Originally Posted by livedarklions

Certainly, the evidence for it is incredibly weak. It's all based on one dataset gathered by a single rider in one city riding with and without a helmet. That data has been reanalyzed twice with different interpretations : https://psyarxiv.com/nxw2k

I think the obvious methodological problem of having the researcher being his own subject is so obvious that I don't really see how any findings based on this could be useful. Obviously, if he knows what hypothesis he's testing, it could affect how he's riding.