A group of Utah first-graders listen and read along in Mandarin.
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But it is the knock-on effects--not how the brain looks but how it functions--that argue most for learning additional languages, and it appears that the bilingual brain is simply more efficient. The constant toggling that comes from having to choose between two words for every object or concept in your world is a total-immersion exercise in what cognitive scientists call task switching and what the rest of us call trying to do 17 things at once. Every time you interrupt an e-mail to pick up the phone, then interrupt the phone call to respond to someone who pops into your office, and then go back to the phone and the e-mail, the tracks in your brain must clank one way or the other. It's more challenging still when you're handling multiple tasks not sequentially but simultaneously.
How deftly any one person responds to these messy real-world challenges is hard to measure, but there are some good experimental proxies. In one, known as the Stroop test, subjects are flashed the names of colors on a screen, with the word matching the actual color of the letters, and are told to say the color's name or hit a key indicating what it is--a task nearly anyone can do instantaneously. Next they are flashed mismatches--the word red printed in blue, say--and told to ignore what the word says and announce only the color. This is a lot harder than you think, especially when you don't know when you'll get a matched example and when you'll get an unmatched one. Almost universally, bilinguals are faster and make fewer mistakes than monolinguals. Related studies have shown that the multilinguals' advantage is especially pronounced not in young adulthood, when the brain's executive functions are operating at their peak, but among kids and seniors, whose cognitive capabilities have either not fully come online or are starting to slip.
"The loss of efficiency when we rotate among tasks is called the global switch cost," says Bialystok. "Everyone slows down some or makes more errors, but multilinguals in all age groups have less of a drop-off." If that increased efficiency plays out in people's lives outside the lab--and there is no reason to think it doesn't--that would confer a real advantage over monolingual classmates, colleagues and others.