>> 1. First of all, it is mandatory in my country. > >This is a bit of a circular argument, isn't it?

Sorry, I should have been more explicit. I stated that fact to give more context and because I wanted to link the mandatory helmet to the fact that bikers not wearing one in my country are more likely to be bad bikers than those wearing it. Moreover, I wanted to also state (I should have make 2 different bullet points) that we (in my country) must wear helmets because it's the law (not only because I, in my personnal opinion, think it's better - I often answer to people who asks me why I wear a stupid helmet that "it's just the law").

Wearing a helmet is a pretty strong hint but it's not a proof.