Since we were considering a motorcycle to save on gas, I looked into this a while back. Three states have no helmet law at all. 20 have a law requiring helmets on all riders, regardless of age. The rest have varying ages of required helmet use (14 in Maine, 17 in 18 states, 18 in Delaware, and 20 in the remaining 7 states.) Three of the ones with exemptions for 21 and up require that you carry valid medical insurance if you ride without a helmet.
That being said, my sister is a paramedic, and a properly fitted motorcycle (or football) helmet can actually function as a cervical collar in case of a collision. Assuming you survive the impact, you are more likely to escape brain damage and paralysis if you're wearing a helmet.