Head circumference, education and risk of dementia: findings from the Nun Study.

To examine the prevalence of dementia associated with having a smaller brain, lower education or both of these characteristics, 294 Catholic sisters were assessed annually for dementia. Sixty participants died and their brains were evaluated to determine fulfillment of neuropathological criteria for Alzheimer's disease (AD). Lower educational attainment and the interaction of smaller head circumference with lower education were associated with the presence of dementia, controlling for age and the presence of one or more apolipoprotein E-epsilon 4 alleles. By contrast, neither low educational attainment nor head circumference was significantly associated with fulfillment of neuropathological criteria for AD. Individuals having both low education and small head circumference were four times as likely to be demented as the rest of the sample. The findings suggest that higher education and larger head size, alone or in combination, may reduce the risk of expressing dementia in late life.