Update: A reader has pointed out that a substantial fraction of gun deaths are suicides (homicide data are here), raising a complication. However, this point is well-established in peer-reviewed literature (PDF): gun ownership increases death rates across the board: suicides, homicides, accidents… For more discussion of the relationship between guns and homicide, see the work of Harvard’s David Hemenway and collaborators (via Adam Gopnik). Also see Mark Duggan, who shows that changes in gun ownership are linked with changes in homicide rate; the relationship is stronger than for other forms of crime, as expected for a causal link.

Given the strength of US gun culture and the enormous number of guns, legislation will likely focus on secondary regulation (background checks, ammunition control, regulation of private sales, limits on magazine capacity) as detailed below. More research on gun safety would be welcome, but…

My last posting on guns made a simple point: in 1994-2004, restrictions on assault-style weapons probably reduced the number of people targeted in mass killings. However, a rash of comments nominated other “main” causes: violent video games, increasing population density, mental illness, lack of Christianity…it was an impressive array of alternative explanations.

Scientific American’s John Horgan enters the fray with a contrarian headline: More Guns Have Not Produced More Killings, But We Still Need Gun Control. However, the argument supporting the headline has a .45-sized hole in it.

First, let me give a general framework. A relatively simple (and obvious) way to examine the data is as follows:

Individuals’ baseline tendencies toward violence are variable. People from all groups are prone to it, some more than others.

Technology amplifies human intentions.

Guns are a cheap, widely available, and powerful technology for killing.

Based on this, framework, one model for the per-capita gun death rate D would be D = V * G * k, where V is the average tendency toward violence and G is availability of guns, with killing power k factored in. One could get fancier by through stratification: different groups have a different rate of V, and different guns have different effects, and so on. But the core idea is the same.

Previously, I showed evidence that regulating assault weapons (k) reduces D. What about the other variables?

Horgan points out that since 1993, the number of gun homicides has decreased, despite the fact that there are more guns than ever. Therefore, he asserts, more guns (G) do not lead to more killing.

The logical hole here is that the parameter V has declined over time, as part of an overall decrease in crime:



A similar decline in property crime occurred too (click the image for more data). Nobody knows the reason for this decline, though a very plausible explanation is reduction in environmental lead (not the Freakonomical idea, please).

A fairer comparison would be to compare gun deaths in different places at the same time. States are laboratories for democracy, culture, and social policy. For example, my home state of New Jersey is #49 out of 50 in per-capita gun ownership. Only Hawaii is lower. And we are ranked #47 out of 50 in the rate of gun death. (These are good things – I am not enthusiastic at the thought of the cast of Jersey Shore packing heat.)

Based on statistics on gun ownership and deaths, the overall tendency is clear:

The three states with the highest rate of gun ownership (MT, AK, WY) have a gun death rate of 17.8 per 100,000, over 4 times that of the three lowest-ownership states (HI, NJ, MA; 4.0 gun deaths per 100,000). The relationship is a near-perfect linear proportion: on average, as G goes up, D goes up (r=+0.63). These data suggest that whether or not our society finds it desirable, gun safety/control is a plausible means of reducing gun deaths.

A striking aspect of this graph is that the rate of gun ownership varies by almost tenfold across states. Residents of different states are in very different environments, gunwise. When opponents of regulation, who are usually in gun-rich states, say that a sufficiently-determined evildoer could get a gun even under a heavy regulatory regime, that could be correct. Think of this measure as an index of “gun culture.”

At its heart, this is not a surprising result. In gun deaths, the direct cause of death is a gun. In most cases, the death occurs as an intentional act of one person against another person. The strength of the correlation suggests that other factors of undoubted interest – socioeconomic factors, background screening – are secondary. Many of those variables go together: hunters, preppers/survivalists, and urban dwellers are different groups. For example, it would be interesting to see what is so special about South Dakota (unusually low gun death rate) or Arizona (unusually high).

An example of a secondary factor is mental illness. Much has been made of the mental illness of shooters like Adam Lanza and Jared Loughner. Violence is higher among the mentally ill (see here and here). In one study, about 1 in 5 imprisoned violent offenders were identified as mentally ill. Therefore even near-perfect detection of violent tendencies in this population would lead to at most a 20% reduction in violence. For this and other reasons, one could ask (NYTimes) whether mental health screening should be the principal approach to gun violence.

It has been suggested that action video games might incite violence. However, the evidence for this is surprisingly scanty. Indeed, such games can also reduce stress hormone secretion and act as a safety valve. So this is a mixed bag at best.

At this point, legislation on gun safety is a distinct possibility. Possible legislative actions include limits on magazine capacity and regulation of ammunition sales. In all cases, the challenge will be in identifying which actions are likely to reduce gun deaths. To reach this goal, empirical evidence is our friend.