The main thing is that it&#x27;s not just a medical issue, so citing medical studies isn&#x27;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 <i>overall</i>, it&#x27;s still a bad law.<p>The main negative effect of helmet laws that I&#x27;ve seen cited is that they reduce the popularity of cycling as a transportation mode, and <i>that</i> 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).<p>[I think Australia&#x27;s helmet law is one of the main examples people talk about.]