The larger analysis also need to consider that some people won't ride a bike if required to wear a helmet; eg, to prevent helmet hair while on the way to an opera.
In Denmark a more common reason isn't hair so much as where to put the helmet. People bike everywhere, and usually you just leave your bike in some random location on a sidewalk, with the rear wheel immobilized with a built-in device that most bikes have (minor theft deterrent). Then you're left either having to carry the helmet with you everywhere, or to try to lock it to the bike somehow (presumably with an extra lock you carry just for that). Also, it drizzles a lot, so your helmet will be very wet if you leave it outside, unless you carry an extra bag or cover for it that works together with a lock.
For commuting between home and office it's not an issue, and more people wear helmets in that case. Of my coworkers, I would say about 1/3 wear a helmet when commuting, skewed towards those who live further (~10km), and those who are somewhat older (>40 y/o). Parents with young kids also tend to wear helmets, rather than trying to explain to the kids that they have to but mom and dad don't.
I think people make the "where to put the helmet" into a bigger problem that it is. I wear a helmet when biking, and I ride my bike everywhere (I'm Danish). I always just hang the helmet on the handlebar, my reasoning "who would steal a helmet". I've been right the last 10 years.
Also really, do you care it's wet? It has holes in it, your going the get your head wet anyway.
In Denmark a more common reason isn't hair so much as where to put the helmet. People bike everywhere, and usually you just leave your bike in some random location on a sidewalk, with the rear wheel immobilized with a built-in device that most bikes have (minor theft deterrent). Then you're left either having to carry the helmet with you everywhere, or to try to lock it to the bike somehow (presumably with an extra lock you carry just for that). Also, it drizzles a lot, so your helmet will be very wet if you leave it outside, unless you carry an extra bag or cover for it that works together with a lock.
For commuting between home and office it's not an issue, and more people wear helmets in that case. Of my coworkers, I would say about 1/3 wear a helmet when commuting, skewed towards those who live further (~10km), and those who are somewhat older (>40 y/o). Parents with young kids also tend to wear helmets, rather than trying to explain to the kids that they have to but mom and dad don't.