Learning a second language leaves a physical trace in the brain. Adults who speak two languages carry more grey matter than monolinguals in the frontal and parietal regions that handle executive control. That is the mental work of paying attention, holding back unwanted responses and keeping information in mind.
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That finding comes from a 2015 study of Spanish-English bilinguals at Georgetown University Medical Center, led by Olumide Olulade and senior author Guinevere Eden. The team wanted to know what, specifically, was driving the change. Their answer: it is not just having a large vocabulary in two languages. It is managing them at the same time.
Why two spoken languages, not just two vocabularies
To test that, Olulade and Eden’s team also scanned bilinguals of American Sign Language and spoken English. ASL-English bilinguals share the vocabulary size and general bilingual experience of Spanish-English speakers, but they can sign and speak simultaneously rather than constantly switching between two spoken channels.
The ASL-English group showed no increase in grey matter. Only the group juggling two spoken languages, which compete for the same vocal output, showed the structural difference. The researchers concluded that managing two spoken languages drives the difference. A larger vocabulary alone does not.
It starts before adulthood
The effect is not confined to grown-up brains that have had decades to change. A 2020 study in Brain Structure and Function scanned children and adolescents. An international team from the University of Reading and Georgetown University found that bilinguals lost less grey matter as they developed than monolinguals did.
Grey matter volume naturally declines from an early age as the brain matures. Lead researcher Christos Pliatsikas, of the University of Reading, said the study found “less such shrinkage in bilinguals than monolinguals during development”. Bilinguals also gained more white matter, the insulated connections that let brain regions communicate.
Georgetown’s Michael Ullman, senior author on the paper, said the adult effects researchers had already documented “may have their roots in childhood”.
The change reaches below the cortex
Grey matter volume on the brain’s surface is not the only structure that shifts. A 2017 study by Pliatsikas and colleagues found that bilingualism also reshapes subcortical structures, including the basal ganglia and thalamus, which are involved in speech monitoring and choosing which language to use.
That study’s more specific finding is about immersion rather than age. Bilinguals who learned their second language later in life, but used it constantly in an immersive environment, showed reshaping of these deep brain structures. It closely matched what researchers had previously seen in people who grew up bilingual from birth. Bilinguals with similar skill levels and a similar age of learning, but without that sustained immersion, did not show the same change. The brain keeps adapting as long as a second language stays in active, continuous use.
An earlier study from King’s College London had already located one part of this pattern in the cortex itself. Learning a second language increases grey matter density in the left inferior parietal cortex. How much depends on the proficiency reached and the age at which the person acquired the language.
What the structural change buys a bilingual brain
These are not changes without consequence. Ellen Bialystok, a psychologist at York University in Toronto, tested 104 people aged 30 to 88. Half spoke only English, from Canada. Half spoke English and Tamil, from India.
The bilingual group responded faster on cognitive tests at every age tested, and showed a slower rate of decline for some processes. Bialystok’s team described the pattern as bilingualism helping “to offset age-related losses”.
Bialystok later reported the finding at a meeting of the American Association for the Advancement of Science. Alzheimer’s symptoms began more than four years later in elderly bilingual adults than in monolingual adults with identical brain damage. “It’s not that being bilingual prevents Alzheimer’s,” she said. “It’s just that you are better able to cope.”
Other research described in the same report ties back to the mechanism Georgetown’s team identified in brain scans. Both of a bilingual’s languages stay active even when they mean to speak only one. The brain constantly practises suppressing the language it is not using.
That repeated exercise in executive control is what researchers now find written into the brain’s structure. It runs from the cortex down to the basal ganglia.
