Bilingualism—broadly defined as a speaker’s ability to use two (or more) languages (or dialects) in their everyday life—characterizes the language experience of a large and heterogeneous part of the world’s population. Reliable figures about the number of bilingual speakers are hard to find, but with approximately 8 billion people and just over 7,000 languages in active use, bilingualism is pervasive. Despite the different theoretical stances and methodological approaches followed in the field, the shared goals of research on bilingualism are to investigate what makes the bilingual experience unique and what it can tell us about the mechanisms of language learning, use, processing, and impairment.

History

Over the last 100 years, research perspectives on bilingualism have shifted from considering bilingual individuals as deficient compared to monolinguals (e.g., Smith, 1923) to a view that holds being bilingual is more than the sum of two monolinguals in one mind (e.g., Grosjean, 1989). This has led to the argument that bilingual individuals should not be systematically compared to the standard of a monolingual native speaker (e.g., Ortega, 2018).

Direct comparisons with monolingual speakers may not always be appropriate or relevant because of the different experience of language exposure and use of these two groups. For example, bilingual children tend to have smaller vocabularies in each of their languages when they are compared to their monolingual peers (e.g., Bialystok et al., 2010). This is not surprising given that bilingual children divide their time—often unevenly—between two or more languages and therefore do not always have the same opportunities to learn any one language as monolingual children. Nevertheless, when we consider bilingual children’s total number of words across both languages, their vocabulary can be larger than that of monolinguals of the same age (e.g., Byers-Heinlein et al., 2024).

It is equally important to consider variation when it comes to bilinguals’ particular language combinations, since very little is known about the acquisition and processing of most of the languages of the world. According to one survey (Kidd & Garcia, 2022), the four major academic journals in the field of language development research have published articles covering a total of 103 languages, a coverage of about 1.47% of the world’s languages. Research in the fields of multilingualism and adult psycholinguistics is equally dominated by a relatively small number of languages and by researchers in North America and Western Europe (e.g., Bylund et al., 2024). On a more positive note, there is increasing awareness of the importance of greater linguistic and cultural diversity in the language and cognitive sciences (e.g., Blasi et al., 2022) and an acknowledgment of the need to study bilingualism in a more inclusive way (e.g., Higby et al., 2023).

Core concepts

Are all languages the same for bilinguals?

Individuals are bilinguals in many different ways: some are born in a country where there is more than one official language (e.g., Spanish and Catalan, or Spanish and Basque, in two autonomous regions of Spain) and may grow up with two societal languages. Others may live in a country where there is only one officially recognized societal language (e.g., Portuguese in Brazil) but have one or more additional languages at home, that is, their heritage languages. The acquisition of a heritage language typically does not enjoy the same opportunities as the societal language because it tends to be heard and used in a more restricted number of contexts and potentially with fewer speakers, or there may be difficulties in accessing books or digital media in that language that foster the acquisition of literacy. As a result, bilinguals’ proficiency in the heritage language may be more limited than in the societal language, and it is not uncommon for bilinguals to be able to speak but not read or write their heritage language when it is not supported by the mainstream education system.

Who is bilingual?

Regardless of their theoretical approach, one issue common to all researchers of bilingualism is how to determine who is bilingual. The extent to which an individual self-identifies as a bilingual, or is categorized as one, varies according to their perceived or actual proficiency and fluency, their patterns of language use, and what counts as a language (e.g., Wagner et al., 2022). Age of first exposure, for instance, determines whether someone is a simultaneous bilingual who has been exposed to two or more languages since birth or soon after, or whether they are a sequential bilingual who was exposed to a second or third language typically after the age of 3 or 4 years. The length of time someone has been exposed to an additional language also matters for the degree of bilingualism. The bilingual profile of a first language speaker of Mandarin who has been studying in Argentina for two years is very different from that of someone who was brought up with Mandarin and Spanish from birth. Because of the many factors that affect bilingualism, it is hard to delimit the concept and to categorize individuals as bilinguals or monolinguals, and the field has moved to viewing bilingualism as a continuum rather than as dichotomous (i.e., monolingual or bilingual) (e.g., Luk & Bialystok, 2013).

Language differentiation in bilinguals

One core question in bilingualism research concerns the extent to which bilingual individuals separate and integrate their languages. Bilingual infants can discriminate between their input languages from as early as 3.5 months on the basis of rhythmic properties that set the two languages apart (Molnar et al., 2014). By the time they start to learn words, bilingual children will typically use language-appropriate words with their interlocutors, for example, German words with their German-speaking parent and Italian words with their Italian-speaking parent (Taeschner, 1983), a sign that they can differentiate between languages. At the same time, all bilinguals, children and adults, will switch to the other language if they only have a word for a concept in that language.

There is clear evidence that bilinguals integrate their grammatical systems. In development, young children show cross-linguistic influence, whereby they may produce grammatical structures that are either ungrammatical or nonstandard, such as using the word order from one language in their other language (e.g., Müller & Hulk, 2001). Evidence from cross-linguistic structural priming shows that hearing a structure in one language (e.g., a Spanish passive, such as el camion fue perseguido por el taxi, ‘the truck was chased by the taxi’) can lead bilinguals to use the same or a similar structure in their other language (e.g., in English, ‘the window was broken by the ball,’ Hartsuiker et al., 2004). This evidence points to shared or connected syntactic structures across bilinguals’ two languages. Priming strength tends to vary as a function of language proficiency, structural overlap across the two languages, and the relative amount of language exposure. These findings support interactive, non-modular models of bilingual syntax where both languages dynamically influence structure building during production and comprehension.

Questions, controversies, and new developments

Over the last 100 years, the pendulum has swung from a deficient view of bilingualism in the early twentieth century to one in which bilingualism has been identified as the source of a cognitive advantage. Many studies have reported a positive association between bilingualism and various components of attentional control, that is, the ability to focus on what is important and ignore distractions (e.g., Bialystok & Craik, 2022) [see Attention]. At the same time, the findings have been inconsistent. Recent meta-analyses have reported no effects or extremely selective effects of bilingualism on executive function in adults (Lehtonen et al., 2018) and in children (Gunnerud et al., 2020). Others have reported an advantage for bilinguals under certain circumstances (Grundy, 2020). Given the heterogeneity of bilingual populations, it is hardly surprising that even the results of meta-analyses should be mixed. Historically, the classification of bilingual speakers was not as nuanced as it should be. While the main advantage of being bilingual is having access to more than one language and culture, the field needs to develop a more comprehensive theory of when to expect effects of bilingualism on cognition and why so that principled hypotheses can be tested.

Broader connections

Research on bilingualism encompasses a wide spectrum of interests in the field of education (e.g., institutional language planning, language teaching), psycholinguistics (e.g., language development in infants and young children, language learning and processing in older children and adults, language attrition/loss) [see Language Acquisition, Psycholinguistics], neurolinguistics (developmental and acquired language disorders, cognitive function, cognitive aging) [see Neuroscience of Language, Acquired Language Disorders], and sociolinguistics (e.g., family language policy, linguistic and cultural identity).

Further reading

• Bialystok, E. (2021). Cognitive effects of bilingualism: An evolving perspective. Bilingualism Across the Lifespan, 9–28. https://doi.org/10.4324/9781315143996-3

• Hoff, E. (2021). Why bilingual development is not easy. In Advances in Child Development and Behavior (Vol. 61, pp. 129–167). JAI. https://doi.org/10.1016/bs.acdb.2021.03.002

• Montrul, S. (2023). Heritage languages: Language acquired, language lost, language regained. Annual Review of Linguistics, 9(1), 399–418. https://doi.org/10.1146/annurev-linguistics-030521-050236