MOD Note: This thread split from Bike Question thread. Please keep the helmet debate here.
Sorry, they don't.
I'm amazed that we now think it criminal to send out a kid without a helmet, when 90% of us rode bikes as kids, and 0% of us rode helmets. I never knew anybody who ever got a serious head injury of any kind.
If helmets provided the protection people claim, the safety benefit would be detectible by now in the general population. It's not, and if you put aside the hysteria for a minute and look at the physical construction of a helmet, you can understand why - it's a small chunk of STYROFOAM. Motorcycle helmets, on the other hand, might actually work - but nobody would wear them, so that's why we ended up with these useless pieces of plumage.