Being able to speak more than one language may help you think more clearly in later life, even if you’ve learned the second language as an adult, according to a new study.

Thomas H. Bak of the University of Edinburgh Center for Cognitive Aging and Cognitive Epidemiology, the author of the new paper, said previous studies had shown that “bilinguals” who suffered dementia began showing symptoms four to five years later than people who spoke only one language. But causality was not clear: Does mastering a second tongue keep brains active longer, or do people with that skill start off with healthier brains than those who don’t?

“That is a very difficult question to address,” Bak said, “and we needed a very special population to do it.”

In his new study, older bilinguals performed better on cognitive tests than monolinguals, even when they had not scored better on intelligence testing decades earlier. That means that, at least in part, learning another language does predict brain health in old age, Bak said.

“Probably the causality is going in both directions, but we showed that there is certainly an effect of bilingualism that cannot be explained by previous differences,” he said.

He and his team used a data set including 853 Scottish participants who were given an intelligence test at age 11 and retested between 2008 and 2010 while in their 70s.

Of those, 262 had learned a language in addition to English, most before age 18, though only 90 were actively using the second language in 2008.

Even taking early intelligence scores into account, people who had learned a second language scored higher on reading, verbal fluency and general intelligence in old age than those who never had.

The relationship was the same for the 65 people who learned their second language after age 18, and seemed to get stronger with third, fourth and fifth languages, according to results published in the Annals of Neurology.

“I think it’s a study that could only have been done really with this cohort, this Scottish group,” said Fergus Craik, a senior scientist at the Rotman Research Institute, which is affiliated with the University of Toronto. “I’m not surprised at the effect, but it’s excellent to have this evidence.”

There’s always a question of which comes first, bilingualism or better brains, said Craik, who was not involved in Bak’s study. People by and large become bilingual not because of interest or intelligence but because they have to.

Learning a second language improves certain mental functions, mostly those connected to the frontal lobe of the brain, Craik said. “It does improve fluid intelligence and ‘executive functioning,’ because you have to control the two languages you know,” he said. “While you communicate in one language, you’ve got to manage and control the other language.”

Speaking more than two languages may improve thinking even more, but that’s still a bit of an open question, Craik said.

For people who are beyond school age but are still interested in learning a second language, these results should be encouraging, Bak said. He wouldn’t recommend that people worried about dementia go out and learn another language as a precaution, but for those who are interested in language, there may be unexpected benefits.

“I would not push it forward, but if you leave the chance of many languages being spoken in the house, it could be great brain training and great fun,” he said.