I am no fan of sports in general, and football in particular. So I'm sure I'll offend a lot of people here.
But isn't this just a chicken-egg controversy? Yes, maybe repeated blows to the head can make a football player dumb and crazy. But, in our society, that seems to put so much importance on football, kids from an early age (junour-high school through college) who show talent in football are given a free pass. They can skate by most academics if they have some ability to help the school win games.
So, the more important question is how do you tell whether a football has suffered brain damage due to the game, or if the typical player is just dumb as a stump to begin with?