In one of his sketches, comedian Eddie Izzard talks about how English speakers see bilingualism: “Two languages in one head? No one can live at that speed! Good lord, man. You’re asking the impossible,” he says. This satirical view used to be a serious one. People believed that if children grew up with two languages rattling around their heads, they would become so confused that their “intellectual and spiritual growth would not thereby be doubled, but halved,” wrote one professor in 1890. “The use of a foreign language in the home is one of the chief factors in producing mental retardation,” said another in 1926.

A century on, things are very different. Since the 1960s, several studies have shown that bilingualism leads to many advantages, beyond the obvious social benefits of being able to speak to more people. It also supposedly improves executive function—a catch-all term for advanced mental abilities that allow us to control our thoughts and behavior, such as focusing on a goal, ignoring distractions, switching attention, and planning for the future.

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