Muito bom o artigo de Eugene Volokh criticando um estudo da Universidade da Pennsylvania, que tenta provar que a posse de armas de fogo em casa aumenta a probabilidade de ser morto com uma arma. Um trecho:

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.