[reinout]

What do you do more often? Fall exactly on the top of your head or on your side? In the latter case, the helmet makes your head bigger and can cause your head to rotate. And swift rotation is worse for your brains than a head-on hit.

So only use a helmet when roadracing or mountainbiking, not for a simple city ride.

[rlpb]

You seem to be assuming that the only thing you're going to hit with your head is the ground.