The cognitive benefits of multilingualism
One theory on why this might be is that there’s increased psychological distance when speaking a language that isn’t your mother tongue. Researchers in the spending study posited that subjects had less of an emotional reaction to things heard in their second (or third, or fourth) language, perhaps allowing for a more levelheaded decision.
More recently and perhaps most importantly, it’s been found that people who learn a second language, even in adulthood, can better avoid cognitive decline in old age. In fact, when everything else is controlled for, bilinguals who come down with dementia and Alzheimer’s do so about four-and-a-half years later than monolinguals.