A growing body of research shows that regularly speaking two languages comes with certain types of improved mental performance.
In a Perspective article appearing in the journal Science, Jared Diamond of the University of California, Los Angeles, and author of Guns, Germs and Steel highlights studies of bilingualism that show this effect.
Diamond began wondering about the effects on the brain of multilingualism while camping with New Guinea Highlanders, all of whom could speak between five and 15 languages.
"What are the cognitive effects of such multilingualism?" asks Diamond in the new article.
"Being able to use two languages and never knowing which one you're going to use right now rewires your brain," says Ellen Bialystok of York University in Toronto, whose work Diamond cited repeatedly in the article.
"The attentional executive system which is crucial for all higher thought - it's the most important cognitive piece in how we think - that system seems to be enhanced," she says.
Executive functioning allows us to keep a goal in mind, take actions to achieve that goal, and to ignore other information that might distract us from that goal, says Albert Costa, who studies bilingualism at the Universitat Pompeu Fabra in Barcelona, Spain.
"The question is: Would it be the case that bilinguals, by the constant need for controlling the two languages, develop a more efficient executive functioning system?" he says. "The results suggest that bilinguals may have this positive collateral effect."
"The effects are much stronger when you go to kids and older people," he says. These are ages where executive functioning is worse.
Bialystock has shown that bilinguals do better at tests that require multitasking, including ones that simulated driving and talking on a phone.
"Make no mistake: Everybody is worse," says Bialystock, "but the bilinguals were less worse."
Bialystock's studies focused on people who were truly bilingual. The longer people have spoken multiple languages, the greater the cognitive effects. There are even benefits when languages were taken up at later ages. "We have not seen a cut-off," she says.
But Bialystock and Costa say bilingualism comes with some cost.
"For bilinguals, there are a couple of milliseconds before you can target the right word in the right language. Bilinguals have more 'tip-of-the-tongue' problems," says Bialystock.
"Bilingual children have on average a smaller vocabulary in each of their languages than monolingual children," she says. "There is a smaller vocabulary in each language, but they probably know more words altogether."
But having improved executive functioning, Bialystock argues, is more important than small differences in vocabulary or millisecond lags in word retrieval.
Still, all of these findings are somewhat abstract. It is difficult to take laboratory findings showing better executive functioning in bilinguals and demonstrate that they translate into better performance in the workplace or some other practical environment.
In one real-world application, Bialystock's recent work shows that multilingualism can provide health benefits to Alzheimer's patients.
"We have demonstrated in at least two separate studies of several hundred people altogether that - all else being equal - people who have spent their lives speaking two languages are better able to cope with the pathology of Alzheimer's," she says. "They show symptoms of the disease up to four years later than monolinguals. Once the disease starts to destroy areas of the brain, bilinguals are able to keep functioning."
"It's the same argument that you hear for doing crossword puzzles and such," she says, though she argues that language provides a more intense and varying type of mental exercise, which is why the effect is so strong.
In total, the evidence suggests attitudes bilingualism should be better accommodated in monolingual societies, says Bialystock. "When people come from somewhere and join you in your country, don't make them give up their language."
While Costa says that the findings on Alzheimer's patients should be taken cautiously, he agrees that there are social benefits to be had from better accommodation of bilingualism in an increasingly international world.
"For a while it has worked to be monolingual," he says. "I don't think it's tenable anymore."