Bilingualism has a positive effect on cognition later in life, even for those who acquired the second language in adulthood, researchers found.
While prior research has investigated the impact of learning more than one language, ruling out "reverse causality" has proven difficult.
The crucial question is whether people improve their cognitive functions through learning new languages or whether those with better baseline cognitive functions are more likely to become bilingual, researchers said.
"Our study is the first to examine whether learning a second language impacts cognitive performance later in life while controlling for childhood intelligence," said lead author Dr Thomas Bak from the Centre for Cognitive Aging and Cognitive Epidemiology at the University of Edinburgh.
For the study, researchers relied on data from the Lothian Birth Cohort 1936, comprising 835 native speakers of English who were born and living in the area of Edinburgh, Scotland.
The participants were given an intelligence test in 1947 at age 11 years and retested in their early 70s, between 2008 and 2010.
Of the participants reported to be able to communicate in at least one language other than English, 195 learned the second language before age 18, and 65 learned it thereafter.
Findings indicated that those who spoke two or more languages had significantly better cognitive abilities compared to what would be expected from their baseline.
The strongest effects were seen in general intelligence and reading. The effects were present in those who acquired their second language early as well as late.
"These findings are of considerable practical relevance. Millions of people around the world acquire their second language later in life. Our study shows that bilingualism, even when acquired in adulthood, may benefit the ageing brain," said Bak.
The findings are published in Annals of Neurology, a journal of the American Neurological Association and Child Neurology Society.