I thought this article today reported by forbes.com on 12 month old babies exposed to two languages was really interesting. I haven’t yet read the article in Science, but I will. In this study babies who were learning language in a bilingual environment seemed to be able to demonstrate more flexibility in learning new words. This finding could be due to the fact that they have to learn different words (at least one in each language) for the same thing.
In work I’ve been involved with we found that preschool and early school-age children produced some words in both languages and produced other words in only one language. But, it could well be that they know (or would recognize) the words in the other language even if they only use them in one. Is it just as easy to learn two as one as the title of the news report implies? I don’t think so. I think actually that the fact that it’s harder is what gives bilingual some of the cognitive advantages reported by researchers including Ellen Bialystok.