Even heading a football can cause some brain damage. I beleive ex England player Jeff Astle`s early death was brought on through brain damage from heading the ball. Gary Lineker also used to avoid heading the ball as much as possible in training for the same reason.

Boxers know the risks. While its a shame for any fighter to lose control of their faculties, fans become less sympathetic if they fight on until they are well past it and bring on permanent damage themselves.