"The advantages of bilingualism are even more fundamental than being able to converse with a wider range of people. (...)
It forces the brain to resolve internal conflict, giving the mind a workout that strengthens its cognitive muscles. Bilinguals, for instance, seem to be more adept than monolinguals at solving certain kinds of mental puzzles. (...)
The collective evidence from a number of such studies suggests that the bilingual experience improves the brain’s so-called executive function — a command system that directs the attention processes that we use for planning, solving problems and performing various other mentally demanding tasks. These processes include ignoring distractions to stay focused, switching attention willfully from one thing to another and holding information in mind — like remembering a sequence of directions while driving. (...)
The key difference between bilinguals and monolinguals may be more basic: a heightened ability to monitor the environment."