content: More like "in 1 out of 100 cases, seat belts are actually the cause of death in a car crash rather than preventing it. That's still 99 out of 100 cases where wearing a seat belt will save you." It doesn't have to be perfect. It only has to be better than the alternative to be worth using, but human intuition is terrible at that kind of statistical judgement and so people will avoid things that will almost certainly save them, on the tiny chance that they won't.