Despite the evidence I published yesterday (statistical) that most mass shootings are not stopped by people with concealed weapons (who aren't off-duty cops, in a survey that is fundamentally flawed) is a matter of statistical fact. But Ann Coulter makes a solid point on The Daily Caller that the idea that everyone is packing is a definite deterrent:

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.

The effect of concealed-carry laws in deterring mass public shootings was even greater than the impact of such laws on the murder rate generally.

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.

That makes some sense.