Why do some bikers not wear helmets?

There are no laws mandating helmet use for motorcyclists in many developing countries and some US states. In the first case, such laws are likely unenforceable, in the second I suppose the American urge for "freedom" makes such laws inappropriate and the hope is that motorcyclists would have the common sense to use helmets. So what determines why some choose not to wear a helmet?

Michael Grimm and Carole Treibich went to Delhi and surveyed motorcyclists, focusing on helmet use and speeding. Some of the answers are not surprising: the bare-headed ones are less risk-averse, younger, less educated and less informed about accident and fatality rates. More interesting is that speeding and not using a helmet seem to be strong substitutes. Imposing helmets alone would thus not necessarily improve safety. One would have to impose helmets and enforce speed limits. That is likely too much to expect from India though, where just informing about the true risks may be more effective.