A bilingual–monolingual comparison of young children's vocabulary size: Evidence from comprehension and production

What is it about?

This study looks at an important measure of very young children's language development: it looks at the number of words that children understand and say, so at their vocabulary size. It compares vocabulary size in children who heard two languages from birth (bilinguals) and children who heard only one of those languages from birth (monolinguals). In fact, the number of languages children were hearing was the only controllable difference between groups. They were the same age, were all first born, and all came from middle to upper middle class backgrounds. They all lived in the same country, all lived with both their biological fathers and mothers, and were all carried to term. All children heard Dutch from birth, and the bilingual children in addition heard French. We established that by age 13 months the bilingual children understood many more words than did the monolingual children. By the time the same children were 20 months of age we found that if you add up all the Dutch words that children knew (combining those they just understood with those they said) the monolingual children's vocabulary size was larger than the bilinguals'. Of course, the bilinguals in addition spoke and understood French. Their total word knowledge was no different from that of the monolinguals. We looked at many other things but again found no group differences.

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Why is it important?

This study is important because of two things. It is the first one to compare bilingual and monolingual children who are fully matched on all kinds of aspects that might play a role in the number of words they know. This allows us to really focus on the extent to which the number of languages that children are hearing affects their vocabulary size. Earlier studies have suggested that the number of languages heard really matters for vocabulary size. In addition, earlier studies have claimed that bilinguals are at the short end, and that somehow, hearing two languages leads to smaller vocabulary size compared to monolinguals. As a second important point, we show that this is not the case. In fact, we show the opposite for the youngest age: bilingual infants understand a total number of words at age 13 months that is normally expected for monolingual children who are 4 months older! So hearing two languages doesn't slow children's lexical development. True, when they are 20 months of age, monolingual children know more words in their single language than bilinguals do in one of them, but of course bilingual children also know a second language. And when you look at their total word knowledge any differences with monolinguals vanish. Unfortunately many people in the medical profession still believe that a bilingual environment leads to language delay. This means that if a bilingual child doesn't know that many words they will blame it on the bilingual setting, and they won't investigate and check whether, for instance, the child's hearing is optimal. Our study shows that bilingualism does not lead to delay, and that if there is a suspicion of language delay, one should check for possible reasons, just as one would for a monolingual child. Only then can children be properly helped. We also found that in each of our groups there was a great deal of variation. Some children knew many more words than others, regardless of whether they were bilingual or monolingual. We think it is high time to try and explain these dfferences, especially amongst bilinguals, and we think the explanation should first be sought in the way that parents speak to children.

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