A University of Washington study, published in the Journal of the American Medical Association, found that helmets provide substantial protection for cyclists of all ages, not just children. Dr. Andrea Gielen, of the Johns Hopkins Center for Injury Research, says social pressure is to blame. They're basically telling us that it's the social consequences of wearing a helmet that stands between them and protecting their heads. It's not their belief that they are not going to get into a crash or get injured so much as it is that their friends will think they look goofy if they wear a helmet, or that wearing a helmet is uncomfortable, that it's just not the socially acceptable thing to do.
Accidental injury is the leading killer of children, and a major killer of adults.