Source: "Motorcycle fatalities have risen sharply in Florida since the state repealed its mandatory helmet law."
Sharply is from 515 to 933 killed. As the article points out it is an "81 percent increase". I find a few problems with this.
One, the big bad bikers would follow the law (wear a helmet while riding their motorcycle), but as soon as the law is abolished they would stop wearing their helmet. They have already bought the helmet, so there is no additional cost for the riders. Is the threat of a ticket really enough to convince people to wear a safety device? I know that the threat of the ticket for not wearing a seat belt does not change my behavior at all. If anything, it decrease the chance I will wear a seat belt.
Two, what if there was an increase in the number of motorcycles on the road. Gas prices have only risen during the study. So more (inexperienced) bikers, are on the roads. Secondly, the main people I know who ride bikes are old people, who have slow reflexes.
Also, the study did not study to see if motorcycle deaths rose in states whose laws remained unchanged.
For me it really comes down to not relating to the psychology that is assumed in the study.