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.
Logically what they are practicing is the fallacy of post hoc, ergo propter hoc – “after this, therefore because of this” – and it doesn’t work that way, as he deftly illustrates. But here’s the money quote, and the thing I find so angering about “studies” of this type:
Unfortunately, this is how conventional wisdom is molded. A badly flawed study leads to an even more flawed New York Times article. Readers read it and say “Wow, it’s dangerous for me to own a gun” — or “Since guns endanger even their owners, there’s really no reason to keep them legal.” Precisely because the study seems so authoritative, so scientific, it’s likely to be influential, even when it’s misdesigned and misreported. And this is especially so when these flaws are repeated in study after study, as they have routinely been in the gun debate. Bad social science leads to bad legal policy.
His piece concludes with a comment on the suggestion that medical professionals should make recommendations to their patients:
Finally, the study concludes with a recommendation to the medical profession: Physicians should “discuss with all patients” “the consequences of having access to guns.” But “discussions” are only helpful if the physician actually knows what he’s talking about. Many physicians have little personal knowledge about guns, and haven’t read the rebuttals to these studies. If they start spreading this erroneous information to their patients, the results won’t be good either for the patients or for the reputation of the medical profession.
They’re way ahead of the curve on this one, Professor. Just look below at my post “This is the Kind of Thing That REALLY IRRITATES ME!”