I just posted below about the NFL concussion study. This is a bit personal. My father, now 86, has Alzheimers (or perhaps some similar memory impairment). It's unclear what this is due to, but it's possible it was due to his occupation. Dad worked in construction for years as a sprinkler fitter. He did suffer several falls and blows to the head. Much of his work career was before the hardhat era. Family lore is that two people fell off scaffolds on a particular job. One guy died; Dad had such a hard head he went back to work the following week.
So, are Dad's problems due to blows on the head due to construction work, or due to genetic/diet or other factors? As I get older, that becomes an important personal question to me.