boxing
I had the same type of questions before I started to train in full contact for awhile. I asked a teacher I had, a clinical neuropsychologist about it. He said that getting hit at all, even lightly over a long period of time can cause damage. Of course, I wasn't getting whacked in the head with a bat, so I wasn't too worried about it. I think that getting hit in the head every once in a while won't cause too much damage, at least I hope not, haha.
"I'm here to wipe the floor with your ass, and you know it, and everybody here knows it, and you deserve it."-Duncan, "Some Kind of Wonderful"
"I used to be with it, then they changed what it was. Now what I'm with isn't it, and what's it seems wierd and scary to me. It'll happen to you!!!" -Grandpa Simpson
"Once while walking through the mall a guy came up to me and
said 'Hey, hows it going?'. So I grabbed his arm and twisted
it up behind his head and said 'Now whose asking the questions?'" - Jack Handey
"There's no doubt in my mind that we should allow the world worst leaders to hold America hostage, to threaten our peace, to threaten our friends and allies with the world's worst weapons."George Bush—South Bend, Ind., Sept. 5, 2002