The biggest benefits seem to come to those who master their second languages fully. That in turn is usually because they speak the two as natives, or at least have spoken them on a near-daily basis for a long time. A bit of university French does not, unfortunately, convey the same advantages as deep knowledge and long experience. Switching languages frequently in the course of a day (or conversation) may be particularly important. Studies of interpreters and translators have provided some of the strongest evidence for a bilingual advantage. For example, they are faster at repeatedly jumping back and forth between simple addition and subtraction problems than monolinguals, suggesting generally better cognitive control.
But elsewhere is “a forest of confounding variables”, says Mark Antoniou of Western Sydney University. Bilinguals are not like monolinguals in lots of ways. The child of diplomats, raised in a foreign language abroad, may have cognitive and educational advantages that have nothing to do with bilingualism. At the other end of the socioeconomic ladder, though, studies have found striking evidence that in poorer parts of the world multilingual people show the strongest advantages from speaking several languages. Where schooling is scant, researchers surmise that bilingualism exercises children’s brains in a way that their schooling may not.