"Being bilingual may delay Alzheimer's and boost brain power..."

[Östen Dahl]

For those who like me cannot access the link Lise Menn provided, here is one that does not require a subscription to the journal: http://intraspec.ca/CraikBialystokFreedman2010.pdf

The authors give a fairly clear definition of what they mean by "bilingual":

"The criterion for classification as bilingual was having spent the majority of life, at least from early adulthood, regularly using at least 2 languages."

In other words, it is regular use rather than knowledge that is criterial, and the authors treat bilingualism that originates in childhood and early adulthood alike. Yet, the abstract, rather misleadingly in my opinion, uses the term "lifelong bilingualism". Another study reports slightly different results (Chertkow et al., Multilingualism (But Not Always Bilingualism) Delays the Onset of Alzheimer Disease: Evidence From a Bilingual Community; Alzheimer Disease & Associated Disorders, 24:118-125; doi: 10.1097/WAD.0b013e3181ca1221). In this case, I have regrettably only been able to access the abstract. Chertkow et al. found that the protective effect showed up in a group of "multilingual immigrants to Canada" but not among nonimmigrants "raised in both official languages of Canada-French and English". To judge from the title of the paper, the authors seem to want to attribute their result to differences in the number of languages known. The obvious alternative is an explanation in terms of the age when the second language is learnt -- which is compatible with the idea that it is the extra effort involved in using a non-native language that is responsible for the protective effect. 

However, Chertkow et al. also report that there was a tendency for "nonimmigrants whose first language was French" to behave somewhat like the immigrants, apparently meaning that there was some protective effect there. I am not sure how this is to be interpreted; maybe someone who has access to the full paper can give more information here. In any case, I think their paper shows that it is not irrelevant how bilingualism is defined. 

- östen