Reams of papers have been published on the cognitive advantages of multilingualism. Beyond the conversational doors it can open, multilingualism is supposed to improve “executive function”, a loose concept that includes the ability to ignore distractions, plan complex tasks and update beliefs as new information arrives. Most striking, numerous studies have even shown that bilinguals undergo a later onset of dementia, perhaps of around four years, on average. But some of these studies have failed to replicate, leaving experts wondering whether the effect is real, and if so, what exactly it consists of.