I'd say helmet use is about 95% cultural. You get a few people who won't wear them because they get claustrophobic or because they get too hot (poor, but plausible excuses), but what I've observed is:
Harley riders don't wear helmets unless required to do so by law or when participating in a charity ride where the organizer requires helmets. I don't get the no helmet thing, but have to give them credit for turning out in droves for fundraiser rides.
Other American cruiser riders (Victory, Indian, etc.) mostly follow the Harley culture.
Japanese cruiser riders sometimes wear helmets - use seems to increase with age.
People riding big touring bikes (Gold Wings, etc.) almost all wear helmets.
People that ride standards wear helmets more often than not.
People that regularly ride to work wear helmets.
People that race bikes usually wear helmets, even when they aren't racing.
The worst of all offenders are the people you see wearing a muscle shirt, shorts, and flip flops on crotch rockets. These are usually young guys, often seen with their girlfriends on the pillion. Even the vehement anti-helmet folks at least have the sense to wear boots, gloves, and heavy leathers.
My favorite of all excuses, however, is the "helmet hair" defense. It's always fun to remind these riders that 1) 70 MPH wind doesn't do much for your hair either, and 2) nor do all the bugs you get in your hair with no lid, but most of all 3) your hair isn't going to look too good lying a hundred feet away from the rest of your dead body on the highway.
Having said all that, I still don't like helmet laws for adult riders. I wouldn't leave my driveway without a lid, but that's a decision I feel fully qualified to make all by myself, thanks.
We're in CT where it has been cold and snowed three times already. At least it didn't stay. I've got the road bike. Took it to my favorite bike mechanic for a spring tune up/safety check.
