I saw somewhere (Scientific American...NIH some other laymen's publication? ) that a meta study was recently published examining whether increased helmet use had any correlation with or causal relationship to increased injury rates related to increased risk taking behaviors. The meta study concluded no relationship.
I didn't read any more than the article on the abstract. Nor can I tell you about any of the populations studied in the individual studies reviewed. I just know I read it somewheres. I am in health stats but not public health or populational control. So this isnt exactly my bag. But I'm satisfied that research has been done on this issue. And I'm fairly confident that risks of helmets are negligable at best.