Lilliard Richardson of the University of Missouri Truman School of Public Affairs and David J. Houston of the University of Tennessee in Knoxville said that since 1975 when the majority of states required motorcycle helmets -- more than 100,000 motorcycle riders in America have died in crashes.

Today, 20 states have universal helmet laws that require all riders to wear helmets, 26 states have partial coverage laws -- usually only for young riders.

The study, published in the American Journal of Public Health, said the states where repeals of universal coverage were instituted, the fatality rate increased an average of 12.2 percent. Conversely, in states with universal helmet laws, the fatality rate was 11.1 percent lower than in states with no helmet mandates.

The researchers also found that fatality numbers in states with partial laws were not statistically different from those with no helmet laws.

"Previous studies have been limited to certain states and fail to distinguish the states with partial coverage from those with no legislation," Richardson said in a statement.