In a new working paper called “The Intended and Unintended Effects of Youth Bicycle Helmet Laws” (abstract here; pdf here), Christopher S. Carpenter and Mark Stehr offer a surprising conclusion: while mandatory helmet laws have led to increased helmet use, and while helmet use has been shown to reduce bicycle fatalities, such laws also seem to lead to a decrease in actual bike riding.

Why?

Carpenter and Stehr lay out their case compellingly. They exploit a convenient variable: the fact that helmet laws exist in only 21 states, pictured here:

While conceding that these are primarily “coastal” states, the authors note that they cover more than 50 percent of the U.S. youth population.

Their research first confirms earlier research that “helmet laws significantly reduced bicycling fatalities among youths age 0-15 (i.e., youths who were directly treated by most states’ age-16 helmet laws) by about 19 percent.”

Their research further suggests “that helmet laws significantly increased youth helmet use by 29-35 percent.”

But: “There is also robust evidence for an unintended and previously undocumented mechanism: helmet laws produced modest but statistically significant reductions in youth bicycling participation of 4-5 percent.”

To read more go to the NY Times