Canadian researchers were interested in the relationship between bilingualism and Alzheimer's disease. Their hypothesis was based on the concept of cognitive reserve, a term that represents the attributes of your brain that make it resistant to damage. For example, you might have heard that keeping your mind challenged by doing crossword puzzles can delay cognitive decline. This is one way to increase your cognitive reserve. Presumably, complex mental activity like crossword puzzles but also like speaking more than one language can lead to a lesser chance of developing dementia and, in the event where you do get dementia, a slower rate of decline.
The researchers sifted through the records of over 200 patients from a memory clinic. About half of their sample spoke one language and half spoke two languages. The researchers assessed the relationship between the number of languages each patient spoke and whether each patient had Alzheimer's disease. For the patients who did have Alzheimer's disease, the researchers noted how old the person was when the disease started. Of course, the researchers controlled their results for all the obvious potential biases, such as cultural differences, immigration, formal education and employment status.
The results of this analysis show that the bilingual patients developed Alzheimer's disease much later (4.1 years on average) than the monolingual patients. Since developing an age-associated disease like Alzheimer's later means you have a greater chance of dying before you get the disease, delaying the onset by 4 years means a reduction in the total cases of Alzheimer's disease. Currently, no drugs have an effect that's comparable to bilingualism.
Since no study is ever perfect, let me point out two small caveats before you fish out your old high school Spanish books. First, there is one thing the researchers could not control for, and that is whether cultural differences could lead to delays in seeking medical help for a condition. This could muck up the results because if some patients delayed their first medical visit, then the age at which they received the diagnosis for Alzheimer's disease could be skewed. Second, the protective effect of speaking two languages cannot be generalized to people who have some knowledge of another language but are not fully bilingual. In this study, the patients who were bilingual were true bilinguals, fluent in both languages and having used both languages regularly for most of their lives.
Still, it's a good reminder that a busy mind is a healthy mind. And it's nice to have evidence to justify the occasional weekend in Paris.
Reference: Bilingualism as a protection against the onset of symptoms of dementia. (2007) Bialystok E et al. Neuropsychologia 45:459-464.