A new study from Canada's Concordia University found what many parents of bilingual kids have known intuitively; at one and two years old, bilingual children have smaller vocabularies in both languages than their monolingual peers.
Researchers worked with 181 parents and their infants at a Vancouver daycare to understand the effects of language switching on small children. All the children learned English, but because their second and third languages varied so widely, the study focused on their English vocabulary. What they found was that language mixing was very common in conversations with their kids. But they didn't do it without reason; they switched languages when there was no direct translation, when a word was hard to pronounce or when they weren't sure of a word. Parents also switched languages to make sure their children understood a new word.
The results? The one-year-old bilingual children understood fewer words in both languages and the slightly older children in the study didn't say as many words compared to their monolingual peers. Lots of language mixing makes it hard for young children to categorize what they hear, leading to slower learning. It also seems like it's harder to children to learn new words from a mixed-language sentence than it is to learn from a sentence in a single language.
But those trying to raise bilingual children shouldn't worry too much. Little polyglots will be confused a little at first but the exposure to multiple languages is beneficial. Children will eventually develop the strategies to switch between languages and learn two sets of grammar rules. So keep teaching your