A recent study by four researchers at the University of the Balearic Islands is a good example. They studied 112 bilinguals using three criteria: the age they acquired a second language; fluency in their two languages (most are not equally adept in both); and the frequency with which they switch between the two options. Frequency of switching, it turned out, was the variable that correlated best with improved executive control. Unlike Mr Filippi’s, other studies have hinted that frequent switching may be a good predictor of the bilingual advantage.