Only one public policy has ever been shown to reduce the death rate from such crimes: concealed-carry laws.

Their study controlled for age, sex, race, unemployment, retirement, poverty rates, state population, murder arrest rates, violent crime rates, and on and on. …

Someone planning to commit a single murder in a concealed-carry state only has to weigh the odds of one person being armed. But a criminal planning to commit murder in a public place has to worry that anyone in the entire area might have a gun.

You will notice that most multiple-victim shootings occur in “gun-free zones” — even within states that have concealed-carry laws: public schools, churches, Sikh temples, post offices, the movie theater where James Holmes committed mass murder, and the Portland, Ore., mall where a nut starting gunning down shoppers a few weeks ago.

Guns were banned in all these places. Mass killers may be crazy, but they’re not stupid.