Guest post by John R. Lott Jr., founder of the Crime Prevention Research Center. A recent study in the American Journal of Public Health claimed that there were more police feloniously killed in states that had more guns. The study got extensive news coverage at the TV networks such as NBC News, newspapers such as the Chicago Sun-Times, and international coverage such the UK Guardian.
Yet, it took just a couple of minutes to read the paper and realize that the empirical work was done in a very non-standard way. There is a big benefit to using so-called panel data, where you can more accurately account for differences in crime rates across states or over time. This method is called “fixed effects.” Strangely, the authors, David Swedler, Molly M. Simmons, Francesca Dominici, and David Hemenway, only control for the differences across states and not over time.
A couple of simple examples show why other studies on crime take into account these factors.
Take the differences across places. Many people point out that the UK has both a lower gun ownership rate and a lower homicide rate than the US. Some use this to claim that gun control causes crime rates to fall. But the homicide rate actually went up by 50 percent in the eight years after the 1997 handgun ban went into effect. The homicide rate was still lower than that in the U.S., but there were lots of reasons it was lower to begin with, not the handgun ban.
The same point applies over time. Suppose a state passes a gun-control law at the same time that crime rates are falling nationally. It would be a mistake to attribute the overall drop in national crime rates to the law that was passed. To account for that concern, researchers normally see whether the drop in crime rate for the state that had the change is greater or less than the overall national change.
Unfortunately, the American Journal of Public Health study doesn’t account for this last concern, and it makes a big difference. Just accounting for the changes in crime rates over time completely reverses the claims made in the American Journal of Public Health article.
Click here to see the full review debunking the American Journal of Public Health article.