Does South Dakota have a helmet law for motorcyclists and passengers?
Yes, but South Dakota's statute isn't nearly as broad as the so-called "universal" helmet laws that have been passed in a number of states. In South Dakota, only riders and passengers who are under the age of 18 are required to wear a motorcycle helmet. (By way of comparison, a "universal" helmet law applies to all motorcycle riders, regardless of their age.)
The relevant language of South Dakota's motorcycle helmet law (which you can find at South Dakota Codified Laws 32-20-4) reads: "No person under eighteen years of age may operate or ride upon a motorcycle on the public streets or highways of this state unless the person wears a protective helmet... No person may operate a motorcycle with any person under the age of eighteen as a passenger if the passenger is not wearing a protective helmet."
So, anyone who is under 18 and riding a motorcycle without a proper helmet would be in violation of this law, and a motorcycle operator can be held liable if his or her passenger is under 18 and not wearing a helmet. (In that situation, both the operator and the passenger would be issued a citation.)
Violation of the state's helmet law is classified as a "Class 2 misdemeanor" in South Dakota, meaning that the offense is punishable by a maximum of thirty days imprisonment in a county jail or a five hundred dollar fine, or both. Keep in mind that these are the maximum allowable penalties; you'll most likely be issued a fine of around $100 or less for a motorcycle helmet violation in South Dakota.
For more details on motorcycle licensing, safety regulations, and tips for riders, check out the South Dakota Motorcycle Manual from the South Dakota Department of Public Safety.
What if you get into a motorcycle accident, and you file a personal injury lawsuit against another driver, but you weren't wearing a helmet? In that situation, a South Dakota civil jury could decide that you were negligent -- and they could make such a finding even if you were over 18 and not legally required to wear a helmet. That means, any compensation you could receive will be reduced by a percentage that equals your share of blame for your own injuries. To learn more about these shared fault rules, check out South Dakota Car Accident Laws.