People with more years of education, more intellectually demanding occupations, or higher IQs are less likely to develop Alzheimer's disease. A meta-analysis of 20 studies including more than 30,000 subjects, indicates a fairly close and consistent correlation over an average of seven years. It looks as though some brains have a backup capacity, now called cognitive reserve, that can delay or prevent the onset of dementia. What's unclear is the source of this reserve capacity and its practical significance.
A simple explanation for cognitive reserve is that by virtue of heredity, environment, or both, people with higher education and higher IQs can tolerate more loss because they have larger brains — more neurons, or more synaptic connections among neurons. A larger head size (which usually implies a larger brain size) is associated with a lower risk of dementia. In the Minnesota Nun Study, for example, autopsies revealed that Catholic sisters with a head size greater than average were less likely to develop symptoms of dementia with the same amount of apparently Alzheimer's-induced brain damage (amyloid plaques and neurofibrillary tangles).