[snogglethorpe]

The main thing is that it's not just a medical issue, so citing medical studies isn't sufficient. Even if wearing a helmet decreases your injuries 100% of the time (and there are a few counterexamples to that), if a helmet law causes more deaths overall, it's still a bad law.

The main negative effect of helmet laws that I've seen cited is that they reduce the popularity of cycling as a transportation mode, and that can have extremely negative effects on public health (due to increased driving, reduced physical activity, and indirectly as a general damper on pro-cycling sentiment and infrastructure changes).

[I think Australia's helmet law is one of the main examples people talk about.]

[prawn]

Yes, also that more cyclists means a greater awareness of them amongst drivers, which improves safety. Making helmets non-compulsory can make riding safer in that way, encouraging more people to ride.

[snogglethorpe]

Yup. "Safety in numbers" is definitely an important factor.

In the end, it's far more important (in terms of safety, as well as for other social goals that cycling promotes) to get people on bikes than it is to micromanage the way they do it.

It's worth nothing that helmet usage is extremely low in pretty much all the great cycling nations (Netherlands, Denmark, Japan, etc), all of which have cycling mode-share orders of magnitude greater than any place in the U.S.

[In Japan it is becoming increasingly common to see very young kids wearing bicycle helmets, especially when they're seated on their parent's bike, but they seem to stop wearing them sometime in their grade-school years.]