>There was an interesting study on mental characteristics which were
>related to the chances of developing Alzheimer's disease. They found that
>people who were more intelligent and more mentally active when they were
>young were less likely to later develop Alzheimer's.
>This is surprising because Alzheimer's is thought of as a disease
>condition, involving the destruction of brain cells.
There are two alternative explanations for this, other than that which might at first spring to mind.
The first is that if DAT (dementia alzheimer's type) kills cells, then those with more cells with more connections (and also, therefore, higher IQ) then this alone might explain the protection: simple redunancy and the fact that DAT is measured agaisnt arbitrary psychological thresholds.
The second is that DAT does not just start at age 65, hence the search for neurological and neuropsychological early detection tests. lower original IQ might be because the Alzheimer's is "tricking" from an early age.