No helmet required law in my state, but I wouldn't be foolish enough to ride a motorcycle without one (and the other gear, too). Wearing a helmet once saved me from more serious injury when I had a simple low-side back when I was 17, almost 3 decades ago. Looking at the road rash abrasion on the side/back of the helmet was all the learning I needed to buy another helmet before I did any more riding.
On average, I think less than half the riders/passengers in my state use helmets, though BICYCLE helmets are mandatory for bicycling children under 16 years old. I also wear a bike helmet when I cycle/mtn bike, though I admit to being "old school" and not wearing any helmet when I downhill ski.