> It's a common misconception amongst amateur cyclists that because helmets marginally increase the chance of neck injury and that head injuries are relatively uncommon on bicycles that helmets deliver negative returns in terms of safety but as others have pointed out the data doesn't seem to suggest this is actually the case.

The question is actually more interesting with public health. I have seen several studies that showed that mandatory cycling helmets decrease life expectancy. That's because less people will ride a bike if they have to wear a helmet and the benefits of cycling outweigh the dangers of not wearing the helmet. That doesn't mean it isn't good to wear one anyway if out cycling.

>Also in terms of common sense consider that all professional cyclists wear helmets when they're competing.

Mentioning professional cycling does not help your argument. The riders organisations were fighting strongly against mandatory helmets, and professional cycling was very late with requiring helmets they came many years after they were mandatory for amateur races. There is also the observation that professional cycling has become riskier, although I would not necessarily link that to cyclist wearing helmets, it would be worth investigating though.