On Saturday, February 16, 2019 at 5:36:56 AM UTC, James wrote:

Yet the second study I posted a link to, if you examine the numbers,
shows that the number of cyclists with a head injury of any kind, from
either group, is very close to 30%. That means either the helmet
wearers are taking more risks and are still as likely to hurt their
head, or the helmet isn't doing much of anything.

An inconclusive study would suggest to me that it was honestly done.

Perhaps extra risk taking by those who chose to wear a helmet is the
reason. I guess forcing people who would not volunteer to wear a
helmet, may either stop them from cycling or make them a fraction less
likely to hurt their head, or make them take similar risks to the
voluntary helmet wearers and they are back to 30% risk of a head injury ;-)

Considering the size of the existing studies (which by themselves makes such studies suspect and confusing), and the 3000+ respondents you would need for a proper study with control groups, and the likely cost, I'll take a modest bet on "We won't know in our lifetime."

Andre Jute
Can't say fairer than that, guv