Well, unlike a seat-belt, a helmet is easily stolen and needs to be kept track of by itself. This means that if the net effect of having to keep track of it is

Well, unlike a seat-belt, a helmet is easily stolen and needs to be kept track of by itself. This means that if the net effect of having to keep track of it is to cost you sufficiently that you choose a car or other sedentary mode of transit over a bike you may well lose more quality-adjusted life years. We shouldn't over-weight rare catastrophic events compared to common continuous damage events. In practice this means that I wear a helmet all the time in SF. But most of the time, I wouldn't wear one in Cambridge.