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).
But it may be the software — active reserve — rather than the hardware that provides protection. Brains with the same number of neurons and synapses may differ in efficiency, especially as they age. And just as new blood vessels sometimes grow around blocked arteries, some people may make better use of existing circuits or recruit alternative circuits to compensate for losses and prevent disruption of mental activity.