Post written by François Grosjean.

Although the advantages of being bilingual are numerous (see here), parents and educators are often worried that bilingual children will not know as many words as their monolingual peers. However much time one spends describing and justifying bilingualism (after all, half the world's population, if not more, is bilingual), the question keeps coming back is: But do bilingual children know as many words?

Dr. Barbara Zurer Pearson, a pioneering researcher in the field of childhood language acquisition, and her colleague Sylvia Fernández, studied this question in the 1990s. They examined the vocabulary development of English-Spanish bilingual children, aged between 8 and 30 months. They found that the rate and pace of development of the bilinguals' lexical knowledge were similar to those of monolingual children. In addition, the total vocabulary count of these children (taking into account both languages) was not different to that of the monolinguals, but their single language vocabularies were somewhat smaller. So we have known for some time that bilingual children do have as many words as their monolingual counterparts when both languages are taken into account but maybe not so when one examines only one language.

Almost twenty years later, a study by Diane Poulin-Dubois, Ellen Bialystok, Agnes Blaye, Alexandra Polonia and Jessica Yott, confirmed and extended this line of research. They compared the lexical development of two-year old monolingual and bilingual infants. One of the tasks used was similar to the Pearson and Fernández task; it is based on a vocabulary checklist that parents fill in and that measures a child's expressive vocabulary. They too found that the total vocabulary size obtained for the monolinguals and the bilinguals was not statistically different. As for the vocabulary size in the children's first language, it was once again smaller in the bilinguals than in the monolinguals.

Diane Poulin-Dubois and her colleagues explained the fact that bilingual children have a smaller vocabulary in just one language in the following way: They are exposed to their languages in different environments and hence they may encounter specific items in a context where only one language is used. This decreases the number of words acquired in each language.

This explanation makes a lot of sense. In one of the very first posts I wrote for this blog, I stressed how important the functions of languages are in the life of bilinguals. They usually acquire and use their languages for different purposes, in different domains of life, with different people. Different aspects of life often require different languages. I have called this the complementarity principle (see here).

The principle accounts for many interesting phenomena in bilingualism such as the ultimate fluency one attains in a language (at least at the lexical level), automatic language behaviors such as counting and praying (often done in just one language), the need to switch languages when the "wrong language" is used, the difficulty bilinguals have with translating, and so on.

It would seem that the complementarity principle is also at work in very young bilingual children. The above research points in this direction as does work undertaken by York University researcher and bilingualism specialist, Ellen Bialystok, along with her colleagues Gigi Luk, Kathleen Peets and Sujin Yang. They tested the English receptive vocabulary of a very large number of monolingual and bilingual children, between the ages of 3 and 10 years, whose school language was English. Once again, they found that monolingual children outperformed bilingual children when tested in just one language. To try to understand this finding, they took the step of examining the results by domain: the school domain (with words like "writing", "rectangle", "astronaut", etc.) and the home domain (with words like "squash" and "camper", for example).

The results they obtained confirm the presence of the complementarity principle. The difference that had been found between monolinguals and bilinguals was maintained in the home domain. This is normal as the bilingual children used their other language at home and hence didn't know English home words as well. However, in the school domain, a domain where English is used by both groups, the monolingual and bilingual children showed similar results. The authors concluded that bilingual children are not disadvantaged in academic uses of English.

In sum, the vocabulary of bilingual children will be in a given language for certain domains, in the other language for other domains, and in both languages for some shared domains. Concerning shared domains, back in the 1990s already, Barbara Zurer Pearson and her colleagues had found that a bit more than 30% of words in bilingual children were doublets or translation equivalents (i.e. a particular concept had a label in both languages), and more recently Diane Poulin-Dubois and her colleagues report a very similar percentage (37%).

All these results make perfect sense and reflect the fact that different facets of life in bilingual children and adults often require different languages. Increasingly, sociolinguistic aspects of bilingual language knowledge and use are being taken into account in psycholinguistics research and this can only be applauded.

Photo of a group of children from Shutterstock.


Bialystok, Ellen, Luk, Gigi, Peets, Kathleen F. & Yang, Sujin (2010). Receptive vocabulary differences in monolingual and bilingual children. Bilingualism: Language and Cognition, 13(4), 525-531.

Pearson, Barbara Zurer & Fernández, Sylvia (1994). Patterns of interaction in the lexical growth in two languages of bilingual infants and toddlers. Language Learning, 44, 617–653.

Poulin-Dubois, Diane, Bialystok, Ellen, Blaye, Agnes, Polonia, Alexandra & Yott, Jessica (2012). Lexical access and vocabulary development in very young bilinguals. International Journal of Bilingualism. DOI: 10.1177/1367006911431198

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