Prospero’s R.L.G. surveys research on children raised as bilingual or multilingual:
Many parents once believed that a second language was a bad idea, as it would interfere with developing the first and more important one. But such beliefs are woefully out of date today. Some studies (such as this one) seem to show that bilinguals have smaller vocabularies in each language (at early stages) than monolinguals do. But other studies (such as this one) find no vocabulary shortfall in either language. In any case, the influence of mono- or bilingualism on vocabulary size is later overtaken by the importance of education, socio-economic status, reading and writing habits. In short, there is little evidence that raising a child bilingual will hurt their primary language.
The benefits, by contrast, are both strong and long-lasting. Bilingual children as young as seven months outperform monolinguals at tasks requiring “executive function”: prioritising and planning complex tasks and switching mental gears. This is probably because monitoring the use of two languages is itself an exercise in executive function. Such studies control for socio-economic status, and in fact the same beneficial effects have been shown in bilingual children of poor families. Finally, the effects appear to be lifelong: bilinguals have later onset of Alzheimer’s disease, on average, than do monolinguals.
Previous Dish on Tim Doner, the 17-year-old boy who can speak 20 languages, here. A video profile of 12-year-old Wendy Vo, who can speak 11 languages, is seen above.