mathbabe

A fascinating and timely study just came out about the “Stand Your Ground” laws. It was written by Cheng Cheng and Mark Hoekstra, and is available as a pdf here, although I found out about in a Reuters column written by Hoekstra. Here’s a longish but crucial excerpt from that column:

It is fitting that much of this debate has centered on Florida, which enacted its law in October of 2005. Florida provides a case study for this more general pattern. Homicide rates in Florida increased by 8 percent from the period prior to passing the law (2000-04) to the period after the law (2006-10).By comparison, national homicide rates fell by 6 percent over the same time period. This is a crude example, but it illustrates the more general pattern that exists in the homicide data published by the FBI.

The critical question for our research is whether this relative increase in homicide rates was caused by these laws. Several factors lead us to believe that laws are in fact responsible. First, the relative increase in homicide rates occurred in adopting states only after the laws were passed, not before. Moreover, there is no history of homicide rates in adopting states (like Florida) increasing relative to other states. In fact, the post-law increase in homicide rates in states like Florida was larger than any relative increase observed in the last 40 years. Put differently, there is no evidence that states like Florida just generally experience increases in homicide rates relative to other states, even when they don’t pass these laws.

We also find no evidence that the increase is due to other factors we observe, such as demographics, policing, economic conditions, and welfare spending. Our results remain the same when we control for these factors. Along similar lines, if some other factor were driving the increase in homicides, we’d expect to see similar increases in other crimes like larceny, motor vehicle theft and burglary. We do not. We find that the magnitude of the increase in homicide rates is sufficiently large that it is unlikely to be explained by chance.

In fact, there is substantial empirical evidence that these laws led to more deadly confrontations. Making it easier to kill people does result in more people getting killed.

If you take a look at page 33 of the paper, you’ll see some graphs of the data. Here’s a rather bad picture of them but it might give you the idea:

That red line is the same in each plot and refers to the log homicide rate in states without the Stand Your Ground law. The blue lines are showing how the log homicide rates looked for states that enacted such a law in a given year. So there’s a graph for each year.

In 2009 there’s only one “treatment” state, namely Montana, which has a population of 1 million, less than one third of one percent of the country. For that reason you see much less stable data. The authors did different analyses, sometimes weighted by population, which is good.

I have to admit, looking at these plots, the main thing I see in the data is that, besides Montana, we’re talking about states that have a higher homicide rate than usual, which could potentially indicate a confounding condition, and to address that (and other concerns) they conducted “falsification tests,” which is to say they studied whether crimes unrelated to Stand Your Ground type laws – larceny and motor vehicle theft – went up at the same time. They found that the answer is no.

The next point is that, although there seem to be bumps for 2005, 2006, and 2008 for the two years after the enactment of the law, there doesn’t for 2007 and 2009. And then even those states go down eventually, but the point is they don’t go down as much as the rest of the states without the laws.

It’s hard to do this analysis perfectly, with so few years of data. The problem is that, as soon as you suspect there’s a real effect, you’d want to act on it, since it directly translates into human deaths. So your natural reaction as a researcher is to “collect more data” but your natural reaction as a citizen is to abandon these laws as ineffective and harmful.