Bilingualism delays age at onset of dementia, independent of education and immigration status

From their study of 648 patients with dementia in a clinic in India, Alladi et al. concluded that bilingualism leads to a delay in onset age of dementia compared with monolingualism. [1] The data are not sufficient to draw this conclusion. Age of onset studies conducted in a single disease group (case-only studies) are unable to show associations with risk factors for a simple reason: the age of onset depends on the age distributions of the groups from which the participants are selected in the source population. If monolingual persons die at a younger age on average than their higher educated bilingual counterparts, then the mean age of monolingual people in the source population will be lower than that of bilingual people. This difference will be reflected in the mean age of onset of any disease of later life, including dementing illnesses. To establish that the finding reflects cognitive reserve and is not an artifact of differing age distributions of monolingual and bilingual people in the source population, the age distribution of the source population by mono-/bilingualism needs to be determined. Characteristics that differed considerably between monolingual and bilingual patients in the present study included lower education, greater illiteracy, and more rurality in the monolingual patients???factors known to be related to lower life expectancy. Adjusting for these factors in models restricted to cases cannot address the issue of differences in life expectancy in these groups in the source population.

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