It has been known for some years that learning a foreign language has a good effect on the brain, as it stimulates grey matter and helps the memory. There is also evidence that if you just grow up bilingual this can give you an advantage - apart from the obvious one that you are fluent in two languages, it also seems to help your general learning skills. It's because of findings like these that older people are often advised to take some foreign language lessons to keep their brains active.

There are other advantages to knowing more languages, which may not have a direct effect on IQ but definitely make you more effective as a person. If you learn a language you also learn about another culture, plus the experience of going to another country and trying to manage in that language is one that everybody should have if possible. It makes you much more open and able to get on with people, as well as helping you to speak your own language better - because you just become more aware of words, grammar etc.