Bike helmet laws 

 I'm into biking (mostly road-biking these days) so I was interested to read a post on the New York Times'  "Freakonomics" blog  about  a study  that uses variation in bike helmet laws across US states to show that helmet laws decrease bike riding among kids and teens.  Since I think that most people should ride bikes most of the time AND I have been known to bug people to wear helmets, perhaps I've been working against myself.   


 A few things came to mind while reading the study.  First, the study shows that helmet laws have an effect on bike safety for kids in the same age ranges.  Unless I missed something, it seems like part of this effect could be due to fewer kids riding bikes (in addition to the obvious safety improvement that comes from actually wearing a helmet).   I'd be curious how much the decrease in bike use is influencing the increase in safety, especially if kids are simply deciding to do other things like skateboarding that are perhaps equally dangerous but don't require helmets (a possibility mentioned by the authors).  This may mean that the total effect of helmet laws on child safety is less than the effect estimated in the paper because some of the decreases in bike injuries are counter-balanced by increases in other types of injury that aren't part of the study. 

 Second, the authors use some fixed effects and diff-in-diff models, but I think this paper is calling out for the  synthetic control method  developed by Abadie, Diamond, and Hainmueller.  The policy intervention is clean and there are a reasonable number of states that don't have laws, so building synthetic matches might be feasible.  There might be some interference problems with states that pass helmet laws later, but those are details... 

 I'll end this post with a shameless plug: bike more! (and wear a helmet)