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Domestic Disputes
Bad social science and bad legal policy.

By Eugene Volokh

...What the University of Pennsylvania study found was a statistical correlation: Gun ownership is correlated with gun deaths. But that two things are correlated doesn't prove that one causes the other. The sex-crime rate is correlated over time with the use of air conditioning, but not because air conditioning causes sex crime; rather, both rise during the summer months. Likewise, whether someone in your home has been to the hospital recently is correlated with death in your home, but not because hospital care tends to kill people (though sometimes it does). Rather, both hospital stays and deaths often have a common cause: serious illness.

It turns out that a hugely disproportionate fraction of homicide victims are themselves criminals — criminals do dangerous things, and deal with dangerous people. In a recent San Francisco study, two-thirds of all gun-homicide victims (and one-third of all gun suicides) were found to have had arrest records, and other studies of gun-homicide victims yield similar results. And criminals, especially drug dealers and gang members, are particularly likely to own guns; most gun owners aren't criminals, but many criminals are gun owners. So even if gun ownership and gun homicide are correlated, both may be caused by a common factor: Hardcore criminals are especially likely to own guns — and to be killed by guns.

Social scientists have long recognized that correlation doesn't prove causation, because the two correlated things may actually be caused by a third thing. That's why sound social science requires that scientists use statistical tools that control for a wide variety of such "confounding factors."

This particular study in fact tried to control for various factors. Before controlling for those factors, the study actually found that gun-homicide victims were slightly less likely to own guns than the nonvictims were. The correlation between gun ownership and gun homicide arose when the study controlled for sex, race, age, marital status, education, income, veteran status, region, population of area of residence, and whether the person lived alone.

The study, however, completely failed to control for what might well be the most important factors: whether the household contained violent criminals, gang members, drug dealers, and the like. These are the very factors that might cause both gun ownership and gun death. And because the study didn't control for them, it says nothing about whether gun ownership really "increases the odds" that a law-abiding citizen will be killed. The study's results could easily flow simply from the huge set of homicide victims who are themselves criminals....