Friday, October 10, 2014

Gun Control and Gun Violence

The United States has heard repeated calls for more gun control legislation in the wake of the Sandy Hook Elementary School shooting. Every day it seems there's a new mass shooting, with dire implications for the state of our country. But these mass shootings are isolated events that have almost been tailor-made to provoke disproportionate media attention. The day-to-day assaults, kidnappings, and murders affect a lot more people.

Liberals claim that gun control makes places safer by making guns harder to obtain and be used illegally. Conservatives counterclaim that gun control makes communities more dangerous by eliminating a key method of self defense for law abiding citizens. Each side has their share of talking points. Liberals point to the high rates of gun-related deaths in the United States compared to other developed countries (a point used famously in Michael Moore's Bowling for Columbine), and conservatives point to stories of self-defense by would-be victims of robberies, home invasion, domestic abuse, or other serious crimes. Although I fall on the liberal side of the spectrum, I would much rather take up a position supported by empirical evidence. So I asked the question: Do harsher or stricter gun laws affect crime?

To test this, I decided to do a simple regression of violent crime rates against the relative restrictiveness of gun laws. I am using the FBI's Uniform Crime Report statistics for 2012, and comparing it with's Gun Rights Index. Both datasets come with a few major caveats. has a strongly libertarian viewpoint, and clearly shows some favoritism towards laxer gun laws, while the FBI cautions strongly against using UCR data in order to rank crime in areas, due to different reporting standards by different police agencies. Despite FE's obvious political slant, I don't see a reason to doubt their data; it's unclear whether it would be more in their favor to magnify differences and paint states like New York as overly restrictive, or rather to blur them so as to obscure any supposed effect.

My first regression ended up with a slight negative correlation, suggesting that more restrictive gun laws led to more violence. But when I plotted the results, I found that I had made a classic demographic pitfall... counting DC as a state!

Outlier much? (Look in the top left)
If you remove DC, the correlation breaks down completely:

The lack of a correlation likely means that there are a host of other factors involved (understandable, since I only looked at a single metric of gun freedom). As another quick test, I tried the regression on just the homicide rate, although it doesn't look much better.

For those who haven't taken a statistics class, or need a refresher, the r² is an effect size (essentially what % of gun violence is affected by gun control), and the p is the probability that any correlation is just a product of pure chance. So there appears to be no connection between gun violence and gun laws in the bottom two graphs, and only a slight connection in the first (which would probably go away if I controlled for poverty level, population density, or something similar).

In any case, there are still a number of ways to go on this theme – looking at gun ownership rates, like this analysis from the Violence Policy Center; using a different metric of gun violence, like gun-related deaths (from the CDC) or individual arrest records (through the UCR's successor, the NIBRS) ; or controlling for other possible factors, like economic inequality or education levels; might be promising. For now, I'm unable to draw conclusions either way.

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