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Cognitive Consequences of Bilingualism and Multilingualism: Cross-Linguistic Influences

• Published: 12 April 2021
• Volume 50 , pages 313–316, ( 2021 )

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• Michael C. W. Yip   ORCID: orcid.org/0000-0002-9790-334X 1  

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Bilingualism and multilingualism are common in almost all communities worldwide today. Research studies on the psycholinguistics of bilingualism and multilingualism in East Asia region has developed tremendously in the past 20 years. Along with the new methodologies, innovative approaches, and the development of those state-of-the-art technologies (Altarriba and Heredia (eds) in An introduction to bilingualism: principles and processes, Routledge, 2018), a lot of new research findings on this line of research have been reported.

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Bilingualism and Multilingualism

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Yip, M.C.W. Cognitive Consequences of Bilingualism and Multilingualism: Cross-Linguistic Influences. J Psycholinguist Res 50 , 313–316 (2021). https://doi.org/10.1007/s10936-021-09779-y

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Bilingualism and multilingualism from a socio-psychological perspective.

• Tej K. Bhatia Tej K. Bhatia Department of Linguistics, Syracuse University
• https://doi.org/10.1093/acrefore/9780199384655.013.82
• Published online: 28 June 2017

Bilingualism/multilingualism is a natural phenomenon worldwide. Unwittingly, however, monolingualism has been used as a standard to characterize and define bilingualism/multilingualism in linguistic research. Such a conception led to a “fractional,” “irregular,” and “distorted” view of bilingualism, which is becoming rapidly outmoded in the light of multipronged, rapidly growing interdisciplinary research. This article presents a complex and holistic view of bilinguals and multilinguals on conceptual, theoretical, and pragmatic/applied grounds. In that process, it attempts to explain why bilinguals are not a mere composite of two monolinguals. If bilinguals were a clone of two monolinguals, the study of bilingualism would not merit any substantive consideration in order to come to grips with bilingualism; all one would have to do is focus on the study of a monolingual person. Interestingly, even the two bilinguals are not clones of each other, let alone bilinguals as a set of two monolinguals. This paper examines the multiple worlds of bilinguals in terms of their social life and social interaction. The intricate problem of defining and describing bilinguals is addressed; their process and end result of becoming bilinguals is explored alongside their verbal interactions and language organization in the brain. The role of social and political bilingualism is also explored as it interacts with individual bilingualism and global bilingualism (e.g., the issue of language endangerment and language death).

Other central concepts such as individuals’ bilingual language attitudes, language choices, and consequences are addressed, which set bilinguals apart from monolinguals. Language acquisition is as much an innate, biological, as social phenomenon; these two complementary dimensions receive consideration in this article along with the educational issues of school performance by bilinguals. Is bilingualism a blessing or a curse? The linguistic and cognitive consequences of individual, societal, and political bilingualism are examined.

• defining bilinguals
• conceptual view of bilingualism
• becoming bilingual
• social networks
• language organization of bilinguals
• the bilingual mind
• bilingual language choices
• language mixing
• code-mixing/switching
• bilingual identities
• consequences of bilingualism
• bilingual creativity
• and political bilingualism

1. Understanding Multilingualism in Context

In a world in which people are increasingly mobile and ethnically self-aware, living with not just a single but multiple identities, questions concerning bilingualism and multilingualism take on increasing importance from both scholarly and pragmatic points of view. Over the last two decades in which linguistic/ethnic communities that had previously been politically submerged, persecuted, and geographically isolated, have asserted themselves and provided scholars with new opportunities to study the phenomena of individual and societal bilingualism and multilingualism that had previously been practically closed to them. Advances in social media and technology (e.g., iPhones and Big Data Capabilities) have rendered new tools to study bilingualism in a more naturalistic setting. At the same time, these developments have posed new practical challenges in such areas as language acquisition, language identities, language attitudes, language education, language endangerment and loss, and language rights.

The investigation of bi- and multilingualism is a broad and complex field. Unless otherwise relevant on substantive grounds, the term “bilingualism” in this article is used as an all-inclusive term to embody both bilingualism and multilingualism.

2. Bilingualism as a Natural Global Phenomenon: Becoming Bilingual

Bilingualism is not entirely a recent development; for instance, it constituted a grassroots phenomenon in India and Africa since the pre-Christian era. Contrary to a widespread perception, particularly in some primarily monolingual countries—for instance, Japan or China—or native English-speaking countries, such as the United States, bilingualism or even multilingualism is not a rare or exceptional phenomenon in the modern world; it was and it is, in fact, more widespread and natural than monolingualism. The Ethnologue in the 16th edition ( 2009 , http://www.ethnologue.com/show_country.asp?name=lb ) estimates more than seven thousand languages (7,358) while the U.S. Department of States recognizes only 194 bilingual countries in the world. There are approximately 239 and 2,269 languages identified in Europe and Asia, respectively. According to Ethnologue , 94% of the world’s population employs approximately 5% of its language resources. Furthermore, many languages such as Hindi, Chinese, Arabic, Bengali, Punjabi, Spanish, and Portuguese are spoken in many countries around the globe. Such a linguistic situation necessitates people to live with bilingualism and/or multilingualism. For an in-depth analysis of global bilingualism, see Bhatia and Ritchie ( 2013 ).

3. Describing Bilingualism

Unlike monolingualism, childhood bilingualism is not the only source and stage of acquiring two or more languages. Bilingualism is a lifelong process involving a host of factors (e.g., marriage, immigration, and education), different processes (e.g., input conditions, input types, input modalities and age), and yielding differential end results in terms of differential stages of fossilization and learning curve (U-shape or nonlinear curve during their grammar and interactional development). For this reason, it does not come as a surprise that defining, describing, and categorizing a bilingual is not as simplistic as defining a monolingual person. In addition to individual bilingualism, social and political bilingualism adds yet other dimensions to understanding bilingualism. Naturally then, there is no universally agreed upon definition of a bilingual person.

Bilingual individuals are subjected to a wide variety of labels, scales, and dichotomies, which constitute a basis of debates over what is bilingualism and who is a bilingual. Before shedding light on the complexity of “individual” bilingualism, one should bear in mind that the notion of individual bilingualism is not devoid of social bilingualism, or an absence of a shared social or group grammar. The term “individual” bilingualism by no means refers to idiosyncratic aspects of bilinguals, which is outside the scope of this work.

Relying on a Chomskyan research paradigm, bilingualism is approached from the theoretical distinction of competence vs. performance (actual use). Equal competency and fluency in both languages—an absolute clone of two monolinguals without a trace of accent from either language—is one view of a bilingual person. This view can be characterized as the “maximal” view. Bloomfield’s definition of a bilingual with “a native-like control of two languages” attempts to embody the “maximal” viewpoint (Bloomfield, 1933 ). Other terms used to describe such individuals are “ambilinguals” or “true bilinguals.” Such bilinguals are rare, or what Valdes terms, “mythical bilingual” (Valdes, 2001 ). In contrast to maximal view, a “minimal” view contends that practically every one is a bilingual. “That is no one in the world (no adult, anyway) which does not know at least a few words in languages other than the maternal variety” (Edwards, 2004/2006 ). Diebold’s notion of “Incipient bilingualism”—that is, exposure to two languages—belongs to the minimal view of bilingualism (Diebold, 1964 ). While central to the minimalist viewpoint is the onset point of the process of becoming a bilingual, the main focus of the maximalist view is the end result, or termination point, of language acquisition. In other words, the issue of degree and the end state of second language acquisition is at the heart of defining the concept of bilingualism.

Other researchers such as Mackey, Weinreich, and Haugen define bilingualism to capture language use of bilinguals’ verbal behavior. For Haugen, bilingualism begins when the speakers of one language produce complete meaningful utterances in the second language (Haugen, 1953 ; Mackey, 2000 ; Weinrich, 1953 ). Mackey, on the other hand, defines bilingualism as an “alternate use of two or more languages” (Mackey, 2000 ). Observe that the main objective of the two definitions is to focus on language use rather the degree of language proficiency or equal competency in two languages.

The other notable types of bilingualism identified are as follows: Primary/Natural bilingualism in which bilingualism is acquired in a natural setting without any formal training; Balanced bilingualism that develops with minimal interference from both languages; Receptive or Passive bilingualism wherein there is understanding of written and/or spoken proficiency in second language but an inability to speak it; Productive bilingualism then entails an ability to understand and speak a second language; Semilingualism, or an inability to express in either language; and Bicultural bilingualism vs. Monocultural bilingualism. The other types of bilingualism, such as Simultaneous vs. Successive bilingualism (Wang, 2008 ), Additive vs. Subtractive bilingualism (Cummins, 2000 ), and Elite vs. Folk bilingualism (Skutnabb-Kangas, 1981 ), will be detailed later in this chapter. From this rich range of scales and dichotomies, it becomes readily self-evident that the complexity of bilingualism and severe limitation of the “fractional” view of bilingualism that bilinguals are two monolinguals in one brain. Each case of bilingualism is a product of different sets of circumstances and, as a result, no two bilinguals are the same. In other words, differences in the context of second language acquisition (natural, as in the case of children) and proficiency in spoken, written, reading, and listening skills in the second language, together with the consideration of culture, add further complexity to defining individual bilingualism.

3.1 Individual Bilingualism: A Profile

The profile of this author further highlights the problems and challenges of defining and describing a bilingual or multilingual person. The author, as an immigrant child growing up in India, acquired two languages by birth: Saraiki—also called Multani and Lahanda, spoken primarily in Pakistan—and Punjabi, which is spoken both in India and Pakistan. Growing up in the Hindi-speaking area, he learned the third language Hindi-Urdu primarily in schools; and his fourth language, English, primarily after puberty during his higher education in India and the United States. He cannot write or read in Saraiki but can read Punjabi in Gurmukhi script, and he cannot write with the same proficiency. He has native proficiency in speaking, listening, reading, and writing. A close analysis of his bilingualism reveals that no single label or category accounts for his multifaceted bilingualism/multilingualism. Interestingly, his self-assessment finds him linguistically least secured in his two languages, which he acquired at birth. Is he a semilingual without a mother tongue? No matter how challenging it is to come to grips with bilingualism and, consequently, develop a “holistic” view of bilingualism, it is clear that a bilingual person demonstrates many complex attributes rarely seen in a monolingual person. See Edwards ( 2004/2006 ) and Wei ( 2013 ) for more details. Most important, multiple languages serve as a vehicle to mark multiple identities (e.g., religious, regional, national, ethnic, etc.).

3.2 Social Bilingualism

While social bilingualism embodies linguistic dimensions of individual bilingualism, a host of social, attitudinal, educational, and historical aspects of bilingualism primarily determine the nature of social bilingualism. Social bilingualism refers to the interrelationship between linguistic and non-linguistic factors such as social evaluation/value judgements of bilingualism, which determine the nature of language contact, language maintenance and shift, and bilingual education among others. For instance, in some societies, bilingualism is valued and receives positive evaluation and is, thus, encouraged while in other societies bilingualism is seen as a negative and divisive force and is, thus, suppressed or even banned in public and educational arenas. Compare the pattern of intergenerational bilingualism in India and the United states, where it is well-known that second or third-generation immigrants in the United States lose their ethnic languages and turn monolinguals in English (Fishman, Nahirny, Hofman, & Hayden, 1966 ). Conversely, Bengali or Punjabi immigrants living in Delhi, generation after generation, do not become monolinguals in Hindi, the dominant language of Delhi. Similarly, elite bilingualism vs. folk bilingualism has historically prevailed in Europe, Asia, and other continents and has gained a new dimension in the rapidly evolving globalized society. As aristocratic society patronized bilingualism with French or Latin in Europe, bilingualism served as a source of elitism in South Asia in different ages of Persian and English. Folk bilingualism is often the byproduct of social dominance and imposition of a dominant group. While elite bilingualism is viewed as an asset, folk bilingualism is seen as problematic both in social and educational arenas (Skutnabb-Kangas, 1981 ). One of the outcomes of a stable elite and folk bilingualism is diglossia (e.g., Arabic, German, Greek, and Tamil) where both High (elite) and Low (colloquial) varieties of a language—or two languages with High and Low social distinctions—coexist (e.g., French and English diglossia after the Norman conquest (Ferguson, 1959 ). Diasporic language varieties have been examined by Clyne and Kipp ( 1999 ) and Bhatia ( 2016 ). Works by Baker and Jones ( 1998 ) show how bilinguals belong to communities of variable types due to accommodation (Sachdev & Giles, 2004/2006 ), indexicality (Eckert & Rickford, 2001 ), social meaning of language attitudes (Giles & Watson, 2013 ; Sachdev & Bhatia, 2013 ), community of practice, and even imagined communities.

3.3 Political Bilingualism

Political bilingualism refers to the language policies of a country. Unlike individual bilingualism, categories such as monolingual, bilingual, and multilingual nations do not reflect the actual linguistic situation in a particular country (Edwards, 1995 , 2004/2006 ; Romaine, 1989/1995 ). Canada, for instance, is officially recognized as a bilingual country. This means that Canada promotes bilingualism as a language policy of the country as well as in Canadian society as a whole. By no means does it imply that most speakers in Canada are bilinguals. In fact, monolingual countries may reflect a high degree of bilingualism. Multilingual countries such as South Africa, Switzerland, Finland and Canada often use one of the two approaches—“Personality” and “Territorial”—to ensure bilingualism. The Personality principle aims to preserve individual rights (Extra & Gorter, 2008 ; Mackey, 1967 ) while the Territorial principle ensures bilingualism or multilingual within a particular area to a variable degree, as in the case of Belgium. In India, where 23 languages are officially recognized, the government’s language policies are very receptive to multilingualism. The “three-language formula” is the official language policy of the country (Annamalai, 2001 ). In addition to learning Hindi and English, the co-national languages, school children can learn a third language spoken within or outside their state.

4. The Bilingual Mind: Language Organization, Language Choices, and Verbal Behavior

Unlike monolinguals, a decision to speak multiple languages requires a complex unconscious process on the part of bilinguals. Since a monolingual’s choice is restricted to only one language, the decision to choose a language is relatively simple involving, at most, the choice of an informal style over a formal style or vice versa. However, the degree and the scale of language choice are much more complicated for bilinguals since they need to choose not only between different styles but also between different languages. It is a widely held belief, at least in some monolingual speech communities, that the process of language choice for bilinguals is a random one that can lead to a serious misunderstanding and a communication failure between monolinguals and bi- and multilingual communities (see pitfalls of a sting operation by a monolingual FBI agent (Ritchie & Bhatia, 2013 )). Such a misconception of bilingual verbal behavior is also responsible for communication misunderstandings about social motivations of bilinguals’ language choices by monolinguals; for example, the deliberate exclusion or sinister motives on the part of bilinguals when their language choice is different from a monolingual’s language. A number of my international students have reported that on several occasions monolingual English speakers feel compelled to remind them that they are in America and they should be using English, rather than say Chinese or Arabic, with countrymen/women.

Now let us examine some determinants of language choice by bilinguals. Consider the case of this author’s verbal behavior and linguistic choices that he normally makes while interacting with his family during a dinner table conversation in India. He shares two languages with his sisters-in-law (Punjabi and Hindi) and four languages with his brothers (Saraiki, Punjabi, Hindi, and English). While talking about family matters or other informal topics, he uses Punjabi with his sisters-in-law but Saraiki with his brothers. If the topic involves ethnicity, then the entire family switches to Punjabi. Matters of educational and political importance are expressed in English and Hindi, respectively. These are unmarked language choices, which the author makes unconsciously and effortlessly with constant language switching depending on participants, speech events, situations, or other factors. Such a behavior is largely in agreement with the sociolinguistic Model of Markedness, which attempts to explain the sociolinguistic motivation of code-switching by considering language choice as a means of communicating desired group membership, or perceived group memberships, and interpersonal relationships (Pavlenko, 2005 ).

Speaking Sariki with brothers and Punjabi with sister-in-laws represent unconscious and unmarked choices. Any shift to a marked choice is, of course, possible on theoretical grounds; however, it can take a serious toll in terms of social relationships. The use of Hindi or English during a general family dinner conversation (i.e., a “marked” choice) will necessarily signal social distancing and fractured relations.

Languages choice is not as simple as it seems at first from the above example of family conversation. In some cases, it involves a complex process of negotiation. Talking with a Punjabi-Hindi-English trilingual waiter in an Indian restaurant, the choice of ethnic language, Punjabi, by a customer such as this author may seem to be a natural choice at first. Often, it is not the case if the waiter refuses to match the language choice of the customer and replies in English. The failure to negotiate a language in such cases takes an interesting turn of language mismatching before a common language of verbal exchange is finally agreed upon; often, it turns out to be a neutral and prestige language: English. See Ritchie and Bhatia ( 2013 ) for further details. When the unmarked choice is not clear, speakers tend to use code-switching in an exploratory way to determine language choice and thus restore a social balance.

During a speech event, language choice is not always static either. If the topic of conversation shifts from a casual topic to a formal topic such as education, a more suitable choice in this domain would be English; subsequently, a naturally switch to English will take place. In other words, “complementarity” language domains or language-specific domain allocation represent the salient characteristics of bilingual language choice. The differential domain allocation manifests itself in the use of “public” vs. “private” language by bilinguals, which is central to bilingual verbal repertoire (Ritchie & Bhatia, 2013 ). Often the role of expressing emotions or one’s private world is best played by the bilingual’s mother tongue rather than by the second or prestige/distant language. Research on bilingualism, emotions, and autobiographical memory accounts of bilinguals shows that an account of emotional events is qualitatively and quantitatively different when narrated in one’s mother tongue than in a distant second language (Devaele, 2010 ; Pavlenko, 2005 ). While the content of an event can be narrated equally well in either language, the emotional experience/pain is best described in the first language of the speaker. Particularly, bilingual parents use their first language for terms of endearment for their children. Their first language serves as the best vehicle for denoting emotions toward their children than any other language in their verbal repertoire. Taboo topics, on the other hand, favor the second or a distant language.

Any attempt to characterize the bilingual mind must account for the following three natural aspects of bilingual verbal behavior: (1) Depending upon the communicative circumstances, bilinguals swing between the monolingual and bilingual language modes; (2) Bilinguals have an ability to keep two or more languages separate whenever needed; and (3) More interestingly, they can also carry out an integration of two or more languages within a speech event.

4.1 Bilingual Language Modes

Bilinguals are like a sliding switch who can move between one or more language states/modes as required for the production, comprehension, and processing of verbal messages in a most cost-effective and efficient way. If bilinguals are placed in a predominantly monolingual setting, they are likely to activate only one language; while in a bilingual environment, they can easily shift into a bilingual mode to a differential degree. The activation or deactivation process is not time consuming. In a bilingual environment, this process usually does not require bilinguals to take more than a couple of milliseconds to swing into a bilingual language mode and revert back to a monolingual mode with the same time efficiency. However, under unexpected circumstances (e.g., caught off-guard by a white Canadian speaking an African language in Canada) or under emotional trauma or cultural shock, the activation takes considerable time. In the longitudinal study of his daughter, Hildegard, reported that Hildegard, while in Germany, came to tears at one point when she could not activate her mother tongue, English (Leopard, 1939–1950 ). The failure to ensure natural conditions responsible for the activation of bilingual language mode is a common methodological shortcoming of bilingual language testing, see Grosjean ( 2004 / 2006 , 2010 ). An in-depth review of processing cost involved in the language activation-deactivation process can be found in Meuter ( 2005 ). Do bilinguals turn on their bilingual mode, even if only one language is needed to perform a task? Recent research employing an electrophysiological and experimental approach shows that both languages compete for selection even if only one language is needed to perform a task (Martin, Dering, Thomas, & Thierry, 2009 ; Hoshino & Thierry, 2010 ). For more recent works on parallel language activation and language competition in speech planning and speech production, see Blumenfeld and Marian ( 2013 ). In other words, the potential of activation and deactivation of language modes—both monolingual and bilingual mode—hold an important key to bilingual’s language use.

4.2 Bilingual Language Separation and Language Integration

In addition to language activation or deactivation control phenomena, the other two salient characteristics of bilingual verbal behavior are bilinguals’ balanced competence and capacity to separate the two linguistic systems and to integrate them within a sentence or a speech event. Language mixing is a far more complex cognitive ability than language separation. Yet, it is also very natural to bilinguals. Therefore, it is not surprising to observe the emergence of mixed systems such as Hinglish, Spanglish, Germlish, and so on, around the globe. Consider the following utterances:

Such a two-faceted phenomenon is termed as code-mixing (as in 1 and 2) and code-switching (as in 3). Code-mixing (CM) refers to the use of various linguistic units—words, phrases, clauses, and sentences—primarily from two participating grammatical systems within a sentence. While CM is intra-sentential, code-switching (CS) is an inter-sentential phenomenon. CM is constrained by grammatical principles and is motivated by socio-psychological factors. CS, on the other hand, is subject to discourse principles and is also motivated by socio-psychological factors.

Any unified treatment of the bilingual mind has to account for the language separation (i.e., CS) and language integration (CM) aspects of bilingual verbal competence, capacity, use, and creativity. In that process, it needs to address the following four key questions, which are central to an understanding the universal and scientific basis for the linguistic creativity of bilinguals.

Is language mixing a random or a systematic phenomenon?

What motivates bilinguals to mix and alternate two languages?

What is the social evaluation of this mixing and alternation?

What is the difference between code-mixing or code-switching and other related phenomena?

I. Language mixing as a systematic phenomenon

Earlier research from the 1950s–1970s concluded that CM is either a random or an unsystematic phenomenon. It was either without subject to formal syntactic constraints or is subject only to “irregular mixture” (Labov, 1971 ). Such a view of CM/CS is obsolete since late the 20th century . Recent research shows that CM/CS is subject to formal, functional, and attitudinal factors. Studies of formal factors in the occurrence of CM attempt to tap the unconscious knowledge of bilinguals about the internal structure of code-mixed sentences. Formal syntactic constraints on the grammar of CM, such as The Free Morpheme Constraint (Sankoff & Poplack, 1981 ); The Closed Class Constraint (Joshi, 1985 ), within the Generative Grammar framework; and The Government Constraint and the Functional Head Constraint within the non-lexicalist generative framework, demonstrate the complexity of uncovering universal constraints on CM; for details, see Bhatia and Ritchie ( 2009 ). Recently, the search for explanations of cross-linguistic generalizations about the phenomenon of CM, specifically in terms of independently justified principles of language structure and use, has taken two distinct forms. One approach is formulated in terms of the theory of linguistic competence within the framework of Chomsky’s Minimalist Program (MacSwan, 2009 ). The other approach—as best exemplified by the Matrix Language Frame (MLF) model (Myers-Scotton & Jake, 2001 ) is grounded in the theory of sentence production, particularly that of Levelt ( 1989 ). Herring and colleagues test the strengths and weaknesses of both a Minimalist Program approach and the MLF approach on explanatory grounds based on switches between determiner and their noun complements drawn from Spanish-English and Welsh-English data (Herring, Deuchar, Couto, & Quintanilla, 2010 ). Their work lends partial support to the two approaches.

II. Motivations for language mixing

While research on the universal grammar of CM attempts to unlock the mystery of the systematic nature of CM on universal grounds, it does not attempt to answer Question (II), namely, the “why” aspect of CM. The challenge for linguistic research in the new millennium is to separate grammatical constraints from those motivated by, or triggered by, socio-pragmatic factors or competence. Socio-pragmatic studies of CM reveal the following four factors, which trigger CM/CS: (1) the social roles and relationships of the participants (e.g., dual/multiple identities; social class); (2) situational factors (discourse topic and language domain allocation); (3) message-intrinsic consideration; (4) language attitudes, including social dominance and linguistic security. See Ritchie and Bhatia ( 2013 ) and Myers-Scotton ( 1998 ) for further details. The most commonly accepted rule is that language mixing signals either a change or a perceived change by speaker in the socio-psychological context of a speech event. In essence, CM/CS is motivated by the consideration of “optimization,” and it serves as an indispensable tool for meeting creative and innovative needs of bilinguals (Bhatia, 2011 ). A novel approach provides further insights into a discourse-functional motivation of CM, namely, coding of less predictable, high information-content meanings in one language and more predictable, lower information-content meanings in another language (Myslin & Levy, 2015 ).

III. Social evaluation of language mixing

Now let us return to Question (III). From the discussion of Questions (I and II), it is self-evident that complexity and multifaceted creativity underlies CM/CS in bilingual communication. Surprisingly, though, the social evaluation of a mixed system is largely negative. Even more interestingly, bilinguals themselves do not have a positive view of language mixing. It is the widely held belief on the part of the “guardians” of language (including the media) and puritans that any form of language mixing is a sign of unsystematic or decadent form of communication. Bilinguals are often mocked for their “bad” and “irregular” linguistic behavior. They are often characterized as individuals who have difficulty expressing themselves. Other labels such as “lazy” and “careless” are also often bestowed upon them. Furthermore, the guardians of language often accused them of destroying their linguistic heritage. For these reasons, it is not surprising that even bilinguals themselves become apologetic about their verbal behavior. They blame mixing on “memory lapse,” among other things, and promise to correct their verbal behavior, vowing not to mix languages. In spite of this, they cannot resist language mixing!

Table 1 illustrates the anomaly between the scientific reality of language mixing and its social perception. Social perception translates into the negative evaluation of mixed speech.

Table 1. Language Mixing (CM/CS) Anomaly (Adapted from Bhatia & Ritchie, 2008 , p. 15).

Backlash to mixing is not just restricted to societies and bilinguals; even governments get on the bandwagon. Some countries, such as the newly freed countries of the ex-Soviet Union and France, regulate or even ban mixing either by appointing “language police” or by passing laws to wipe out the perceived negative effects of “bad language” in the public domain. Asia is not an exception in this regard. A case in point is a recent article by Tan ( 2002 ) reporting that the Government of Singapore has banned the movie Talk Cock because it uses a mixed variety of English, called Singlish. Linguistic prescriptivism clearly played a central role in the decision. In spite of the near-universal negative evaluation associated with CM/CS, the benefits rendered by language mixing by far outweigh its negative perception, which, in turn, compels the unconscious mind of bilinguals to mix and switch in order to yield results that cannot be rendered by a single/puritan language use; for a typology of bilingual linguistic creativity, and socio-psychological motivations, see Ritchie and Bhatia ( 2013 ).

IV. Language mixing and other related phenomena

Returning to the fourth question, it should be noted that CM/CS is quite distinct from linguistic borrowing. The primary function of linguistic borrowing is to fill a lexical gap in a borrower’s language (e.g., Internet, satellite). Furthermore, with borrowing, the structure of the host language remains undisturbed. However, CM requires complex integrity of two linguistic systems/grammar within a sentence, which may yield a new grammar. Other mixed systems, such as pidgin and creole languages, often fail to match the complexity and creativity of CM/CS. The distinction between code-mixing and code-switching is controversial for a number of reasons, particularly the integration of the participating grammar’s intrasententially details; see Bhatia and Ritchie ( 2009 ). Additionally, Deuchar and Stammers ( 2016 ) claim that code-switches and borrowings are distinct on the basis of frequency and degree of integration. Specifically, only the former are low in both frequency and integration. For details about contrasting and comparing different positions on this issue, see Myslin & Levy ( 2015 ); Poplack and Meechan ( 1998 ); and Lakshmanan, Balam, and Bhatia ( 2016 ). Furthermore, there is a debatable distinction between CM and Translanguaging (Garcia & Wei, 2014 ).

5. Bilingual Language Development: Nature vs. Nurture

Beyond innateness (e.g., nature, Biolinguistic and Neurological basis of language acquisition), social factors play a critical play in the language development of bilinguals. As pointed out earlier, describing and defining bilingualism is a formidable task. This is due to the fact that attaining bilingualism is a lifelong process; a complex array of conditions gives rise to the development of language among bilinguals. Based on the recommendation of educators, among others, bilingual families usually adopt a “One-Parent/One-Language” strategy with different combinations, such as language allocation based on time and space; for example, using one language in the morning and other in the evening or one language in the kitchen and another in the living room. This is done to maintain minority language. In spite of their obvious potential benefits for language maintenance, such strategies fall short in raising bilingual and bicultural children for a number of reasons, including imparting pragmatic and communicative competence and providing negative and positive evidence to children undergoing heritage language development with sociolinguistically real verbal interactional patterns (Bhatia & Ritchie, 1995 ). Therefore, De Houwer ( 2007 ) rightly points out that it is important for children to be receiving language input in the minority language from both parents at home. This also represents a common practice in non-Western societies in Asia (e.g., India) and Africa (e.g., Nigeria) where both parents, including members of the joint family minority languages, speak in their minority language.

While raising bilingual children does not pose any serious challenge for majority children (e.g., English-speaking children learning French in Canada), it is a different story for minority or heritage children. Sadly, a complex mix of political and social bilingualism leads heritage/minority parents, who themselves experience adverse discrimination in social and work settings, simply to prohibit the use of minority languages in family and educational environments. This practice, no matter how well intended, often results in negative school performance and emotional problems for minority children.

6. Simultaneous vs. Sequential Childhood Bilingualism

Broadly speaking, childhood bilingualism can manifest itself in two distinct patterns: (1) Simultaneous bilingualism and (2) Sequential bilingualism. A child being exposed to two languages to more or less to the same degree from birth onward is described as a simultaneous bilingual; conversely, a child being exposed to one language first followed by a second language, with the latter coming after the age of five, is referred to as sequential bilingual. Sequential bilingualism takes place either in schools or in peer groups and/or family settings. Surely, sequential bilingualism can persist throughout the adulthood. How is early bilingualism different from late bilingualism? Research on sequential and adult language acquisition shows that the pattern of sequential/successive language acquisition falls somewhere in the middle of the continuum between a simultaneous bilingual and an adult language learner.

7. Adult Bilingualism: UG and Native Language Dominance

Why is the task of learning a second language by adults more difficult and time consuming than by children? In spite of considerable motivation and effort, why do adults fall short of achieving native-like competency in their target language? Why do even very competent and balanced bilinguals speak with an “accent”? The Critical Period Hypothesis by Lenneberg ( 1967 ) attempts to answer these questions, and it is sensitive to age (Lenneberg, 1967 ). Children are better equipped to acquire languages because their brains are more “plastic” before they hit maturity. They have access to UG, to which adults have either no access or only partial access. Afterward, the loss of plasticity results in the completion of lateralization of language function in the left hemisphere. Even though adults are more cognitively developed and exhibit a high degree of aptitude, they have to rely on their native language (L1 transference—including “foreign accent” together with morphological features) in the process of learning a second language (Gass, 1996 ). Then there comes a time when their ultimate attainment of L2 falls short of the native language target, termed “fossilization” stage. No amount of training allows them to bypass this stage to free themselves from second language errors. Siegel, for instance, offers an alternative explanation of the language attainment state termed fossilization in second language acquisition research—a stage of falling short of attaining a native-speaker end grammar (Siegel, 2003 ). He argues that fossilization is not biologically driven but is the reflection of learners’ decisions not to clone the native speaker’s norm in order to index their own identity. Some researchers believe that this stage does not have a biological basis; instead, it is the result of bilingual, dual, or multiple identities. Adult learners are not ready to give up their identity and, as a result, this prevents them from having a perfect native-like competency of L2. For alternative theories of language acquisition, see, for example, a usage-based approach by Tomasello ( 2003 ); and the Dynamic System Theory by De Bot, Wander, and Verspoor ( 2007 ).

The differential competencies, as evident from the different types of adult bilinguals, can be accounted for primarily on sociolinguistic grounds. For instance, gender or the period of residency in a host country yields the qualitative and quantitative differences in bilingual language acquisition. Factors such as access to workplace, education, relationship, social networks, exogamic marriage, religion, and other factors lead to differential male and female bilingualism in qualitative grounds (Piller & Pavlenko, 2004/2006 ). Additionally, learners’ type, their aptitude, and attitude also contribute to a variable degree of language learning curves. Instrumental learners who learn a second language for external gains tend to lag behind Integrative learners who aim at integration with the target culture. Similarly, the Social Accommodation Theory (Sachdev & Giles, 2004/2006 ) attempts to explain differences in language choices and consequences on one hand and the social evaluation of speech (good vs. bad accents) on the other, which influence the social-psychological aspects of bilingual verbal interaction in different social settings (Altarriba & Moirier, 2004/2006 ; Lippi-Green, 2012 ).

8. Effects of Bilingualism

Until the middle of the 20th century in the United States, researchers engaged in examining the relationship between intelligence and bilingualism concluded that bilingualism has serious adverse effects on early childhood development. Such findings led to the development of the “factional” view of bilingualism, which was grounded in a flawed monolingual perspective on the limited linguistic capacity of the brain on one hand and the Linguistic Deficit Hypothesis on the other.

Their line of argument was that crowding the brain with two languages leads to a variety of impairments in both the linguistic and the cognitive abilities of the child. Naturally, then, they suggested that bilingual children not only suffer from semilingualism (i.e., lacking proficiency both in their mother tongue and the second language) and stuttering, etc., but also from low intelligence, mental retardation, left handedness, and even schizophrenia.

It took more than half a century before a more accurate and positive view of bilingualism emerged. The main credit for this goes to the pioneering work of Peal and Lambert ( 1962 ), which revealed the actual benefits of bilingualism. The view of bilingualism that subsequently emerged can be characterized as the Linguistic Augmentation Hypothesis (Peal & Lambert, 1962 ). Peal and Lambert studied earlier balanced bilingual children and controlled for factors such as socioeconomic status. Sound on methodological grounds, their result showed bilinguals to be intellectually superior to their monolingual counterparts. Their study, which was conducted in Montreal, changed the face of research on bilingualism. Many studies conducted around the globe have replicated the findings of Peal and Lambert. In short, cognitive, cultural, economic, and cross-cultural communication advantages of childhood and lifelong bilingualism are many, including reversing the effects of aging (Bialystok, 2005 ; Hakuta, 1986 ). Nevertheless, the effects of bilingualism on children’s cognitive development, particularly on executive function and attention, is far from conclusive; see Klein ( 2015 ) and Bialystok ( 2015 ).

9. Bilingualism: Language Spread, Maintenance, Endangerment, and Death

Language contact and its consequences represent the core of theoretical and descriptive linguistic studies devoted to bilingualism, and onto which globalization has added a new dimension. Ironically, in the age of globalization, the spread of English and other Indo-European languages, namely, Spanish and Portuguese, has led to the rise of bilingualism induced by these languages; they also pose a threat to the linguistic diversity of the world. Researchers claim that about half the known languages of the world have already vanished in the last 500 years, and that at least half, if not more, of the 6,909 living languages will become extinct in the next century (Hale, 1992 ; Nettle & Romaine, 2000 ). Research on language maintenance, language shift, and language death addresses the questions of why and how some languages spread and others die. Phillipson and Mufwene attempt to account for language endangerment within the framework of language imperialism ( 2010 ) and language ecology ( 2001 ), respectively. Fishman ( 2013 ) examines the ways to reverse the tide of language endangerment. Skutnabb-Kangas views minority language maintenance as a human rights issue in public and educational arenas ( 1953 ).

Critical Analysis of Scholarship

Advances in our understanding of bilingualism have come a long way since the predominance of the “factional” and linguistically deficient view of bilingualism. The complexity and diverse conditions responsible for lifelong bilingualism has led to a better understanding of this phenomenon on theoretical, methodological, and analytical grounds. A paradigm shift from monolingualism and the emergence of a new, interdisciplinary approach promises new challenges and directions in the future study of bilingualism.

Issues and Conceptualization

Although bilingualism is undoubtedly a widespread global phenomenon, it is rather ironic that, for a number of reasons, including the primary objective of linguistics, multidimensional aspects of bilingualism, and misperception of bilingualism as a rare phenomenon, the study of bilingualism has posed—and continues to pose—a serious of challenges to linguistics for quite some time. This is evident from eminent linguist Roman Jacobson’s observation from more than half a century ago: “bilingualism is for me a fundamental problem of linguistics” (Chomsky, 1986 ). Similarly, Chomsky remarked that the pure idealized form of language knowledge should be the first object of study rather than the muddy water of bilingualism (Grosjean, 1989 ). Consequently, research on bilingualism has taken a backseat to monolingualism, and monolinguals have served as a benchmark to characterize and theorize bilinguals, which, in turn, led to the ill conceptualization of the bilingual person as “two monolinguals in one brain” (Dehaene, 1999 ).

Are bilinguals just a composite or sum of two monolinguals crowded in one brain? A large body of research devoted to the bilingualism and intelligence debate either implicitly or explicitly subscribed to the “two monolinguals in one brain” conception. This set the stage for the “linguistic deficiency hypothesis” about bilingual children and adults on one hand and the limited linguistic capacity of the brain on the other. When looking from the lens of monolingualism, a “factional view” (Bhatia & Ritchie, 2016 ; Nicol, 2001 ), or even distorted view, of bilingualism emerged that portrayed bilinguals as semilinguals with a lack of proficiency not just in one but two languages. Although still in its infant stage, from recent research on bilingualism, a more accurate or holistic view of a bilingual and multilingual person has begun to emerge in 1990s, namely, just as an individual bilingual does not constitute two monolinguals in one brain, a multilingual is not merely a byproduct of bilingualism alone or vice versa. Similarly, the notion that brain capacity is ideally suited for one language is a myth. Additionally and interestingly, no two bilinguals behave the same way all the time since they are not a clone of each other.

Bilingualism, unlike monolingualism, exhibits complex individual, social, political, psychological, and educational dimensions in addition to involving a complex interaction of two or more languages in terms of coexistence, competition, and cooperation of two linguistic systems. Additionally, although bilingualism is a lifelong process, the language development among bilinguals is not merely a linear process; there are turns and twists on the way to becoming bilingual, trilingual, and multilingual. The path to trilingualism is even more complex than growing up with two languages (Bhatia & Ritchie, 2016 ).

The role of sociolinguistic factors in language learning; language use (creativity); language maintenance; and language shift, particularly in trilingual language acquisition and use, opens new challenging areas of future research. The main challenge for theoreticians and practitioners is how to come to grips with various facets of the bilingual brain ranging from language contact, bilingual language interaction, to language modes of the bilingual mind/brain on one hand and methodological issues on the other.

Despite a number of studies on the Critical Period Hypothesis, and other competing hypotheses of bilingual language acquisition, future research in cognitive aptitude, age, and multiple language effects with the lens of interdisciplinary debatable findings and methodologies continues to pose new challenges and promises to the field of bilingualism (Long, 2016 ).

Further Reading

• Auer, P. , & Wei, L. (Eds.). (2007). Handbook of multilingualism and multilingual communication . Berlin: Mouton de Gruyter.
• Bhatia, T. , & Ritchie, W. (2004/2006). The handbook of bilingualism . Oxford: Wiley-Blackwell.
• Bhatia, T. , & Ritchie, W. (2013). The handbook of bilingualism and multilingualism . Oxford: Wiley-Blackwell.
• Ferreira, A. , & Schwieter, J. W. (Eds.). (2015). Psycholinguistic and cognitive inquiries into translation and interpreting . Amsterdam: Benjamins.
• Heredia, R. , & Cieś licka, A. (Eds.). (2015). Bilingual figurative language processing . New York: Cambridge University Press.
• Schwieter, J. W. (Ed.). (2015). The Cambridge handbook of bilingual processing . Cambridge, U.K.: Cambridge University Press.

Foundational Works

• Bialystok, E. , & Hakuta, K. (1994). In other words: The science and psychology of second language acquisition . New York: Basic Books.
• Cummins, J. (2000). Language, power and pedagogy: Bilingual children in the cross-fire . Clevedon, U.K.: Multilingual Matters.
• Edwards, J. R. (1994). Multilingualism . London: Routledge.
• Grosjean, F. (2008). Studying bilinguals . Oxford: Oxford University Press.
• Haugen, E. (1953). The Norwegian language in America: A study in bilingual behavior . Philadelphia: University of Pennsylvania Press.
• Mackey, W. F. (1967). Bilingualism as a world problem/Le bilinguïsme: Phenomène mondial . Montreal, QC: Harvest House.
• Romaine, S. (1995). Bilingualism (2d ed.). Oxford: Blackwell.
• Weinreich, U. (1953). Languages in contact: Findings and problems . The Hague, The Netherlands: Mouton.

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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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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

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