Motorcycles are the most dangerous form of motorized transportation. Per vehicle miles traveled, motorcyclists are about 3 times as likely as passenger car occupants to be injured in a crash, and 16 times as likely to die. Because the majority of these deaths are caused by head injury, safety advocates have recommended mandatory use of motorcycle helmets. Others contend that state laws mandating helmet use infringe on motorcyclists’ rights, and question whether such laws really reduce motorcycle deaths and injury. Scientific evidence cannot address the appropriate balance between personal freedom and public safety, but it can address the effectiveness of mandatory helmet laws. This Issue Brief summarizes a new analysis of the effects of motorcycle helmet laws on death rates, and points out the need to account for other potential factors when comparing death rates across states.