and Bialystok, 2014 ). In other countries, for example, in Arab Countries, bilinguals usually belong to a high social class and often learn more than one language because they receive a bilingual school education ( Abdelgafar and Moawad, 2015 ). It is known that low socioeconomic status leads to lower cognitive functioning ( Rosen et al., 2019 ). Given the high frequency of low socioeconomic status and reduced vocabulary in bilinguals, several authors have indicated the importance of analyzing these aspects and monitoring the effect of these variables statistically if a difference between groups is present. Although many authors considered that statistical control of these variables is the correct procedure (e.g., Carlson and Meltzoff, 2008 ; Blom et al., 2014 ), others believe that these conditions are a specific characteristic of the population of interest ( Buac et al., 2016 ).

This systematic review aims to summarize the findings of studies investigating the relationship between bilingualism and executive functions in children and adolescents. It will be verified whether bilingualism affects one or more specific executive functions. Studies that have used the same task will be compared, highlighting any changes that have been made to the experimental tasks that could influence the results. The studies will be analyzed to identify any factors that may be involved in determining the outcomes. We excluded studies with older adult participants from this systematic review, although they provide the strongest evidence for a bilingual effect ( Antón et al., 2014 ). As Baum and Titone (2014) suggested, older adults experienced a historical and cultural moment in which attitudes toward bilingualism were very different from those of today. This factor could have affected the use of languages at various times in their lives. Moreover, studies with adults would imply the need to consider many other factors (e.g., drug treatment). We believe it is necessary to conduct a systematic review focusing only on this population, considering its specific characteristics.

The review process was conducted according to the PRISMA Statement ( Liberati et al., 2009 ; Moher et al., 2009 ). The PRISMA Statement consists of a 27-item checklist and a four-phase flow diagram and helps authors improve systematic review reporting. This review was registered as PROSPERO CRD42019127965.

Research Strategies

A systematic search of the international literature was conducted in the following electronic databases by selecting articles published in peer-review journals: PsycINFO, PsycARTICLES, MEDLINE, and PubMed. The last research was conducted on 15 April 2020. Restrictions were made limiting the research to academic publications in English, Italian, and Spanish. No restriction of age, gender, or ethnicity was made. The search strategy used Boolean combinations of the following keywords: “bilingual * ,” “second language user,” “executive function * ,” “cognitive flexibility,” “shifting,” “task switching,” “updating,” “working memory,” “inhibition,” and “cognitive inhibition.” Reference lists of the selected articles were screened. A total of 3,785 articles were obtained from the search procedure. Mendeley reference manager software was used for removing duplicates. The first screening was made by reading the title and abstract. The full text of the selected studies was read.

Eligibility Criteria

The studies that respected the following characteristics were included: the presence of at least one bilingual group and one monolingual group, at least one executive function measured, age of participants between 5 and 17 years. Studies on preschool-age children were excluded because the EFs and underlying neural areas are immature and still developing ( Diamond, 2013 ). The age limit has been set at 17 years because, during middle adolescence, the peak of executive functions is reached ( Anderson, 2002 ). Studies on bimodal bilingual, second language learners, and trilingual or multilingual people were excluded. Studies on clinical populations were excluded. All the selected studies were screened to assess the risk of bias using Standard quality assessment criteria for evaluating primary research papers from various fields ( Kmet et al., 2011 ). The studies were included if they reached a score above 70%.

Data Collection

According to the PICOS approach ( Liberati et al., 2009 ), the following information has been extracted from the selected studies: author(s) and year of publication, country, characteristics of participants (age, percentage of females, spoken languages, use of languages, socioeconomic status), criteria used for selecting bilingual participants, the experimental paradigm used, results of the studies. These data are summarized in Tables 1 , 2 .

Table 1 . Main characteristics of the studies included.

Table 2 . Bilingual participants' characteristics in the selected studies.

Selection of Studies

The flowchart ( Figure 1 ) shows the number of studies identified from the databases and the other sources, the number of studies examined by the authors, and assessed for eligibility. The reasons for exclusion are reported.

Figure 1 . Studies selection flow diagram (PRISMA flow chart).

Results of the Selected Studies

Of the 53 studies identified, 24 were conducted in Europe, 10 in America, two in Asia, one in Africa, one in Australia, and 14 did not report the country. Bialystok and Viswanathan (2009) included participants from two different continents (America and Asia).

Twenty-seven studies included bilingual participants who knew a specific language pair while in 23 studies, bilinguals spoke a common language plus another language. Bialystok and Viswanathan (2009) included two groups of bilingual participants, one speaking a specific language pair, the other speaking different languages. Two studies ( Barac and Bialystok, 2012 ; Blom et al., 2017 ) included distinct groups of bilingual participants with different linguistic backgrounds to check if the type of language known, influenced the results.

In most studies, information on the participants' linguistic background was collected through interviews or questionnaires made to their caregivers. In two studies, the information was collected by directly interviewing the participants ( Jalali-Moghadam and Kormi-Nouri, 2015 ; Raudszus et al., 2018 ). The analyzed studies reported different definitions of bilingualism; some of these definitions are based on the assessment of the competences in the two languages; others are founded on the age of acquisition of the two languages. Twenty-five studies reported information on the time of acquisition of the second language (e.g., type of bilingualism, the age range in which the languages were learned), but only 12 studies indicated the age of acquisition. Most of the studies did not indicate the language context in which the children were immersed, and only eight studies defined the language used at home by parents and children. Forty-five studies assessed the participants' language skills using both tests and self-report questionnaires or interviews. In twenty-four studies were assessed both languages known by the bilingual participants. In three studies ( Escobar et al., 2018 ; Dick et al., 2019 ; Zeng et al., 2019 ), objective assessments and self-report questionnaires were used. The use of both tools allows investigating both language proficiency (tests) and language use (self-report), two aspects that can contribute to a better description of the bilingual experience ( Luk and Bialystok, 2013 ). Twenty-four studies reported a reduced vocabulary for bilinguals compared to monolinguals considering only the groups' common language. In three studies, no assessment of the participants' language skills was conducted. Many of the studies provided information on socioeconomic status, and the most used as an indicator of SES the educational level of parents. In nine studies, the group of bilinguals had a lower socioeconomic status than monolinguals. In Veenstra et al. (2018) , the bilinguals had a higher socioeconomic status than monolinguals. Nine studies did not report information on the SES (see Table 2 ).

Bilingualism and Attention (n = 11)

Eleven studies examined the effect of bilingualism on attention. Three studies ( Engel de Abreu et al., 2012 , 2014 ; Blom et al., 2017 ) used the Sky Search task of the Test of Everyday Attention for Children ( Manly et al., 1999 ) to assess selective attention. Participants were asked to identify pairs of identical pictures on a sheet of paper while ignoring the presence of distracting stimuli. In all studies, bilingual participants took less time to solve the task compared to monolinguals.

Calvo and Bialystok (2014) used the Pair Cancellation Subtest of the Woodcock-Johnson Tests of Cognitive Abilities-III ( Woodcock et al., 2001 ) to assess non-verbal visual attention and the cancellation subtest of Wechsler Intelligence Scale for Children-Fourth Edition (WISC-IV; Wechsler, 2003 ) to assess verbal-visual attention. In the task with verbal stimuli, bilinguals performed significantly worse than monolinguals, while in the task with non-verbal stimuli, no differences emerged between the two groups.

Seven studies ( Carlson and Meltzoff, 2008 ; Kapa and Colombo, 2013 ; Antón et al., 2014 ; Ladas et al., 2015 ; Barac et al., 2016 ; Yang and Yang, 2016 ; Veenstra et al., 2018 ) used the child-friendly version of the Attentional Network Task proposed by Rueda et al. (2004) to assess the three attentional networks (alerting, orienting, and executive control).

In Ladas et al. (2015) , the participants also carried out the Attentional Network Task for Interaction ( Callejas et al., 2004 ). Four studies ( Carlson and Meltzoff, 2008 ; Antón et al., 2014 ; Ladas et al., 2015 ; Veenstra et al., 2018 ) found no significant difference in performance between monolinguals and bilinguals. In Yang and Yang (2016) , bilingual children were globally faster and more accurate than monolingual children. No differences were found in the three attention indexes (alerting, orienting, and executive control).

The authors also calculated the global inverse efficiency scores by dividing the mean reaction times by accuracy percentage. This index indicated an advantage for the bilingual group over the monolingual group. In Barac et al. (2016) , no significant differences in RTs or attentional indexes emerged between bilinguals and monolinguals. In Kapa and Colombo (2013) , both reaction times and the percentage of accuracy were analyzed by using age and vocabulary as covariates. For reaction times, the early bilingual group (i.e., children who learned both languages before the age of three) was significantly faster than the monolingual group. At the same time, no significant differences emerged between the later bilingual group (i.e., children who learned Spanish before the age of three and English after three) and the monolingual group. The two bilingual groups did not differ between them. No significant differences were found between the three groups in the percentage of accuracy and the attentional indexes.

Bilingualism and Visual Working Memory (n = 17)

Four studies ( Bialystok and Viswanathan, 2009 ; Gangopadhyay et al., 2016 ; Park et al., 2018 ; Veenstra et al., 2018 ) used the Corsi blocks task to assess visuospatial working memory. No significant differences emerged between the performance of monolinguals and bilinguals. Four studies ( Engel de Abreu et al., 2012 , 2014 ; Blom et al., 2014 , 2017 ) used a modified version of this task, the Dot Matrix Task, and again no significant differences between the two groups' performance were found. In the study of Blom et al. (2014) to verify whether age, socioeconomic status, defined as the average education level of both parents, and vocabulary size, influenced the results, these variables were used as covariates in the statistical analysis and participants were divided into two age groups. Results showed that bilinguals at 6 years had a better performance than monolinguals. Two studies ( Morales et al., 2013 ; Calvo and Bialystok, 2014 ) used a child-friendly version of the Corsi blocks task, the Frog Matrices Task. In Calvo and Bialystok (2014) , bilinguals were more accurate than monolinguals. In Morales et al. (2013) , bilinguals showed a higher proportion score (calculated as the number of remembered elements divided by the total number of elements) than monolinguals in the sequential condition. In the less demanding condition, i.e., the simultaneous condition, no significant differences emerged between the two groups.

Three studies ( Gangopadhyay et al., 2016 ; Arizmendi et al., 2018 ; Janus and Bialystok, 2018 ) used the N-back task to assess non-verbal working memory. In Gangopadhyay et al. (2016) , no significant differences were found between bilinguals and monolinguals. Arizmendi's et al. (2018) study used two N-back tasks (i.e., N-back Auditory task and N-back Visual task), and monolinguals solved the tasks more efficiently than bilinguals. In Janus and Bialystok (2018) , who used a modified version with emotional stimuli, bilinguals were more accurate than monolinguals when they had to indicate that the target was the same as in the previous trial (target trial) than when it was not (non-target trial). Furthermore, bilinguals had slower reaction times than monolinguals when a target trial (2-back condition) or a no target trial was presented (1-back and 2-back conditions).

Three studies ( Engel de Abreu et al., 2012 , 2014 ; Blom et al., 2014 ) used the Odd-One-Out task. No significant differences were found in any of the studies.

Jalali-Moghadam and Kormi-Nouri (2015) used the Concentration task ( Schumann-Hengseler, 1996 ) and the Tower of Hanoi ( Welsh, 1991 ) and no significant differences emerged between bilingual and monolingual participants.

Morales et al. (2013) used the Picture Task. Bilinguals solved the task more efficiently with faster reaction times in all conditions. Bilinguals had the same accuracy score in congruent and incongruent trials, while monolinguals were negatively affected by the incongruent condition.

Two studies ( Bialystok, 1999 ; Carlson and Meltzoff, 2008 ) used the Visually Cued Recall task ( Zelazo et al., 1997 ) and did not find differences between bilinguals and monolinguals.

Bonifacci et al. (2011) used two experimental tasks to assess visual working memory in which participants were required to indicate whether a target stimulus appeared within a string of stimuli. Numerical and unknown alphabetical symbols were used as stimuli. There were no significant differences between the performance of the two groups. Cottini et al. (2015) used the Color-Shape binding task (adapted from Allen et al., 2006 ), bilinguals were more accurate than monolinguals only in the shape condition, while there were no differences in the color condition and the combination of the two conditions. Furthermore, bilinguals presented more false alarms than monolinguals only in the combination condition.

Bilingualism and Verbal Working Memory (n = 21)

Four studies used the listening recall task to assess working memory. In Leikin and Tovli (2014) , participants had to complete sentences with the missing word, and then they have to recall the complete list of words used ( Shani et al., 2005 ). In two studies ( Buac et al., 2016 ; Schröter and Schroeder, 2017 ), participants had to judge whether the sentences were true or false, and then remember the last word ( Daneman and Carpenter, 1980 ). In Bosman and Janssen (2017) , a modified version of this task was adopted in which participants were required to remember the first word because, in the participants' language, the last word of the sentence was always a verb. Within these studies, only Leikin and Tovli (2014) found a significant difference between groups, with bilinguals who named more correct words than monolinguals. The number of the correct sequences (i.e., the number of correct orders of the words) was the same in the two groups. In Bosman and Janssen (2017) , bilingual children's performance was worse than that of monolinguals. In two studies ( Buac et al., 2016 ; Schröter and Schroeder, 2017 ), no significant differences emerged.

Bialystok and Feng (2009) used the Proactive Interference Task, and no significant differences in the performance of the two groups were found.

Eighteen studies ( Danahy et al., 2007 ; Bialystok and Feng, 2009 ; Bialystok and Viswanathan, 2009 ; Bialystok, 2010 ; Engel de Abreu, 2011 ; Kapa and Colombo, 2013 ; Blom et al., 2014 , 2017 ; Engel de Abreu et al., 2014 ; Filippi et al., 2015 ; Garraffa et al., 2015 ; Buac et al., 2016 ; Cockcroft, 2016 ; Bosman and Janssen, 2017 ; Raudszus et al., 2018 ; Veenstra et al., 2018 ; Hartanto et al., 2019 ; Jaekel et al., 2019 ) evaluated working memory by using different versions of the digit span task. In 12 studies ( Danahy et al., 2007 ; Bialystok and Feng, 2009 ; Bialystok and Viswanathan, 2009 ; Engel de Abreu, 2011 ; Kapa and Colombo, 2013 ; Engel de Abreu et al., 2014 ; Filippi et al., 2015 ; Garraffa et al., 2015 ; Cockcroft, 2016 ; Blom et al., 2017 ; Raudszus et al., 2018 ; Veenstra et al., 2018 ) no significant differences between the two groups emerged. In three studies, monolinguals remembered a significantly higher number of digits than bilinguals in the forward digit span task ( Buac et al., 2016 ; Bosman and Janssen, 2017 ) and backward digit span ( Jaekel et al., 2019 ). In Bialystok (2010) , which reported three studies involving three different groups of participants, bilinguals' scores were lower than monolinguals' scores only in the third study. In this study, bilingual participants had a smaller vocabulary size when compared to monolinguals. In Blom et al. (2014) , bilinguals scored were higher in both forward and backward digit span. In Hartanto et al. (2019) , which assessed the performance in four different time waves, bilinguals had better performance than monolinguals only in time 4 (mean age bilinguals: 7.13; mean age monolinguals: 7.05).

Three studies ( Engel de Abreu, 2011 ; Garraffa et al., 2015 ; Cockcroft, 2016 ) evaluated short-term verbal memory using the non-word repetition task. In Engel de Abreu (2011) , the monolinguals repeated a significantly higher number of non-word than bilinguals. To verify whether the difference in vocabulary size between participants affected the results, the author repeated the analysis using the receptive vocabulary score as a covariate, and the difference between the two groups disappeared. In the other two studies, there were no significant differences in the performance of the two groups.

Arizmendi et al. (2018) used the number updating task, and no differences emerged between the two groups of participants.

Bilingualism and Inhibition (n = 28)

Two studies ( Bonifacci et al., 2011 ; Barac et al., 2016 ) used the Go/No-Go Task. In Barac et al. (2016) , bilinguals were faster and more accurate than monolinguals. The d' index indicated a better discriminatory capacity in the bilingual group. In Bonifacci et al. (2011) , which used a modified version of the Go/No-Go task, the No-Go condition consisted of an image accompanied by a sound; the two groups were equal on the number of omissions, the percentage of accuracy and the RTs.

Two studies ( Arizmendi et al., 2018 ; Dick et al., 2019 ) used the Stop-Signal task, and no differences between the performances of the two groups emerged.

Nine studies ( Gathercole et al., 2010 ; Duñabeitia et al., 2014 ; Mohades et al., 2014 ; Abdelgafar and Moawad, 2015 ; Jalali-Moghadam and Kormi-Nouri, 2015 ; Schröter and Schroeder, 2017 ; Arizmendi et al., 2018 ; Escobar et al., 2018 ; Nayak et al., 2020 ) assessed cognitive inhibition by using the Stroop task ( Stroop, 1935 ). Two studies ( Abdelgafar and Moawad, 2015 ; Jalali-Moghadam and Kormi-Nouri, 2015 ) used the pencil and paper version of this task and did not find any significant difference in the performance of monolingual or bilingual participants. Two studies ( Duñabeitia et al., 2014 ; Schröter and Schroeder, 2017 ) adopted the computerized version of the task, and no significant differences between the groups occurred. In two studies ( Duñabeitia et al., 2014 ; Mohades et al., 2014 ), a modified version of the task with numerical stimuli was adopted. In this task, children had to report which number was larger, ignoring the physical size of the digits. In Duñabeitia et al. (2014) , no significant differences between the groups were found. In Mohades et al. (2014) , no significant differences between the groups were found for RTs and accuracy, but the bilingual group showed a higher congruency effect. Nayak et al. (2020) used animal stimuli and did not find significant differences between the two groups, even after controlling for age and socioeconomic status. In Gathercole et al. (2010) , monolingual participants solved the classic Stroop task in English while bilinguals carried out the task in both English and Welsh. There were no significant differences among the three groups of bilinguals in both accuracy and reaction times in the Welsh version. Significant differences in accuracy score in the primary school age group emerged in the English version. The comparison among the three bilingual groups showed a lower accuracy in the group exposed at home to Welsh for 80% of the time from birth (OWH). Monolinguals had significantly fewer accuracy scores than those exposed to both Welsh and English at home from birth (WEH). For reaction times, significant differences emerged only in the teens, and monolingual participants responded significantly slower than all bilingual groups. Escobar et al. (2018) used the Day-Night Stroop Task. The experimental task included congruent trials in which participants named the word corresponding to the presented stimulus (e.g., the word day for the sun) and incongruent trials in which they had to pronounce the word opposite to the presented stimulus (e.g., the word day for the moon). No significant differences emerged between the two groups. In Arizmendi et al. (2018) , two modified versions of the Stroop task were used. In both versions, participants had to respond orally. No significant differences emerged between bilinguals and monolinguals.

Nine studies ( Engel de Abreu et al., 2012 , 2014 ; Calvo and Bialystok, 2014 ; Poarch and Bialystok, 2015 ; Blom et al., 2017 ; Ross and Melinger, 2017 ; Park et al., 2018 ; Struys et al., 2018 ; Dick et al., 2019 ) evaluated the interference suppression ability using the Flanker task ( Eriksen and Eriksen, 1974 ). In four studies ( Engel de Abreu et al., 2012 , 2014 ; Poarch and Bialystok, 2015 ; Park et al., 2018 ), bilingual participants had faster RTs. In two studies ( Poarch and Bialystok, 2015 ; Park et al., 2018 ), this advantage emerged in the incongruent condition indicating a better ability to control conflictual information in the bilingual group.

In Blom et al. (2017) , the performance in the Flanker task correlated negatively with the scores in memory tasks, indicating that children with better results in memory tasks had faster reaction times. Moreover, multiple linear regression results have suggested that a more extended vocabulary size is associated with a better ability to perform this experimental task. However, no significant differences between monolinguals and bilinguals emerged. Three studies ( Calvo and Bialystok, 2014 ; Ross and Melinger, 2017 ; Dick et al., 2019 ) showed no significant difference in RTs between bilinguals and monolinguals, but in Calvo and Bialystok (2014) bilinguals reached a higher percentage of accuracy. Struys et al. (2018) analyzed the speed-accuracy trade-off effect (i.e., an increase in accuracy corresponds to an increase in reaction times and vice versa) to verify whether the participants adopted different resolution strategies in the experimental tasks. The results indicated a speed-accuracy trade-off effect in the older bilingual group (mean age: 11.7) but not in the younger bilingual group (mean age: 6.6) or in the monolingual groups. The authors hypothesized that the effect was not present in both groups of bilinguals because they may have adopted different strategies (preferring speed in some cases and accuracy in others). To highlight an advantage in the speed-accuracy trade-off effect, it seems necessary that most participants adopt the same strategy.

Seven studies analyzed the ability to manage conflictual information by using the flanker task in the experimental context of the Attentional Network Test ( Carlson and Meltzoff, 2008 ; Kapa and Colombo, 2013 ; Antón et al., 2014 ; Ladas et al., 2015 ; Barac et al., 2016 ; Yang and Yang, 2016 ; Veenstra et al., 2018 ). In two studies ( Antón et al., 2014 ; Ladas et al., 2015 ), no significant differences in reaction times and the percentage of accuracy between the monolingual and bilingual groups were observed. In the other two studies ( Barac et al., 2016 ; Yang and Yang, 2016 ), no significant differences in reaction times emerged, while bilinguals were more accurate in congruent and incongruent trials than the monolingual group. In three studies ( Carlson and Meltzoff, 2008 ; Kapa and Colombo, 2013 ; Veenstra et al., 2018 ), the Flanker x Group interaction results were not reported.

Seven studies ( Poarch and van Hell, 2012 ; Gathercole et al., 2014 ; Mohades et al., 2014 ; Ross and Melinger, 2017 ; Raudszus et al., 2018 ; Struys et al., 2018 ; Zeng et al., 2019 ) used the Simon Task ( Simon and Wolf, 1963 ). In two studies ( Poarch and van Hell, 2012 ; Raudszus et al., 2018 ), no significant differences emerged between the monolingual and the bilingual groups. Two studies ( Ross and Melinger, 2017 ; Zeng et al., 2019 ) found a lower percentage of errors in the bilingual group than to the monolingual group, while there were no differences between the two groups in reaction times and the Simon effect. In Gathercole et al. (2014) , there were no significant differences between monolinguals and bilinguals in the primary schoolers and teens groups. In the group of 5-year-olds, no difference emerged for the percentage of accuracy. However, the monolinguals were faster than the bilingual group exposed at home to English for 80% of the time from birth (OEH). The OWH bilinguals were faster than the OEH bilinguals. In Mohades et al. (2014) , bilinguals achieved the same performance as monolinguals in reaction times and accuracy, but they showed a greater congruency effect. In Struys et al. (2018) , a speed-accuracy trade-off effect occurred in the two groups of bilinguals but not in monolingual participants.

Three studies ( Bialystok, 2010 ; Cottini et al., 2015 ; Arizmendi et al., 2018 ) assessed inhibition using the Global Local Task ( Andres and Fernandes, 2006 ). In Bialystok (2010) , the Global-Local task was proposed in three different versions. Overall, bilinguals were faster under all conditions than monolinguals. Bilinguals were more accurate than monolinguals in the global condition while in the local condition, there was no difference between the two groups. Moreover, the mixing costs (the difference between trials alone and trials in mixed condition) were smaller for bilinguals than for monolinguals. In Cottini et al. (2015) , bilinguals were more accurate than monolinguals in incongruent and neutral trials, and the total effect of interference was higher in the monolingual group. In this study, bilinguals were more accurate than monolinguals in the local incongruent trials, while monolinguals performed significantly better than bilinguals in the global incongruent trials. In Arizmendi et al. (2018) , no significant differences were found between monolingual and bilingual participants.

Two studies ( Carlson and Meltzoff, 2008 ; Barac et al., 2016 ) used a delay gratification task to assess the ability to inhibit dominant responses. In both studies, no significant differences were found between the monolingual and bilingual participants.

Two studies ( Garraffa et al., 2015 ; Schröter and Schroeder, 2017 ) used the Opposite World Task from the Test of Everyday Attention for Children ( Manly et al., 2001 ) in which it is required to inhibit a dominant verbal response. In Garraffa et al. (2015) , bilinguals were slower than monolinguals, while in Schröter and Schroeder (2017) , no significant difference between the two groups emerged.

Bilingualism and Shifting (n = 12)

Two studies ( Barac and Bialystok, 2012 ; Veenstra et al., 2018 ) used the Color-Shape task switching. In Barac and Bialystok (2012) , bilinguals were faster and had lower global costs than monolinguals. In Veenstra et al. (2018) , which used a composite inhibition score, considering the ANT interference effect, no significant differences emerged between bilinguals and monolinguals. Arizmendi et al. (2018) used a modified version of the Color-Shape task, the Pirate Sorting task, and did not find significant differences between the two groups.

Six studies ( Bialystok, 1999 ; Carlson and Meltzoff, 2008 ; Garraffa et al., 2015 ; Escobar et al., 2018 ; Park et al., 2018 ; Hartanto et al., 2019 ) used different versions of the Dimensional Change Card Sort Task (e.g., Zelazo et al., 1996 ). In four studies ( Bialystok, 1999 ; Carlson and Meltzoff, 2008 ; Garraffa et al., 2015 ; Hartanto et al., 2019 ), the bilingual group gave more correct responses than the monolingual group. In Park et al. (2018) , bilinguals showed lower mixing costs (the difference between trials in the pre-shift condition and non-switch trials in the mixed condition) compared to monolinguals, while no significant difference emerged between the two groups in the switching costs (the difference between non-switch and switch trials in the mixed condition) and shifting costs (the difference between the pre-shift and the post-shift condition). Escobar et al. (2018) found no differences between the two groups.

Gathercole et al. (2014) used a modified card task. In the teen group, the OWH bilingual group was more accurate than the monolinguals and WEH bilinguals. Monolinguals were faster in the group of 5 years old, whereas bilinguals were faster in the group of teenagers.

Ross and Melinger (2017) used a modified version of the Wisconsin Card Sorting Test, the Berg Card Sorting Test ( Piper et al., 2012 ) and did not find differences between the two groups in perseverative errors, reaction times or the number of trials needed to complete a category. However, bilinguals made more total errors than monolinguals.

Gathercole et al. (2010) used the Tapping Task. Three groups of bilinguals who used different languages at home were included in the study. In the primary age group, the OWH and OEH groups showed better performance in the match condition (i.e., emulation of the experimenter's action) and the switch condition (i.e., to do actions contrary to those of the experimenter). In the teen group, the OWH and WEH groups showed an advantage over the monolingual group.

Bilingualism and Multiple Executive Functions (n = 10)

This section examines the results of experimental tasks that evaluated different executive functions at the same time.

Three studies ( Bialystok and Viswanathan, 2009 ; Bialystok, 2010 ; Abdelgafar and Moawad, 2015 ) used the Trail Making Test, a neuropsychological test that allows evaluating visual attention and switching ability. In all studies, bilinguals completed part A faster than monolinguals. In two studies ( Bialystok and Viswanathan, 2009 ; Bialystok, 2010 ), bilinguals solved part B faster.

Five studies ( Bialystok, 2010 ; Abdelgafar and Moawad, 2015 ; Friesen et al., 2015 ; Escobar et al., 2018 ; Zeng et al., 2019 ) used the verbal fluency task. Verbal fluencies require linguistic ability and executive control during lexical access. In the semantic version of this task, the number of possible responses is higher, requiring a high degree of executive control. This result is due to the need to inhibit spontaneous associations not inherent to the criterion and to comply with the restrictions such as the morphological ones ( Friesen et al., 2015 ). In Abdelgafar and Moawad (2015) , semantic fluency was considered an indicator of inhibition ability while in Bialystok (2010) , categorical fluency was considered a verbal productivity indicator. In both studies, no significant differences between the two groups emerged. Conversely, in the other two studies ( Escobar et al., 2018 ; Zeng et al., 2019 ), bilinguals produced more words than monolinguals in letter fluency tasks. In Escobar et al. (2018) , bilinguals produced more words even in the semantic fluency task. In Friesen et al. (2015) , the authors argue that for the performance of the task, it is necessary to involve different components of the executive functions. In terms of categorical fluency, 10-year-old bilingual children produced fewer words than monolinguals. There was no difference in semantic fluency. For the 7-year-old group, there was no difference in both types of verbal fluency between the two groups. However, bilingual children had a higher mean subsequent-response latency, that is, the time in which half of the responses were produced. This index could indicate a difficulty for bilinguals in the lexical access due to the interference produced by the two languages known.

Bialystok and Viswanathan (2009) used the Face Task ( Bialystok et al., 2006 ) to evaluate simultaneously three components of executive functions, i.e., response suppression, inhibitory control, and cognitive flexibility. No significant differences in the performance of the three groups (two bilingual and one monolingual groups) were found considering both response suppression and accuracy. Monolinguals had higher inhibitory and switching costs than bilinguals. The two bilingual groups evaluated in this study did not differ.

Bialystok (2011) used the Dual modality classification task, an experimental task in which stimuli can be visual and auditory. In the single-modality condition, no significant differences in the performance of the two groups emerged. In the dual-modality condition, bilinguals had a higher accuracy score.

Krizman et al. (2016) used the Integrated Visual and Auditory Continuous Performance Test. Participants were required to respond or inhibit the response depending on the specific auditory or visual stimulus presented. Bilinguals performed better than monolinguals. Furthermore, low-SES bilinguals performed better than low-SES monolinguals and at the same level as participants with high SES.

Carlson and Meltzoff (2008) used a modified version of the Kansas Reflectory/Impulsivity Scale (KRISP; Wright, 1971 ), Statue ( Korkman et al., 1998 ), Simon says ( Strommen, 1973 ), and the Gift Delay. These tasks require to suppress motor action during a delay. No significant differences emerged between the bilingual and monolingual groups.

Jaekel et al. (2019) used the Hearts and Flowers task. No significant differences emerged between the bilingual and monolingual groups.

Bilingualism is the knowledge of two languages. Given the absence of a single definition, it is possible to consider bilinguals with a different degrees of proficiency in the languages they know or who have learned languages in different contexts, such as school or home, or different periods of their lives. According to the Joint Activation Model of Green (1998) , bilingualism involves the activation of both languages in the brain, even when only one language is used. This condition seems to have a positive effect on several cognitive functions, including executive functions ( Bialystok et al., 2012 ). After the publication of positive evidence on the bilingual effect, this hypothesis was questioned, given the difficulty in replicating the previous results. This difficulty seems to be due to particular circumstances in which different factors (e.g., age of participants, socioeconomic status, experimental tasks) are involved (i.e., Paap et al., 2015 ).

The current systematic review summarizes the results of 53 studies published between 1999 and 2020 that investigated the effect of bilingualism on executive functions. Analyzing the selected studies, it emerged that the participants had very different characteristics and wide variability in the sample size, ranging from a minimum of 12 participants ( Carlson and Meltzoff, 2008 ) to a maximum of 1740 ( Dick et al., 2019 ). Furthermore, the studies adopted various tasks for the assessment of executive functions. These methodological differences could explain the mixed results found, making it difficult to draw definitive conclusions about the existence of the bilingual effect.

Evidence supporting the existence of the bilingual effect appears when inhibitory control and cognitive flexibility are assessed. In particular, the Sky Search task, the Flanker task, the Dimensional Change Card Sort task, and the Trail Making Test seem to indicate the existence of a bilingual effect. A deeper analysis of the characteristics of the studies included reveals several differences that should lead to a cautious interpretation of the results. The great variability of the experimental tasks becomes evident when considering the studies that used the Stroop task. In particular, the nine studies adopted six different versions of the task. Six studies used different versions of the task with verbal stimuli (i.e., pencil-paper version; computerized version; oral responses version), and found no significant differences between different groups. Two studies used two different versions with non-verbal stimuli, and no significant differences emerged between monolinguals and bilinguals. Two studies used the numerical version, and mixed results were found. However, determining the degree of incidence of the type of stimulus is not possible since no study included both verbal and non-verbal versions of the task. Furthermore, it is not possible to exclude the incidence of the linguistic aspect in the numerical version of the task. As pointed out by Duñabeitia et al. (2014) , it is possible that the linguistic representations of the numbers in the two known languages were active in bilingual brains, and the same may have happened in the non-verbal version since stimuli were used that can be easily verbalized.

Different versions of the task were included in the studies that adopted the Flanker task. The most variable feature was the type of stimulus used (i.e., fish; chevron). Mixed results also emerged in three studies where the same version of the Flanker task was used. Two studies ( Engel de Abreu et al., 2012 , 2014 ) confirmed the bilingual effect, while in Blom et al. (2017) no significant differences emerged. It can be hypothesized that the mixed results may be caused by differences in the participants' linguistic and cultural backgrounds. In two studies ( Engel de Abreu et al., 2012 , 2014 ) bilingual participants were recruited in the Grand Duchy of Luxembourg, a trilingual country with a trilingual education system where children start formal education in the first language at age 4, are exposed to the second language at age six and to the third language at age 7. As the participants in the studies were, on average, eight years old, the bilingual participants included participants that could be considered “trilingual.” In Blom et al. (2017) , three groups of bilingual participants who knew three different language pairs were included. The monolinguals' characteristics may also have influenced the results since, in two studies ( Engel de Abreu et al., 2012 , 2014 ), they were recruited in a different country than the bilinguals. It cannot be excluded that cultural aspects influenced the results.

Most studies that used ANT to evaluate attentive networks did not reveal significant differences between the monolingual and bilingual groups. Again, different factors may have influenced the results. Some authors (e.g., Mullane et al., 2016 ; Lewis et al., 2018 ) highlighted that the child version of the ANT could generate a lower interference effect than the adult version despite the fact that increasing the level of motivation of children to perform the experimental task. When children are evaluated with the adult version, developmental differences emerge that are not visible with the child version. Future studies may adopt the adult version for the assessment of attention in bilinguals. In Yang and Yang (2016) , which found faster reaction times and better accuracy in bilinguals, bilingual participants' cultural. and linguistic background may have influenced the results. Bilingual participants knew a language pair composed of two languages belonging to two different language families, characterized by significant orthographic differences (i.e., Korean-English). This factor seems to have a positive effect on visuospatial abilities ( Yang and Yang, 2016 ). Furthermore, belonging to certain cultures (e.g., Chinese culture) seems to positively influence the development of executive functions ( Carlson and Meltzoff, 2008 ). Also, in Kapa and Colombo (2013) , the bilingual participants' characteristics seem to have a role in the differences that emerged. In the study, the early bilinguals showed better attentive abilities than the monolinguals, but this advantage did not characterize the late bilinguals.

Even in the studies that evaluated the shifting ability with DCCS, some conflicting results emerged. In Park et al. (2018) , significant differences in reaction times emerged between the two groups of participants in the most demanding condition. Other studies using this task confirmed a bilingualism effect. However, it is important to note that in almost all the other studies only the participants' accuracy was assessed. The study of Park et al. (2018) would indicate that the task is too simple for the age considered: the participants included in this study were older compared to the other studies. In Escobar et al. (2018) , the bilinguals had faster reaction times than the monolinguals, but this difference was not significant. The small number of participants (i.e., 17 bilinguals and 17 monolinguals) may have reduced the statistical power of the results.

Another task that showed mixed results is the verbal fluency task. Once again, it is important to highlight that the studies included adopted different versions of this task. Most of the studies that assessed executive functions using category fluency required the participants to name words belonging to the “animals” category. Friesen et al. (2015) used the category “clothing items.” This factor seems to have influenced the results since only in Friesen et al. (2015) did the monolingual group outperformed the bilinguals, whereas, in the other studies, there were no significant differences between the two groups or better performance in the bilinguals. Regarding the letter fluency, several methodological differences emerged. The studies adopted different letters, modalities of administration of the task (oral vs. written production), duration of the test (5 min vs. 1 min), or modalities of calculation of the final score (inclusion or exclusion of incorrect words). Concerning verbal and visual working memory, the evidence for better performance of the bilingual group is limited. In some studies, bilingual participants presented lower performance than monolinguals in the verbal working memory. This result would seem to be mediated by the linguistic abilities of the participants: in Bialystok (2010) , bilinguals showed worse performance than monolinguals only when bilinguals showed a reduced vocabulary size than monolinguals.

Ladas et al. (2015) suggested that, in experimental tasks using verbal stimuli, the absence of a significant result could be interpreted as a bilingual advantage because it is well-known that the vocabulary size of bilinguals, if it is calculated considering only one language, is reduced when compared to that of monolinguals. For example, in Blom et al. (2014) , when the difference in vocabulary size was statistically controlled, a bilingual effect emerged in both the Dot Matrix task and the Digit Backward Recall. However, the absence of significant differences in the performance of bilinguals and monolinguals also emerges in non-verbal tasks, and sometimes even studies using the same experimental task did not observe the same results. These findings suggest that other factors, such as the characteristics of the experimental tasks and the participants, influence the results. The wide variety of tests used for assessing executive functions, which are frequently modified by research groups, makes it difficult to compare the results directly. In several cases, a specific test is used in a single study, or when more than one experimental task is used, the tests chosen had low convergent validity. As suggested by Paap et al. (2015) , each study should include a minimum of two tasks to evaluate each executive function. This methodological choice would make it possible to confirm that controlling that the results are not due to task-specific characteristics. Another point to clarify is whether the bilingual effect only emerges when the task requires a specific degree of complexity or the coordination of several executive functions. In Barac et al. (2016) , which included tasks of increasing difficulty, no differences were observed in the easier task (gift delay), while bilinguals showed an advantage in the more complex tasks (Flanker task and Go/No-Go task). Conversely, in the studies using the Corsi test, the bilingual effect emerges only when an easier version of the task was used (Frog Task Matrix).

The studies included in this systematic review provide an overview of the variability of the population considered in studies on bilingualism. Some studies include bilingual participants who know different language pairs (e.g., Engel de Abreu, 2011 ; Friesen et al., 2015 ), and other participants who are children of immigrants who may face different cultural, family and social contexts (e.g., Leikin and Tovli, 2014 ; Ladas et al., 2015 ). Moreover, information about the acquisition and the use of known languages is not always given, and it does not allow determining the type of bilingualism (i.e., simultaneous or sequential) or the interactional context. Information, such as the age of acquisition of the first and second language, the degree of exposure, and the daily use of the languages, would lead to select better bilinguals. It could allow verifying the possible effects of these characteristics. Knowing the same languages does not determine having shared the same bilingual experience because the interactional contexts in which languages are used may not be the same ( Antoniou, 2019 ). Most studies included in this review do not include information about the context in which language exchanges occur, and linguistic contexts can be very different.

For the classification of participants in bilinguals and monolinguals, parental and self-reports are usually used as they are considered reliable instruments for evaluating experience related to second language acquisition ( Gutiérrez–Clellen and Kreiter, 2003 ; Bedore et al., 2011 ). The lack of detailed information about the bilingual experience could lead to an incorrect classification of the participants, not allowing them to detect any differences. This problem is highlighted by Poarch and Bialystok (2015) , who included a group of partial bilinguals (i.e., native speakers of English who had been learning French for about 2 years) that achieved the same performance as monolinguals. The inclusion of these participants in the bilingual group would have nullified the difference in performance between bilinguals and monolinguals. Another aspect to consider is when children begin formal school education. When children begin school, they are exposed to one or more foreign languages depending on the educational program. Therefore, information on the weekly frequency of exposure and use of the foreign language should be collected.

Some sociodemographic factors, such as low socioeconomic or immigrant status, affect the development of executive functions. Frequently migrant population has a low socioeconomic status, and their bilingualism is often secondary to the migration in a foreign country. In America, there is a high association between low SES and bilingualism. Several studies confirm that belonging to families with low socioeconomic status has negative consequences on the development of different cognitive functions and language skills. In this adverse situation, bilingualism seems to act as a protective factor ( Hartanto et al., 2019 ); in fact, some studies (e.g., Engel de Abreu et al., 2012 ; Krizman et al., 2016 ) reported an advantage of bilingual participants when the socioeconomic status was controlled. The cognitive advantage of bilingualism can be developed independently by the SES ( Blom et al., 2014 ; Calvo and Bialystok, 2014 ).

Further, it needs to clarify at which specific point in the lifespan the bilingual effect should be studied. The strongest evidence supporting the bilingual effect comes from studies that have included participants with executive functions that are not at a maximum level (e.g., older people). The bilingual effect should be evident in children because they have not yet reached the full development of cognitive functions ( Antón et al., 2014 ). Most of the studies in this review investigated the existence of the bilingual effect in children between 5 and 9 years of age. Only thirteen studies included early adolescent participants (10–14 years), while none included middle adolescent participants (15–17 years). The longitudinal study by Park et al. (2018) showed that results could be influenced by time points when individuals are tested and that the various components of the executive functions would seem to follow different trajectories of development. In this study, the bilinguals and monolinguals achieved the same performance when individuals were tested for updating abilities while a bilingual effect in inhibition skills emerged at time 2 but not at time 1. Finally, an advantage was found for the bilingual group in terms of shifting abilities at both times 1 and 2 for mixing cost, while no advantage was found for shifting and switching cost. In addition to age, the test used would also seem to influence the results: in Struys et al. (2018) in which groups of participants of different ages were compared, a smaller congruency effect was found in the group of younger bilinguals (mean age 6.6 years) on the Simon task and a smaller congruency effect for older bilinguals (mean age 11.7 years) on the flanker task. Longitudinal studies should be conducted to investigate whether bilingualism affects the development trajectories of executive functions. It is still unclear how much “training” of the executive functions (in terms of years or time spent on the use of the two languages) is necessary to produce a difference between bilinguals and monolinguals and, therefore, when the condition of bilingualism generates an advantage.

Limitations

This systematic review of the literature has not reached a definitive conclusion about the bilingual effect. This limitation is due to the high variability of the results observed by the different studies. Moreover, as Leivada et al. (2020) recently pointed out, systematic reviews assume that a comparison is made among studies that include similar populations, which is often not the case with these bilingual studies. In the studies on bilingualism, the adoption of a dichotomous “monolingual vs. bilingual” approach and the absence of a shared definition of bilingualism has led to an oversimplification of reality and the inclusion of individuals with very different characteristics in the same group. DeLuca et al. (2019) suggested the need to consider bilingualism as a spectrum of experiences that can affect neural plasticity. Moreover, the monolingual group also presents a degree of variability that should not be ignored ( Baum and Titone, 2014 ). Several aspects of the experience of individuals or groups would seem to affect brain adaptation differently. A quantitative analysis of the literature would have allowed stronger conclusions, but it was impossible to use a metanalytic approach because of the variability of the experimental tasks adopted in the different researches. Comparing the effects size and statistical analysis of the various studies could help to understand the results better. Future studies should analyze the characteristics of the participants more, and verify which factors, such as the AoA or the daily use of each language, influence the results.

Conclusions

The results summarized in this systematic review indicate the need for further studies that should consider the factors that have been identified as possible modulators of the observed results. Future studies should provide more information about the language context in which bilingual participants are immersed. It would be useful to establish guidelines identifying the minimum information needed to be included in the studies for the description of the bilingual population. Several researchers have highlighted the need to adopt a new approach to the study of this topic. Large-scale research projects involving several laboratories worldwide would provide clearer answers about the existence of a positive effect of bilingualism and identify the variables involved in this process ( Baum and Titone, 2014 ; Leivada et al., 2020 ). From the summary of the studies included in this systematic review, it emerges that current evidence does not make it possible to establish the existence of a bilingual effect or to identify the factors involved in determining the bilingual effect. Since bilingualism is a reality concerning a substantial percentage of the population, it is important to clarify this topic. A result in favor of the existence of the bilingual effect would provide the incentive for the implementation of bilingual school programs that could lead to extensive and regular use of more than one language. On the contrary, a reduction in performance linked to the condition of bilingualism would indicate the need to develop support programs aimed at those who, due to various circumstances, such as immigrant status or bilingual school education, are facing this situation. Executive functions are included in life skills, i.e., psychosocial skills that, if properly trained, enable the prevention of social and health problems, the promotion of social and personal development, and the protection of human rights. The absence of specific tests for the evaluation of bilinguals suggests the need to develop ad hoc instruments or to provide the validation of existing tests for this specific population. Tests containing verbal stimuli, used to make diagnoses, could lead to an overestimation of the problems. It would be useful to conduct a further systematic review focusing on the adult population to analyze the effect of bilingualism on those who have reached a peak or are in a phase of decline of executive functions.

Data Availability Statement

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author/s.

Author Contributions

JG and MC: conceptualization of the review, literature search, writing of the original draft, revision, and editing of the manuscript. DM: conceptualization of the review, writing of the original draft, revision, and editing. FF and SP: revision and editing of the manuscript. All authors contributed to the article and approved the submitted version.

This work was supported by FONDECYT 1181472 of the National Commission for Scientific and Technological Research of Chile, and project 21172/IV/19 granted by Fundación Séneca-Agencia de Ciencia y Tecnología de la Región de Murcia (Spain).

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Zelazo, P. D., Jacques, S., Burack, J. A., and Frye, D. (2002). The relation between theory of mind and rule use: evidence from persons with autism-spectrum disorders.? Infant Child Dev. Int. J. Res. Pract. 11, 171–195. doi: 10.1002/icd.304

Zeng, Z., Kalashnikova, M., and Antoniou, M. (2019). Integrating bilingualism, verbal fluency, and executive functioning across the lifespan. J. Cogn. Dev. 20, 656–679. doi: 10.1080/15248372.2019.1648267

Keywords: bilingualism, executive functions, bilingual advantage, inhibition, shifting, working memory

Citation: Giovannoli J, Martella D, Federico F, Pirchio S and Casagrande M (2020) The Impact of Bilingualism on Executive Functions in Children and Adolescents: A Systematic Review Based on the PRISMA Method. Front. Psychol. 11:574789. doi: 10.3389/fpsyg.2020.574789

Received: 21 June 2020; Accepted: 24 August 2020; Published: 06 October 2020.

Reviewed by:

Copyright © 2020 Giovannoli, Martella, Federico, Pirchio and Casagrande. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY) . The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Jasmine Giovannoli, jasmine.giovannoli@uniroma1.it ; Maria Casagrande, maria.casagrande@uniroma1.it

Disclaimer: All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.

Bilingualism as a Life Experience

• Posted October 1, 2015
• By Bari Walsh

What do we know about bilingualism? Much of what we once thought we knew — that speaking two languages is confusing for children, that it poses cognitive challenges best avoided — is now known to be inaccurate. Today, bilingualism is often seen as a brain-sharpening benefit, a condition that can protect and preserve cognitive function well into old age. 

Indeed, the very notion of bilingualism is changing; language mastery is no longer seen as an either/or proposition, even though most schools still measure English proficiency as a binary “pass or fail” marker.

A growing body of evidence suggests that lifelong bilingualism is associated with the delayed diagnosis of dementia. But the impact of language experience on brain activity has not been well understood.

It turns out that there are many ways to be bilingual, according to HGSE Associate Professor Gigi Luk , who studies the lasting cognitive consequences of speaking multiple languages. “Bilingualism is a complex and multifaceted life experience,” she says; it’s an “interactional experience” that happens within — and in response to — a broader social context.

Usable Knowledge spoke with Luk about her research and its applications.

Bilingualism and executive function

As bilingual children toggle between two languages, they use cognitive resources beyond those required for simple language acquisition, Luk writes in a forthcoming edition of the Cambridge Encyclopedia of Child Development . Recent research has shown that bilingual children outperform monolingual children on tasks that tap into executive function — skills having to do with attention control, reasoning, and flexible problem solving.

Their strength in those tasks likely results from coping with and overcoming the demand of managing two languages. In a bilingual environment, children learn to recognize meaningful speech sounds that belong to two different languages but share similar concepts.

In a paper published earlier this year , she and her colleagues looked at how bilingualism affects verbal fluency — efficiency at retrieving words — in various stages of childhood and adulthood. In one measure of verbal acumen called letter fluency — the ability to list words that begin with the letter F, for instance — bilinguals enjoyed an advantage over monolinguals that began at age 10 and grew robust in adulthood.  

Bilingualism and the aging brain

Luk and her researchers are looking at the neuroscience of bilingualism — at how bilingualism may affect the physical structure of the brain in its different regions.    

What they’ve found so far shows that older adults who are lifelong bilinguals have more white matter in their frontal lobes (important to executive function) than monolinguals, and that their temporal lobes (important to language function) are better preserved. The results support other evidence that persistent bilingual experience shapes brain functions and structures.

A growing body of evidence suggests that lifelong bilingualism is associated with the delayed diagnosis of dementia. But the impact of language experience on brain activity is not well understood, Luk says.

In a 2015 paper, she and her colleagues began to look at functional brain networks in monolingual and bilingual older adults. Their findings support the idea that a language experience begun in childhood and continued throughout adulthood influences brain networks in ways that may provide benefits far later in life.

Who is bilingual?

Monolingualism and bilingualism are not static categories, Luk says, so the question of what it means to be bilingual, and who is bilingual, is nuanced. There are several pathways to bilingualism. A child can become bilingual when parents and caregivers speak both languages frequently, either switching between the two. A child can be bilingual when the language spoken at home differs from a community’s dominant language, which the child is exposed to in schools. Or a child can become bilingual when he or she speaks the community’s dominant language at home but attends an immersion program at school.  

Bilingualism is an experience that accumulates and changes over time, in response to a child’s learning environments, says Luk.

Language diversity in schools

In one of her projects, Luk works with a group of ELL directors to help them understand the diverse needs of their language learners and to find better ways to engage their parents. She’s looking at effective ways to measure bilingualism in schools; at connections between the science of bilingualism and language and literacy outcomes; and at the long-term relationship between academic outcomes and the quality and quantity of bilingual experience in young children.

Part of her goal is to help schools move beyond binary categorizations like “ELL” and “English proficient” and to recognize that language diversity brings challenges but also long-term benefits.

“If we only look at ELL or English proficient, that’s not a representation of the whole spectrum of bilingualism,” she says. “To embrace bilingualism, rather than simply recognizing this phenomenon, we need to consider both the challenges and strengths of children with diverse language backgrounds. We cannot do this by only looking at English proficiency. Other information, such as home language background, will enrich our understanding of bilingual development and learning.”  

Additional Resources

• A Boston community organization that runs a bilingual preschool spoke with Luk about her work and its applications to practice. Read the interview.

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Babies' Bilingualism Begins in the Womb, Scientists Reveal

B abies who hear more than one language while they're growing in the womb may be more able to hear a wider range of speech after they're born.

While babies with mothers that only spoke one language were found to be tuned into a specific pitch—corresponding with their mother's language—babies of bilingual mothers were more sensitive to a wider range of pitches, and less tuned into a specific one, according to a new paper in the journal Frontiers in Human Neuroscience.

This implies that bilingual babies start to learn about languages differently before they're even born.

Previous research over the past few decades has found that babies learn about speech in the womb, especially in the third trimester, with newborns preferring their mother's voice to others and even recognizing stories told to them while they were in the womb.

However, not much research has yet been done into how fetuses process speech if their mother is bilingual and speaks multiple languages while pregnant.

"Languages vary in the timing aspects of speech, such as rhythm and accentuation, but also pitch and phonetic information. This means that fetuses from bilingual mothers are expected to be immersed in a more complex acoustic environment that those from monolingual mothers," study co-author Carles Escera, a professor at the University of Barcelona's Institute of Neurosciences, said in a statement.

In the paper, the researchers—from the University of Barcelona in Spain—describe how they studied 131 sets of mothers and babies aged between 1 and 3 days old in Catalonia, where 42 percent of the population regularly speak both Catalan and Spanish.

The mothers completed a questionnaire, where it was found that 41 percent of them spoke only one language during their pregnancy (9 percent of which was in Catalan and 91 percent in Spanish), while the remaining 59 percent spoke both, or one and another language.

The researchers then measured the babies' brain responses using electrodes on their foreheads, which they used to determine an electrophysiological brain response known as the "frequency-following response" (FFR). They then tested the babies' reactions to varying speech sounds that represented sounds common to the different languages: the vowel /a/ at a steady pitch (like in the word "wash"), /a/ rising in pitch, the vowel /o/, and a transition.

"The contrasting vowels /o/ and /a/ belong to the phonetic repertoire of both Spanish and Catalan, which is partly why we chose them," co-author Sonia Arenillas-Alcón, also a researcher at the University of Barcelona, said in the statement.

"Low frequency sounds like these vowels are also transmitted through the womb reasonably well, unlike mid- and high- frequency sounds that reach the fetus in a degraded and attenuated manner."

The results showed that the monolingual babies' brains showed a higher response to the the /o a/ sound, meaning that they were more sensitive to this specific pitch found in their mother's language.

The bilingual babies, on the other hand, were more sensitive to all the sounds in general, with no specific uptick in response for any given sound.

"Here we show that exposure to monolingual or a bilingual speech has different effects at birth on 'neural encoding' of voice pitch and vowel sounds: that is, how information about these aspects of speech has been initially learned by the fetus," study co-author Natàlia Gorina-Careta, a researcher at the University of Barcelona, said in the statement.

"At birth, newborns from bilingual mothers appear more sensitive to a wider range of acoustic variation of speech, whereas newborns from monolingual mothers seem to be more selectively tuned to the single language they have been immersed in."

This may represent a trade-off between selectivity and efficiency in how newborns and fetuses learn about pitch and speech.

"Our data show that prenatal language exposure modulates the neural encoding of speech sounds as measured at birth. These results emphasize the importance of prenatal language exposure for the encoding of speech sounds at birth, and provide novel insights into its effects," said Escera.

The researchers stress that the languages spoken while pregnant likely have very little impact on how the baby learns about speech after birth, so monolingual and bilingual parents need not worry about the impact their speech has on their children.

"Based on our results, we cannot make any recommendation to multilingual parents. The sensitive period for language acquisition lasts long after birth, and thus postnatal experience may well overshadow the initial changes undertaken in the womb," said Jordi Costa Faidella, an associate professor at the University of Barcelona.

"Future investigation into how a bilingual language environment modulates sound encoding during the first years of life will shed more light into this issue."

Do you have a tip on a science story that Newsweek should be covering? Do you have a question about speech? Let us know via [email protected].

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Computer Science > Artificial Intelligence

Title: generation and human-expert evaluation of interesting research ideas using knowledge graphs and large language models.

Abstract: Advanced artificial intelligence (AI) systems with access to millions of research papers could inspire new research ideas that may not be conceived by humans alone. However, how interesting are these AI-generated ideas, and how can we improve their quality? Here, we introduce SciMuse, a system that uses an evolving knowledge graph built from more than 58 million scientific papers to generate personalized research ideas via an interface to GPT-4. We conducted a large-scale human evaluation with over 100 research group leaders from the Max Planck Society, who ranked more than 4,000 personalized research ideas based on their level of interest. This evaluation allows us to understand the relationships between scientific interest and the core properties of the knowledge graph. We find that data-efficient machine learning can predict research interest with high precision, allowing us to optimize the interest-level of generated research ideas. This work represents a step towards an artificial scientific muse that could catalyze unforeseen collaborations and suggest interesting avenues for scientists.

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Bilingualism: Consequences for Mind and Brain

Ellen bialystok.

York University

Fergus I.M. Craik

Rotman Research Institute of Baycrest

Harvard Graduate School of Education

Building on earlier evidence showing a beneficial effect of bilingualism on children’s cognitive development, we review recent studies using both behavioral and neuroimaging methods to examine the effects of bilingualism on cognition in adulthood and explore possible mechanisms for these effects. This research shows that bilingualism has a somewhat muted effect in adulthood but a larger role in older age, protecting against cognitive decline, a concept known as “cognitive reserve”. We discuss recent evidence that bilingualism is associated with a delay in the onset of symptoms of dementia. Cognitive reserve is a crucial research area in the context of an aging population; the possibility that bilingualism contributes to cognitive reserve is therefore of growing importance as populations become increasingly diverse.

Why bilingualism?

It is generally believed that more than half of the world’s population is bilingual [ 1 ]. In each of the U.S. [ 2 ] and Canada [ 3 ], approximately 20% of the population speaks a language at home other than English. These figures are higher in urban areas, rising to about 60% in Los Angeles [ 4 ] and 50% in Toronto [ 3 ]. In Europe, bilingualism is even more prevalent: In a recent survey, 56% of the population across all European Union countries reported being functionally bilingual, with some countries recording particularly high rates, such as Luxembourg at 99% [ 5 ]. Bilinguals, therefore, make up a significant portion of the population. Importantly, accumulating research shows that the development, efficiency, and decline of crucial cognitive abilities are different for bilinguals than for monolinguals. What are these cognitive differences and how does bilingualism lead to these changes?

The context for examining how bilingualism affects cognitive ability is functional neuroplasticity, the study of how experience modifies brain structure and brain function. Such modifications have been found following experiences as diverse as juggling [ 6 ], video-game playing [ 7 ], careers in architecture [ 8 ] taxi-driving [ 9 ], and musical training [ 10 , 11 ]. Bilingualism is different from all of these: like juggling and playing video games it is intense, and like architecture and driving taxis in London it is sustained, but unlike these experiences, bilinguals are not typically pre-selected for talent or interest. Although bilinguals undoubtedly differ from monolinguals in certain ways, they generally did not choose bilingualism. Rather, the circumstances of their family, place of birth, or immigration history simply required that they learn more than one language.

What is different about bilingual minds?

It has long been assumed that childhood bilingualism affected developing minds but the belief was that the consequences for children were negative: learning two languages would be confusing [ 12 ]. A study by Peal and Lambert [ 13 ] cast doubt on this belief by reporting that children in Montreal who were either French-speaking monolinguals or English-French bilinguals performed differently on a battery of tests. The authors had expected to find lower scores in the bilingual group on language tasks but equivalent scores in nonverbal spatial tasks, but instead found that the bilingual children were superior on most tests, especially those requiring symbol manipulation and reorganization. This unexpected difference between monolingual and bilingual children was later explored in studies showing a significant advantage for bilingual children in their ability to solve linguistic problems based on understanding such concepts as the difference between form and meaning, that is, metalinguistic awareness [ 14 – 20 ] and nonverbal problems that required participants to ignore misleading information [ 21 , 22 ].

Research with adult bilinguals built on these studies with children and reported two major trends. First, a large body of evidence now demonstrates that the verbal skills of bilinguals in each language are generally weaker than are those for monolingual speakers of each language. Considering simply receptive vocabulary size, bilingual children [ 23 ] and adults [ 24 ] control a smaller vocabulary in the language of the community than do their monolingual counterparts. On picture-naming tasks, bilingual participants are slower [ 25 – 28 ] and less accurate [ 29 , 30 ] than monolinguals. Slower responses for bilinguals are also found for both comprehending [ 31 ] and producing words [ 32 ], even when bilinguals respond in their first and dominant language. Finally, verbal fluency tasks are a common neuropsychological measure of brain functioning in which participants are asked to generate as many words as they can in 60 seconds that conform to a phonological or semantic cue. Performance on these tasks reveals systematic deficits for bilingual participants, particularly in semantic fluency conditions [ 33 – 37 ], even if responses can be provided in either language [ 38 ]. Thus, the simple act of retrieving a common word is more effortful for bilinguals.

In contrast to this pattern, bilinguals at all ages demonstrate better executive control than monolinguals matched in age and other background factors. Executive control is the set of cognitive skills based on limited cognitive resources for such functions as inhibition, switching attention, and working memory [ 39 ]. Executive control emerges late in development and declines early in aging, and supports such activities as high level thought, multi-tasking, and sustained attention. The neuronal networks responsible for executive control are centered in the frontal lobes, with connections to other brain regions as necessary for specific tasks. In children, executive control is central to academic achievement [ 40 ], and in turn, academic success is a significant predictor of long term health and well being [ 41 ]. In a recent meta-analysis, Adesope et al. [ 42 ] calculated medium to large effect sizes for the executive control advantages in bilingual children and Hilchey and Klein [ 43 ] summarized the bilingual advantage over a large number of studies with adults. This advantage has been shown to extend into older age and protect against cognitive decline [ 25 , 44 , 45 ], a point we return to below.

In this review, we examine the evidence for bilingual advantages in executive control and explore the possible mechanisms and neural correlates that may help to explain them. Our conclusion is that lifelong experience in managing attention to two languages reorganizes specific brain networks, creating a more effective basis for executive control and sustaining better cognitive performance throughout the lifespan.

Language Processing in Bilinguals

Joint activation of languages.

A logical possibility for the organization of a bilingual mind is that it consists of two independently-represented language systems that are uniquely accessed in response to the context: A fluent French-English bilingual ordering coffee in a Parisian café has no reason to consider how to form the request in English, and a Cantonese-English bilingual studying psychology in Boston does not need to recast the material through Chinese. Yet, substantial evidence shows that this is not how the bilingual mind is organized. Instead, fluent bilinguals show some measure of activation of both languages and some interaction between them at all times, even in contexts that are entirely driven by only one of the languages.

The evidence for this conclusion comes from psycholinguistic studies using such tasks as cross-language priming (in which a word in one language facilitates retrieval of a semantically related word in the other language) and lexical decision (in which participants decide whether a string of letters is a legal word in one of the languages) that show the influence of the currently unused language for both comprehension and production of speech [ 46 – 52 ]. Further evidence comes from patient studies showing intrusions from the irrelevant language or inappropriate language switches [ 53 ], and imaging studies indicating involvement of the non-target language while performing a linguistic task in the selected language [ 54 – 56 ]. Using eye-tracking technology, for example, Marian, Spivey, and Hirsch [ 57 ] reported that English-Russian bilinguals performing a task in English in which they had to look at the named picture from four alternatives were distracted by a picture whose name shared phonology with Russian even though there was no connection to the meaning of the target picture and no contextual cues indicating that Russian was relevant. Similarly, Thierry and Wu [ 58 ] presented English monolinguals, Chinese-English bilinguals, and Chinese monolinguals with pairs of words in English (translated to Chinese for Chinese monolinguals) and asked participants to decide if the words were semantically related or not. The manipulation was that half of the pairs contained a repeated character in the written Chinese forms, even though that orthographic feature was unrelated to the English meaning. Waveforms derived from analyses of electroencephalography (EEG) are used to indicate the neuronal response to language on a millisecond by millisecond scale. This event-related potential (ERP) signals the effort associated with integrating the meaning of words in a negative waveform about 400 milliseconds after the stimulus, a waveform called the N400. The more similar the words are to each other, the smaller is the amplitude of the N400. In the study by Thierry and Wu, semantic relatedness was associated with significantly smaller N400 amplitude in all groups as expected, but the repeated character also led to smaller N400 for the two Chinese groups. Thus, although irrelevant to the task, participants were accessing the Chinese forms when making judgments about the semantic relation between English words. Subsequent research has refined these results by showing their basis in the phonology rather than the orthography of spoken language [ 59 ] and extended the phenomenon to the phonological hand forms of American Sign Language [ 60 ].

This joint activation is the most likely mechanisms for understanding the consequences of bilingualism for both linguistic and nonlinguistic processing. For linguistic processing, joint activation creates an attention problem that does not exist for monolinguals: In addition to selection constraints on such dimensions as register, collocation, and synonymy, the bilingual speaker also has to select the correct language from competing options. Although joint activation creates a risk for language interference and language errors, these rarely occur, indicating that the selection of the target language occurs with great accuracy. However, this need to select at the level of language system makes ordinary linguistic processing more effortful for bilinguals than monolinguals and explains some of the costs in psycholinguistic studies described above. For nonlinguistic processing, the need to resolve competition and direct attention is primarily the responsibility of general cognitive systems, in particular executive functions. The possible influence of linguistic processes on nonlinguistic executive control has enormous consequences for lifespan cognition and is discussed in the next section.

Consequences of joint activation

An appealing suggestion for how the executive control system achieves linguistic selection in the context of joint activation is through inhibition of the non-target language. At least two influential models have been proposed that place inhibition at the center of this selection. The first, the Inhibitory Control model [ 61 ] is based on the Supervisory Attentional System [ 62 ] and extends a domain-general and resource-limited attention system to the management of competing languages. The second, the Bilingual Interactive Activation Model (BIA+) [ 63 ], uses computer simulation to model lexical selection from both intralingual and extralingual competitors. Although both models assign a primary role to inhibition, they are quite different from each other and address a different aspect of the selection problem. It is useful, therefore, to consider the distinction between global inhibition and local inhibition proposed by De Groot and Christoffels [ 64 ]. Global inhibition refers to suppression of an entire language system, as in inhibiting French when speaking English, and local inhibition refers to inhibition of a specific competing distractor, such as the translation equivalent of the required concept. Both processes are required for fluent language selection but the two are carried out differently. Guo, Liu, Misra, and Kroll [ 65 ] used functional magnetic resonance imaging (fMRI) to demonstrate the recruitment of different systems for each of global inhibition (dorsal left frontal gyrus and parietal cortex) and local inhibition (dorsal anterior cingulate cortex, supplementary motor area) in a sample of Chinese-English bilinguals, and validated their distinct roles in bilingual language control. While Green’s inhibitory control model is consistent with both types of inhibition, Dijkstra’s BIA+ modeling item selection in local inhibition.

These types of inhibition also differ in their primary domain of influence, with local inhibition largely affecting linguistic performance and global inhibition affecting both linguistic and cognitive performance. The linguistic outcomes of inhibition are reduced speed and fluency of lexical access for bilinguals as described above. However, performance also requires a selection bias towards the target language, showing a role for activation [ 66 , 67 ] as well as inhibition. These alternatives are not mutually exclusive but indicate the need for a more complete description of how attention is managed in bilingual language processing. Ultimately the degree of both inhibition and activation are relative rather than absolute and will be modulated by contextual, linguistic, and cognitive factors. The cognitive outcomes of linguistic inhibition are enhanced attentional control and will be described more fully in the next section. Importantly, the cognitive and linguistic outcomes are related. Three studies have reported a relationship between inhibition and ability in verbal and nonverbal tasks by showing a correlation between Stroop task performance and competing word selection [ 68 ], Simon task performance and language switching in picture naming [ 69 ], and cross-language interference and a variety of executive control measures [ 70 ]. Such results point to an extensive reorganization of cognitive and linguistic processes in bilinguals.

Cognitive networks in bilinguals

Bilingual performance on conflict tasks.

Early evidence that bilingual children solved nonverbal conflict tasks differently from monolingual children was reported in a study by Bialystok and Majumder [ 21 ]. Eight-year-old children were given a variety of nonverbal problems to solve, some of which contained perceptual distraction (block design from the Wechsler Intelligence Scale for Children (WISC) [ 71 ] and some which did not (Noelting's Juice Task [ 72 , 73 ]). Bilingual children outperformed monolinguals on the conflict tasks, but children in the two groups were comparable on tasks that did not include distracting perceptual information. This pattern has been confirmed in studies of both children and adults using a flanker task (children: [ 74 , 75 ]), theory of mind task (children: [ 76 , 77 ]; adults: [ 78 ]), Simon task (children: [ 79 ]; adults: [ 44 ]). Other studies with adults have shown better performance by bilinguals in naming the font color in a Stroop task [ 25 ], smaller costs in task switching [ 80 ], better ability to maintain task set in an attention task [ 81 ], and more susceptibility to negative priming, presumably because of greater inhibition [ 82 ].

Some studies have extended these bilingual advantages into older age. Bialystok, Craik, Klein and Viswanathan [ 44 ] reported an experiment in which middle-aged and older adults who were either monolingual or bilingual were given a version of the Simon task. Participants were shown either a green or a red square on each trial, and the task was to press an associated response key as rapidly as possible. The keys were located at each side of the presentation screen. In one condition, the squares appeared centrally on the screen, so there was no spatial conflict between the location of stimuli and responses; in this condition there were no reaction-time (RT) differences between language groups. In a second condition, the colored squares appeared laterally on the screen, either directly above the appropriate response key (congruent condition) or on the other side of the screen, above the incorrect response key (incongruent condition). The RT difference between congruent and incongruent response trials (the Simon effect) is a measure of attentional control. Bilinguals produced smaller Simon effects than monolinguals at all ages.

Three other results from this study are noteworthy. First, the decrease in attentional control in older adults was reduced in the bilingual groups, suggesting that bilingualism may be protective against the effects of cognitive aging. Second, whereas a bilingual advantage was expected for incongruent stimuli, it was also found for congruent stimuli. This result has been replicated in subsequent studies [ 43 ] and is difficult to account for in terms of response conflict or inhibition. Third, prolonged practice reduced both the Simon effect and the size of the bilingual advantage. Apparently all participants can learn to disregard the distracting effects of interfering stimuli given sufficient practice on a task, but it seems that bilinguals can learn this type of inhibition more rapidly. One interesting question in this regard is the extent to which this attenuation of attentional control is specific to the practiced situation, or whether it generalizes to tasks tapping attentional control in a different manner. Our conjecture is that the attenuation effect is context specific.

A complication that has emerged as more results are reported is that the bilingual advantage is not always found in samples of young adults. For example, a study examining performance on the Simon task in 5-year-olds, young, middle-aged and older adults found a bilingual advantage in RT in the 5-year-olds and in the older adults, but not in the young adult group [ 83 ]. Similarly, a study of the Stroop effect in younger and older adults found a bilingual advantage in both age groups but when the same participants performed the Simon arrow task the bilingual advantage was found only in the older adults [ 25 ]. Similarly, Salvatierra and Rosselli [ 45 ] used a simple version of the Simon task and reported a bilingual advantage for older but not younger adults. There is thus some evidence that the bilingual advantage is greatest in children and in older adults, but less constantly present in young adults – perhaps because the young adult group is at the developmentally peak age for cognitive control.

It appears that bilingual advantages for young adults tend to emerge on tasks or conditions that are difficult. For example, Bialystok [ 84 ] found that bilingual young adults outperformed their monolingual counterparts on the directional arrow Simon task, but only on the condition that included more monitoring and switching than a simpler condition. Similarly, several studies by Costa and colleagues have reported a bilingual advantage in young adults [ 75 , 85 , 86 ] but only under some conditions. For example, Costa et al. [ 85 ] demonstrated that the bilingual advantage on a flanker task held only under high monitoring conditions. In versions where most of the trials were of one type (congruent or incongruent) no bilingual advantage was observed; the advantage was found, however, in a condition involving 25% incongruent and 75% congruent trials, although even there the advantage decreased over blocks of the experiment (cf., [ 44 ]). Costa et al. [ 85 ] concluded that the bilingual advantage reflects a more efficient monitoring system for conflict resolution, in that bilinguals may be better at determining when the misleading information can be safely ignored. Finally, Hernández et al. [ 86 ] used a nonlinguistic version of the Stroop effect and found a trend towards both reduced interference and enhanced facilitation in young adult bilinguals compared with monolinguals (cf., older participants in [ 25 ]). One interesting aspect of the studies by Costa, Hernández and colleagues is that the monolinguals were Spanish speakers and the bilinguals’ two languages were Catalan and Spanish. Most of the participants were undergraduate students and were not immigrants, so the two groups were well equated apart from the language difference. In summary, the evidence for a bilingual advantage in younger adults is more sporadic than in other age groups, although at all ages there are some reports of studies showing no difference between monolinguals and bilinguals performing a conflict task.

Neural correlates of cognitive reorganization

Recently, studies have begun to investigate the neural correlates of bilingual processing examined in the behavioral research. The majority of this research has used fMRI to study bilinguals performing a linguistic task in their two languages. Typically, participants name pictures or generate words in response to a cue signaling the required language and performance is compared for single language and mixed language conditions. Two early studies revealed promising results. The first led to the surprising finding that language switching was accompanied by activation in the dorsolateral prefrontal cortex (DLPFC), an area known to be part of the general executive control system [ 28 ]. Less surprising was a study showing the involvement of Broca’s area as well as a left frontal area in a language switching task [ 87 ]. Subsequent research has corroborated the involvement of these systems and shown that language switching elicits a spatially-distributed activation pattern involving bilateral frontal regions, bilateral precentral areas, bilateral caudate, bilateral (or midline) pre-supplementary areas (pre-SMA), and bilateral temporal regions. This pattern has been found for German-French bilinguals [ 88 ], Spanish-Catalan bilinguals [ 89 ], Chinese-English bilinguals [ 65 , 90 , 91 ] and Spanish-English bilinguals [ 92 ]. A few studies [ 65 , 88 ] have also reported activation in anterior cingulate cortex (ACC), but activation in this area is not consistently observed. Abutalebi and colleagues [ 93 ] extended this finding to show activation of ACC for both language switching and nonverbal switching. Importantly, these studies confirm that frontal systems involved in executive control are recruited by bilinguals to manage attention to language.

Abutalebi and Green [ 94 ] conducted a qualitative review of these studies and proposed that the ACC, left prefrontal cortex, left caudate and bilateral supramarginal gyri (SMG) constitute the neural correlates of the control mechanism for bilingual language production. This model was confirmed in a quantitative meta-analysis examining bilingual language switching [ 95 ] ( Figure 1 ). Both the qualitative and quantitative analyses point to multiple cortical regions in which functional activity is altered by bilingualism, but an outstanding question is whether activity in these regions is synchronous, forming a neural network that is responsive to bilinguals’ experience of managing two languages. To this end, a study by Nakamura and colleagues [ 96 ] showed strong connectivity between left inferior frontal gyrus (IFG) and left middle temporal gyrus (MTG) in a group of Japanese-English bilinguals performing a cross-language priming task. The connectivity was stronger in the frontal-temporal coupling than in the reverse direction. This pattern was replicated using transcranial magnetic stimulation (TMS) with Japanese-English bilingual participants performing the same cross-language priming task. Nakamura and colleagues [ 96 ] interpreted the results as indicating top-down control from left IFG to left MTG in a bilingual context.

Bilingual influence on brain function and structure. Transparent brains showing the left and right hemispheres. Green voxels depict grey matter regions showing high activation during bilingual language switching in a meta-analysis ( 94 ). Red-yellow voxels indicate regions of higher white matter integrity in bilingual older adults relative to monolinguals ( 111 ). Together, the functional and structural data indicate that neural correlates of bilingualism are observed in the frontal lobes, generally responsible for higher cognition such as executive functions.

Taken together, fMRI research on bilingual language switching has implicated distributed cortical activation that converges in the frontal regions. Intriguingly, the brain regions related to bilingual switching are also critical for general attention and cognitive control [ 97 , 98 ]. This overlap in brain regions activated for bilingual switching and cognitive control implies that the same mechanisms may be involved in both activities, and that these shared processes might help to explain the superior performance of bilinguals on nonverbal conflict tasks. In other words, using these cognitive control networks for bilingual language processing may reconfigure them for other purposes, providing part of the explanation for the behavioral differences between monolinguals and bilinguals found in nonverbal conflict tasks. Specifically, the evidence suggests that cognitive control networks may be more broadly based in bilinguals as a result of their dual function. However, fMRI studies on language switching in bilinguals can only show that these networks are included in bilingual language selection. Determining whether or not such reconfiguration occurs can only be evaluated by comparing monolinguals and bilinguals performing nonverbal conflict tasks. The hypothesis is that monolinguals and bilinguals will perform nonverbal control problems using somewhat different networks, specifically, that the network used by bilinguals will be more broadly based.

Only a few studies have contrasted the neural correlates of non-linguistic cognitive control in bilinguals and monolinguals. Garbin and colleagues [ 99 ] gave a color-shape switching task to Spanish monolingual and Spanish-Catalan bilingual young adults in fMRI. A bivalent stimulus (e.g., a red circle) and a cue (e.g., “color” or “shape”) were shown, and participants responded to the indicated dimension. Both RT of switch costs and accuracy favored the bilingual participants but activation patterns were also different for the two groups: monolinguals showed increased activation in the right IFG, whereas bilinguals showed increased activation in left inferior frontal gyrus. More interestingly, higher levels of activation in left IFG and left striatum were associated with smaller switch costs for the bilingual participants, but increased activation in the right IFG was associated with larger switch costs. In light of the lack of switching effect in the behavioral data, it is possible that the bilinguals relied more on the left IFG and striatum in face of the demand to switch between responses associated with a bivalent stimulus. The left IFG was identified in both the qualitative [ 94 ] and quantitative [ 95 ] meta-analyses of bilingual language switching. This region is central to speech production [ 100 ] and has been shown to have higher activation for bilinguals than monolinguals during speech production [ 101 , 102 ]. Thus, left IFG appears to be one of the overlapping brain regions in bilinguals handling both language switching and non-linguistic cognitive control.

A study by Luk and colleagues [ 103 ] used an adaptation of a flanker task to compare activation in monolingual and bilingual participants. The stimuli consisted of a string of five chevrons, and the task was to indicate the direction of the red one (that could appear in one of three positions) while ignoring the four black ones. In a previous behavioral study with these stimuli, bilinguals performed this task more rapidly than monolinguals [ 104 ]. The fMRI data were analyzed using a multivariate statistical technique for neuroimaging data (Partial Least Squares; for review, see [ 105 ]) to identify integrated neural networks. The results showed that monolinguals and bilinguals recruited different neural networks for both congruent and incongruent trials. Another condition that tested ‘no-go’ responses indicated no difference between groups. Importantly, greater activity in the bilingual network, including areas identified in the meta-analysis [ 95 ], was related to smaller RT costs for incongruent trials. There are two implications of these results. First, bilingualism alters functional neural network at the response-selection level (congruent and incongruent trials), but not at the motor execution level (response inhibition no-go trials), a pattern consistent with previous results for both adults [ 106 ] and children [ 79 , 107 ]. Second, bilinguals showed a brain-behavior correlation when suppressing interference from conflicting flankers, replicating a previous study using magnetoencephalography (MEG) [ 108 ].

Although bilingualism is a language experience, managing attention to two languages imposes demands on the cognitive system that require brain regions not typically used for language processing. From studies of bilingual language switching and non-linguistic cognitive control, and from the meta-analysis cited earlier, it seems likely that the neural locus of cognitive control in bilinguals lies in bilateral frontal regions. In order to facilitate information transfer between the hemispheres, it is also possible that prolonged bilingual experience alters anatomical structures in addition to cortical functional networks. Cortical activity assessed by fMRI is limited to blood-oxygenated-level-dependent (BOLD) signal in the grey matter. However, when investigating domain-general neural changes (cognitive control) in response to domain-specific experience (bilingualism), it is important to use methods that allow not only the identification of functional networks but also their underlying anatomical structures [ 109 , 110 ].

There is some evidence for the plasticity of cortical grey matter in response to bilingualism. Mechelli and colleagues [ 111 ] reported higher grey matter density in left inferior parietal regions in a group of Italian-English bilinguals relative to English monolinguals. Strikingly, proficiency in English, the second language, correlated positively with grey matter density in this region. A recent study has extended brain plasticity to white matter. Luk, Bialystok, Craik and Grady [ 112 ] ( Figure 1 ) used diffusion tensor imaging (DTI) to measure resting-state functional connectivity in monolingual and bilingual older adults. The results showed higher white matter integrity in bilingual older adults, primarily in the corpus callosum connecting the two hemispheres but also extending to bilateral superior longitudinal fasciculi, right inferior frontal-occipital fasciculus and uncinate fasciculus. Identifying a seed close to the white matter voxels showing a group difference, Luk et al. conducted a resting-state functional connectivity analysis and showed that while both monolinguals and bilinguals had correlating brain activity with contralateral regions at rest, bilinguals had increased anterior-posterior connectivity. This evidence suggests that bilingualism is associated with better maintenance of white matter structures in the course of normal aging [ 113 ]. Similar DTI results have also been recently reported in bilingual children around the left inferior frontal-occipital fasciculus [ 114 ].

Nature of the bilingual advantage

Why might bilingualism be associated with an advantage in attentional control? The need to manage two jointly activated languages apparently leads to an enhancement of frontal-posterior attentional control mechanisms with the consequence that other types of cognitive control are also enhanced. Inhibitory control was suggested as the relevant mechanism in early studies [ 44 , 61 ] and continues to be endorsed by some researchers [ 51 , 115 ]. One problem with this account, however, is the recurrent finding of a bilingual advantage in congruent trials (for which there is no conflict) as well as incongruent trials [ 44 , 75 ]. Minimally, therefore, inhibition alone is insufficient to explain bilingual processing differences. The inhibition view is also challenged by evidence from preverbal infants who demonstrate early effects of bilingualism but for whom language inhibition is not a plausible explanation ( Box 1 ).

Box 1: Bilingualism in infancy

Research with infants being raised in bilingual homes has produced dramatic evidence for very early effects of bilingualism and challenges some standard explanations for the mechanism underlying these effects. It has long been known that children being raised with two languages do not confuse the languages when learning to speak, even though they may borrow from one when speaking the other [ 131 ]. It is also well known that monolingual infants lose the ability to make phonetic discriminations not present in their language by about 10-months old whereas bilingual infants continue to distinguish between phonetic categories relevant to all languages. Thus, it is not surprising that bilingual infants can differentiate between their two languages essentially from birth [ 132 ]. What is surprising, however, is the extension of this discrimination to non-acoustic properties of language. Weikum and colleagues [ 133 ] showed silent video clips to 8-month old infants who were being raised in homes that were either monolingual English or English-French bilingual. Using a habituation paradigm, the speaker switched languages after habituation and the researchers measured whether or not infants regained interest. The results showed renewed attention among the bilingual but not the monolingual infants. To determine whether the bilingual infants had learned about the facial structures that accompany each language or something more general, the same materials were presented to monolingual Spanish (or Catalan) infants and bilingual Spanish-Catalan infants [ 134 ]. Again, only the bilingual infants noticed the change in language, even though the children in this study had no experience with either language. The authors concluded that bilingualism enhances general perceptual attentiveness through the experience of attending to two sets of visual cues.

This enhanced perceptual attentiveness may help explain the results of a study in which 7-month old monolingual and bilingual infants learned a head-turn response to a cue to obtain a visual reward and then had to replace that with a competing response for the same reward [ 135 ]. Again, only the bilingual infants could learn the new response. Even before children have productive language ability, the experience of building two distinct representational systems endows them with greater perceptual and attentional resources than their monolingual peers. In light of such evidence for bilingual advantages in the first year of life, explanations for the mechanism responsible for the advantages found later may need to be reconsidered to include a role for such perceptual processes.

An alternative to inhibition is to consider the demands imposed by a mixed set of congruent and incongruent trials: there is always some probability that the next display may be an incongruent trial. Thus, even on congruent trials the display must be evaluated before the participant commits to a response. Congruent responses will typically be faster than incongruent responses, but individuals with superior attentional control processes (e.g., bilinguals) will be able to carry out such evaluative decisions more rapidly and effectively. Therefore, a different account of the bilingual advantage is in terms of conflict monitoring [ 43 , 85 , 86 ]. Evidence supporting this view comes from situations in which monitoring demands are low – if the majority of trials are of one type only [ 84 , 85 ], the potentially misleading information (spatial position in the Simon task, flanker items in the flanker paradigm) can be treated as a valid cue, even if the participant must respond in the direction opposite to that indicated by the cue. In such low-monitoring conditions the bilingual advantage is typically not found. More generally, Hernández, Costa, and Humphreys [ 116 ] argue for a bilingual advantage in the deployment of attention, enabling them to resist “capture” by irrelevant information; such differences in attentional control may be the consequence of superior conflict monitoring. Conflict monitoring and inhibition are not mutually exclusive: Although monitoring is consistent with evidence that pure blocks of congruent trials are performed equivalently by monolinguals and bilinguals [ 106 , 117 ], an inhibition account is still required to explain evidence that pure blocks of incongruent trials are sometimes performed faster by bilinguals, notably by older adults for whom the task is more effortful [ 106 ].

Another problem with a pure inhibition account is that bilingual advantages are only found with some types of inhibition. The relevant distinction is captured by the contrast between the concepts of response inhibition and interference suppression [ 79 , 118 ]. In response inhibition, a univalent stimulus is associated with a prepotent response that must be overruled, such as say “day” to a picture of night or press “left” when the arrow points right. Bilinguals typically show no advantage in these situations [ 79 , 107 ]. In interference suppression, a bivalent stimulus contains two cues, each associated with a different response, such as the word “red” written in blue ink, so attention must be selectively focused on the relevant cue. Bilinguals typically outperform monolinguals on these tasks [ 25 ]. The hallmark of univalent response inhibition tasks is that the correct response can be pre-programmed before the cuing stimulus appears (e.g., if sun appears I’ll say ‘night;’ if the arrow points right I’ll respond left). On bivalent tasks, in contrast, the nature of the interfering information is not revealed until the display appears; for example, in the Simon task the participant prepares to respond on the left if the stimulus is green, but cannot prepare to deal with possible competing information until the display is shown. Bilinguals are more efficient at dealing with this online interference, in much the same way as the picture of a horse presented to a French/English bilingual would evoke both ‘ horse ’ and ‘ cheval ’, one of which must be suppressed. In a sense, the bilingual must constantly maintain the set of ‘respond in one language, suppress the other language’ whenever the possibility of two languages exists (cf., global inhibition). Further, this set maintenance must coexist with processing the stimuli and responses of the language currently utilized in a fluent and appropriate manner (cf., local inhibition). Thus, language use for bilinguals involves interference suppression, and the online monitoring required in both nonverbal task switching and language selection is similar.

The suggestion that bilinguals are particularly adept at maintaining the appropriate one of two (or more) relevant task goals or attentional sets in working memory has much in common with the notion of selection of wanted stimuli as opposed to inhibition of unwanted ones. The net effect is the same, but by this view the suppression of potentially interfering information is essentially a consequence of active selection of the relevant information, rather than a primary mechanism of direct inhibition. This view is consistent with that proposed by Colzato and colleagues [ 81 ] who concluded that the bilingual advantage is not due to the constant exercise of inhibition, but that learning to keep two languages separate leads to an improvement in selecting goal-relevant information from goal-irrelevant information.

The sum of the evidence places the bilingual advantage beyond the explanatory power of a single process, a simple neural network, or a single executive control component. Instead, the ongoing experience of monitoring two languages, in conjunction with the need to monitor context, speaker, and other environmental cues while inhibiting attention to the currently unused but active language modifies how the mind and brain engage in ordinary conversation for bilinguals. The more effortful any of these components become, the more likely it is for bilingual advantages to emerge on nonverbal tasks. However, the impact of this modification may be seen most clearly on tasks that bear the closest resemblance to bilingual language use, such as task switching. In this case, it is easy to see how the task of attending to the shape of a stimulus instead of its color resembles the task of retrieving the name for an object in French instead of in English. Not surprisingly, these tasks are typically performed better by bilinguals than by monolinguals, although the details of those performances are not yet well understood: Some studies report bilingual advantages on mixing costs indicating set shifting [ 86 , 106 ] while others report the advantage in local switch costs indicating response switching [ 80 , 119 ]. More generally, it is important to point out that bilingual advantages are not always found, even on tasks for which such performance differences would be expected. Some of the conditions that support the appearance of a bilingual advantage have been discussed, such as the need for monitoring and difficulty of the conditions, but others are still unknown. Another factor in determining performance outcomes is likely the nature or degree of bilingualism in the participants ( Box 2 ). More details about the specific tasks and precise language histories of the bilingual participants may resolve these differences.

Box 2: How bilingual?

Bilingualism is not a categorical experience but experimental research designs require it is treated as such – participants are monolingual or bilingual and differences in performance are assessed for members of the two groups. However, individuals can never be perfectly monolingual or bilingual: even the most monolingual people have had some experience with another language, for example as a school subject or a travel necessity, and all bilinguals have preferred languages or preferred contexts for each. These gradations raise three questions about the research results.

The first question is the possibility of a cumulative benefit for multiple languages. If managing two languages enhances cognitive control processes, then does further enhancement accrue from the management of three or more languages, as explicitly proposed by Diamond [ 136 ]? Research by Chertkow et al. [ 123 ] on Alzheimer’s disease and Kavé et al. [ 127 ] on normal aging showed better outcomes for multilinguals than for bilinguals, but there may be significant differences between multilinguals and bilinguals that do not exist between bilinguals and monolinguals. As we have suggested, bilinguals are typically not pre-selected for talent or interest but multilinguals may often be individuals with high ability and motivation to learn other languages, factors which may impact as well on cognitive performance.

The second question is the degree of bilingualism required for these benefits to emerge. If bilingualism is protective against some forms of dementia, then middle-aged people will want to know whether it is too late to learn another language, or whether their high-school French will count towards cognitive reserve. A related question concerns the age of acquisition of a second language; is earlier better? The best answer at present is that early age of acquisition, overall fluency, frequency of use, levels of literacy and grammatical accuracy all contribute to the bilingual advantage, with no single factor being decisive [ 137 , 138 ]. Increasing bilingualism leads to increasing modification of cognitive outcomes.

Finally, if the benefits of bilingualism are at least partly explained by the joint activation of two languages, does the similarity of the two languages matter? Does Spanish-English bilingualism require more (or less?) attentional control to maintain separation than say Chinese-English bilingualism? In a study with children who spoke English plus one of French, Spanish, or Chinese, there was no effect of the type of bilingualism, and all bilingual children outperformed monolingual children on tests of executive control [ 139 ].

Bilingualism and Dementia

The finding that bilingualism enhances cognitive control raises the possibility that lifelong bilingualism protects against age-related cognitive decline, and may even postpone the onset of symptoms of dementia. In this case, bilingualism may be one of the environmental factors that contribute to cognitive reserve or brain reserve [ 120 ]. Cognitive reserve is the idea that engagement in stimulating physical or mental activity can act to maintain cognitive functioning in healthy aging and postpone the onset of symptoms in those suffering from dementia. These factors include education, occupational status, higher socio-economic class, and the continuing involvement in physical, intellectual and social activities [ 121 – 123 ]. If bilingualism contributes to cognitive reserve, then bilinguals should maintain higher levels of cognitive functioning and cope better with symptoms of dementia than monolinguals who are otherwise equivalent.

To test this idea, Bialystok, Craik, and Freedman [ 124 ] examined the hospital records of monolingual and bilingual patients who had been diagnosed with various types of dementia. In spite of being equivalent on a variety of cognitive and other factors, the bilinguals experienced onset symptoms and were diagnosed approximately 3 – 4 years later than the monolinguals. Specifically, monolingual patients were diagnosed on average at age 75.4 years, and bilinguals at age 78.6. A replication from a new set of patients all diagnosed with probable Alzheimer’s disease (AD) [ 125 ] confirmed the results.

Three questions about these results are their reliability, validity, and causality. For reliability, several studies have replicated these findings. Chertkow et al. [ 126 ] reported partial support for the original results and showed that multilinguals were diagnosed with AD later than comparable monolinguals, although a more limited effect was found when monolinguals were compared with bilinguals. A similar positive relationship between multilingualism and high-level cognitive functioning was reported by Kavé et al. [ 127 ] in a study of elderly Israelis. Gollan et al. [ 128 ] reported a study with Spanish-English bilinguals who had been diagnosed with probable AD and found that a higher degree of bilingualism was associated with later age of onset and diagnosis, although only in the less-educated patients.

Second, validity requires demonstrating the specific relation between the predictor and outcome variables. Previously, socioeconomic status, cultural differences and immigration status have been suggested as contributors to or even causes of the bilingual advantage. However, in both Toronto studies, educational level and occupational status favored the monolingual group and immigration status was ruled out as a contributing factor.

Third, regarding cause and effect, is it possible the people that people with ‘good brains’ are both resistant to dementia and also more likely to learn a second language? This is unlikely: most people do not become bilingual because they are bright or have a flair for learning languages, but rather out of necessity. Supporting this interpretation, a recent study showed that in a sample of monolingual and bilingual AD patients matched on age, cognitive level, and other factors, CT scans showed more AD pathology in the brains of the bilinguals, consistent with the idea that they are better able to cope with the disease and can function longer without showing symptoms [ 129 ].

In the first study reporting the surprising outcome of an advantage in cognitive and linguistic performance by bilingual children, Peal and Lambert [ 13 ] concluded: “Intellectually [the bilingual child’s] experience with two language systems seems to have left him with a mental flexibility, a superiority in concept formation, a more diversified set of mental abilities” (p. 20). Peal and Lambert did not explain what they meant by “mental flexibility” but the description works well to describe the data accumulated in the 50 years since their original study. Bilinguals do sometimes have an advantage in inhibition, but they also have an advantage in selection; bilinguals do sometimes have an advantage in switching, but they also have an advantage in sustaining attention; and bilinguals do sometimes have an advantage in working memory, but they also have an advantage in representation and retrieval. Together, this pattern sounds like “mental flexibility”, the ability to adapt to ongoing changes and process information efficiently and adaptively.

It should not be surprising that intense and sustained experience leaves its mark on our minds and brains – the functional connections that come from practice are surely changed by massive experience, and the structural regions that are recruited for specific activities undoubtedly change as well through use. These responses to experience are precisely what we mean by neuroplasticity. Yet, in the case of bilingualism, the assumption has long been that any such effects would be deeply negative: As one influential educational researcher commented in 1926, “This might be considered evidence that the use of a foreign language in the home is one of the chief factors in producing mental retardation as measured by intelligence tests” ([ 130 ], p. 393). Almost a century later, and in the face of substantial evidence to the contrary, there remains resistance to the idea that bilingualism can enhance aspects of cognitive function. Educational and clinical practitioners routinely advise parents to “simplicify” their children’s linguistic environment when there are signs of academic struggle, and language professionals prescribe optimal timetables (and methods) for introducing languages to children to minimize the inevitable confusion. But such views are based on fear and anecdote – the weight of scientific evidence supports the promise of “mental flexibility”. There is still much we do not know about the effect of bilingualism on the mind, the neural correlates of those effects, and the causal components of the experience that lead to them. But it is too late to turn back: it is now clear that the bilingual mind has been uniquely shaped by experience.

Box 3: Outstanding questions

• Nature of the Bilingual Advantage: What are the limits and boundary conditions for the bilingual advantage and why are bilingual advantages not always found? What is the role of the standard components of executive control – inhibition, shifting, and working memory – in bilingual differences in processing? Do these relations change over the lifespan?
• Cognitive Reserve: Is the bilingual protection against cognitive decline similar to other types of cognitive reserve in terms of mechanism and neural correlates?
• Brain Correlates: What changes occur in the frontal lobes? Are there effects on other brain regions? What is the mechanism for these experience-dependent changes in frontal networks?
• Psychopathology: What are the neural correlates of the protective effects for patients with dementia? Does bilingualism have differential effects on various types of dementia?

Acknowledgement

Preparation of this manuscript was supported by grant R01HD052523 from the US National Institutes of Health and grant A2559 from the Natural Sciences and Engineering Research Council of Canada to EB, grant A8261 from the Natural Sciences and Engineering Research Council of Canada to FIMC, and grant MOP57842 from the Canadian Institutes of Health Research to EB and FIMC. We thank Steven Lovasz for his assistance in preparing the manuscript.

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Contributor Information

Ellen Bialystok, York University.

Fergus I.M. Craik, Rotman Research Institute of Baycrest.

Gigi Luk, Harvard Graduate School of Education.

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Adhesive coatings can prevent scarring around medical implants

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When medical devices such as pacemakers are implanted in the body, they usually provoke an immune response that leads to buildup of scar tissue around the implant. This scarring, known as fibrosis, can interfere with the devices’ function and may require them to be removed.

In an advance that could prevent that kind of device failure, MIT engineers have found a simple and general way to eliminate fibrosis by coating devices with a hydrogel adhesive. This adhesive binds the devices to tissue and prevents the immune system from attacking it.

“The dream of many research groups and companies is to implant something into the body that over the long term the body will not see, and the device can provide therapeutic or diagnostic functionality. Now we have such an ‘invisibility cloak,’ and this is very general: There’s no need for a drug, no need for a special polymer,” says Xuanhe Zhao, an MIT professor of mechanical engineering and of civil and environmental engineering.

The adhesive that the researchers used in this study is made from cross-linked polymers called hydrogels, and is similar to a surgical tape they previously developed to help seal internal wounds. Other types of hydrogel adhesives can also protect against fibrosis, the researchers found, and they believe this approach could be used for not only pacemakers but also sensors or devices that deliver drugs or therapeutic cells.

Zhao and Hyunwoo Yuk SM ’16, PhD ’21, a former MIT research scientist who is now the chief technology officer at SanaHeal, are the senior authors of the study, which appears today in Nature . MIT postdoc Jingjing Wu is the lead author of the paper.

Preventing fibrosis

In recent years, Zhao’s lab has developed adhesives for a variety of medical applications, including double-sided and single-sided tapes that could be used to heal surgical incisions or internal injuries. These adhesives work by rapidly absorbing water from wet tissues, using polyacrylic acid, an absorbent material used in diapers. Once the water is cleared, chemical groups called NHS esters embedded in the polyacrylic acid form strong bonds with proteins at the tissue surface. This process takes about five seconds.

Several years ago, Zhao and Yuk began exploring whether this kind of adhesive could also help keep medical implants in place and prevent fibrosis from occurring.

To test this idea, Wu coated polyurethane devices with their adhesive and implanted them on the abdominal wall, colon, stomach, lung, or heart of rats. Weeks later, they removed the device and found that there was no visible scar tissue. Additional tests with other animal models showed the same thing: Wherever the adhesive-coated devices were implanted, fibrosis did not occur, for up to three months.

“This work really has identified a very general strategy, not only for one animal model, one organ, or one application,” Wu says. “Across all of these animal models, we have consistent, reproducible results without any observable fibrotic capsule.”

Using bulk RNA sequencing and fluorescent imaging, the researchers analyzed the animals’ immune response and found that when devices with adhesive coatings were first implanted, immune cells such as neutrophils began to infiltrate the area. However, the attacks quickly quenched out before any scar tissue could form.

“For the adhered devices, there is an acute inflammatory response because it is a foreign material,” Yuk says. “However, very quickly that inflammatory response decayed, and then from that point you do not have this fibrosis formation.”

One application for this adhesive could be coatings for epicardial pacemakers — devices that are placed on the heart to help control the heart rate. The wires that contact the heart often become fibrotic, but the MIT team found that when they implanted adhesive-coated wires in rats, they remained functional for at least three months, with no scar tissue formation.

“The formation of fibrotic tissue at the interface between implanted medical devices and the target tissue is a longstanding problem that routinely causes failure of the device. The demonstration that robust adhesion between the device and the tissue obviates fibrotic tissue formation is an important observation that has many potential applications in the medical device space,” says David Mooney, a professor of bioengineering at Harvard University, who was not involved in the study.

Mechanical cues

The researchers also tested a hydrogel adhesive that includes chitosan, a naturally occurring polysaccharide, and found that this adhesive also eliminated fibrosis in animal studies. However, two commercially available tissue adhesives that they tested did not show this antifibrotic effect because the commercially available adhesives eventually detached from the tissue and allowed the immune system to attack.

In another experiment, the researchers coated implants in hydrogel adhesives but then soaked them in a solution that removed the polymers’ adhesive properties, while keeping their overall chemical structure the same. After being implanted in the body, where they were held in place by sutures, fibrotic scarring occurred. This suggests that there is something about the mechanical interaction between the adhesive and the tissue that prevents the immune system from attacking, the researchers say.

“Previous research in immunology has been focused on chemistry and biochemistry, but mechanics and physics may play equivalent roles, and we should pay attention to those mechanical and physical cues in immunological responses,” says Zhao, who now plans to further investigate how those mechanical cues affect the immune system.

Yuk, Zhao, and others have started a company called SanaHeal, which is now working on further developing tissue adhesives for medical applications.

“As a team, we are interested in reporting this to the community and sparking speculation and imagination as to where this can go,” Yuk says. “There are so many scenarios in which people want to interface with foreign or manmade material in the body, like implantable devices, drug depots, or cell depots.”

The research was funded by the National Institutes of Health and the National Science Foundation.

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Nikhil is an intern consultant at Marktechpost. He is pursuing an integrated dual degree in Materials at the Indian Institute of Technology, Kharagpur. Nikhil is an AI/ML enthusiast who is always researching applications in fields like biomaterials and biomedical science. With a strong background in Material Science, he is exploring new advancements and creating opportunities to contribute.

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