To be fair to the previous poster, there were some studies (from Australia I believe), which showed an increase in bike accidents with helmet laws. IIRC they didn't have a good explanation, but thought it could be due to more risky behaviour, but also cars being less considerate if people wear helmet.

There were also some studies that showed that for population health bike helmet laws might be a net negative, because it prevents more people riding bikes and the positive health benefits of riding outweigh the slightly increased risk of biking even without a helmet.