People who already speak more than one language tend to be better at learning other languages than people who only speak one. It's often thought that people who master more than one language have a natural aptitude for languages. Rather than studying adult language learners researchers at Northwestern University in Chicago studied children who had grown up bilingual and compared them to monolingual English speakers; they did this to see whether it was bilingualism itself, rather than any special talent for languages, that made it easier for bilingual people to learn another tongue. They asked participants in the study to learn words in a made-up language which bore no relation to any of the languages - Mandarin, Spanish or English - they already spoke. Bilingual people were able to master nearly twice as many words as monolingual ones. Other research has shown that bilingual people develop Alzheimer's disease on average four years later than monolingual ones.
You can find out more about this research at