I am reading the fascinating book "Traffic" by Tom Vanderbilt. In it, he cites a study in which a researcher rode a bicycle around with or without a helmet and measured the space that passing cars gave him. He discovered that when he wore a helmet, vehicles tended to pass closer than when he did not wear a helmet. Can you think of why this might be?
It seems as though a bicyclist who does not wear a helmet might be less likely to get hit by a car, while a bicyclist who is wearing a helmet is more likely to survive/reduce serious injuries if a collision occurs.
Another thing to consider is that Dutch bicyclists wear helmets more rarely than Americans do. At the same time, the Netherlands has a much lower fatality rate per mile traveled for cyclists than does the USA.