I like it that this thread is more thoughtful than dogmatic.

My experience and Susan's (in Austria) resonate - unsurprisingly.

I've had a concussion, too -- by fainting and falling over. Indoors. On carpet. I also once hit my head falling off my bike -- no concussion, but a wound that needed a stitch. My friend had one slipping on the stairs down to the underground train. Normal daily activities bring with them the danger of injury and death. My brother hit his head badly playing on his tricycle as a child in the garden at age 3, yet I think putting helmets on playing toddlers, which some do, will do more harm than good.

(And I rode horses, too, and interestingly while I would bike to the stable without a helmet, I'd in nearly all cases wear one on the horse...)

The kind of cycling I used to do -- slow, utility or pleasure transport mostly on separate bike lanes, quiet side roads and multi-use path shared with pedestrians -- counts or at least counted as one of those activities. Of course I fell. But cycling as fast as a slow car, with feet attached to pedals on roads also used by either a high number of cars or very fast cars, in an environment where drivers tend to be hostile? Or going down bumpy dirt trails? This is a different situation, so a helmet it is.

I'm sensitive to the "cost to the collectivity argument" too of course, but would like to see statistics -- what's the percentage of injuries on non-helmeted cyclists vs. helmeted cyclists vs. people just walking on the street.

And thanks again for the warm welcome.