The researchers -- led by Dr. Ellen Bialystok of the Rotman Research Institute of the Baycrest Centre for Geriatric Care in Toronto -- recruited 184 Toronto-area residents for their study. Bialystok and colleagues examined the participants for knowledge of languages and the age at which signs of dementia began to appear.
Men who spoke only one language were found to develop dementia at an average age of 70.8, while uni-lingual women developed the disorder at 71.9. However, among men who spoke at least two languages, onset of dementia was delayed until age 76.1, on average, while multi-lingual women developed dementia at an average of 75.1 years old.
When groups of men and women were combined, multi-lingual people experienced a delay in onset of dementia of 4.1 years, compared to those who spoke only one language.
"It's a much larger effect than I expected," Bialystok said. "You do research because you hope that your ideas are right. But I am always surprised; I always have the 'wow' reaction. And in this case the results were so clear."
According to Bialystok, the benefit likely comes from fluently knowing at least one non-native language, as method of learning and grammatical correctness did not seem to affect results. "What matters is that you have to manage two complete language systems at once," she said.
Bialystok and colleagues believe that the benefits of knowing multiple languages are not influenced by level of education, occupation, cultural upbringing or immigration history. The study revealed that participants with the highest education tended to speak only one language, which is typically thought to postpone the onset of dementia.
Neuropsychologist Fergus Craik, a member of the research team, said the study was an example of how lifestyle can affect mental functioning in old age. "It's not like it stops dementia, but ... it's deferred," he said. "That, in and of itself, is hugely important."