American politicians, some parents, and a few others have previously expressed the concern that learning more than one language muddles the mind. This is, of course, absurd, and it is hard to believe why anyone really thought this. In fact, it could be said that having more than one language under your belt makes it easier to learn yet another language, a demand Americans often place on foreigners or immigrants to the US which is less often placed on the Americans (see this discussion).
Now, to support the idea that having more languages is good for the mind is being demonstrated at the Language Acquisition Lab at Cornell. According to Barbara Lust, “Cognitive advantages follow from becoming bilingual… These cognitive advantages can contribute to a child’s future academic success.”
There is a press report providing some of the lab’s results here.