New "Study" by 300 Mayors claims to find evidence that gun regulations reduce crime

The Washington Post had an article on this rsearch today. Anyone who has read my books on crime and gun control will immediately see the problbems with this research. The study compares the average crime rates for states with and without certain laws and claims that for certain states the crime rates are lower in states with certain regulations. Many of these laws I have looked at myself using panel level data (e.g., requiring background checks on the private transfer of guns), and I couldn't find an effect.

Let me give you a simple example of the problems with using purely cross-sectional data as used in this study. Suppose that high crime rate cities are the ones that adopt the strictest gun controls (which is generally true) and that crime rates fell ((just for the sake of argument assume that is the case), but the drop is not enough so that the crime rate in these cities is still higher than the low crime rate cities that didn't adopt the laws. Looking across the cities it would appear that the cities with the gun control laws had the higher crime rates, but does that mean that gun control caused higher crime? No. Obviously in the example that we gave the opposite really happened, but looking at simple averages gave a very misleading result. The only way that you can figure out what happened is to follow those cities over time and see how their crime rates changed relative to what they were in the past and to see how they changed over time ralative to the places that didn't chnage their laws