In the wake of a monstrous crime like a madman’s mass murder of defenseless women and children at the Newtown, Conn., elementary school, the nation’s attention is riveted on what could have been done to prevent such a massacre.

Luckily, some years ago, two famed economists, William Landes at the University of Chicago and John Lott at Yale, conducted a massive study of multiple-victim public shootings in the United States between 1977 and 1995 to see how various legal changes affected their frequency and death toll.

Landes and Lott examined many of the very policies being proposed right now in response to the Connecticut massacre: waiting periods and background checks for guns, the death penalty and increased penalties for committing a crime with a gun.

None of these policies had any effect on the frequency of, or carnage from, multiple-victim shootings. (I note that they did not look at reforming our lax mental health laws, presumably because the ACLU is working to keep dangerous nuts on the street in all 50 states.)

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.

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 gunned down shoppers a few weeks ago.

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

If the deterrent effect of concealed-carry laws seems surprising to you, that’s because the media hide stories of armed citizens stopping mass shooters. At the Portland shooting, for example, no explanation was given for the amazing fact that the assailant managed to kill only two people in the mall during the busy Christmas season.

It turns out, concealed-carry-holder Nick Meli hadn’t noticed that the mall was a gun-free zone. He pointed his (otherwise legal) gun at the shooter as he paused to reload, and the next shot was the attempted mass murderer killing himself. (Meli aimed, but didn’t shoot, because there were bystanders behind the shooter.)

In a nonsense “study” going around the Internet right now, Mother Jones magazine claims to have produced its own study of all public shootings in the last 30 years and concludes: “In not a single case was the killing stopped by a civilian using a gun.”

This will come as a shock to people who know something about the subject.

The magazine reaches its conclusion by simply excluding all cases where an armed civilian stopped the shooter: They looked only at public shootings where four or more people were killed, i.e., the ones where the shooter wasn’t stopped.

If we care about reducing the number of people killed in mass shootings, shouldn’t we pay particular attention to the cases where the aspiring mass murderer was prevented from getting off more than a couple rounds?

It would be like testing the effectiveness of weed killers, but refusing to consider any cases where the weeds died.

In addition to the Portland mall case, here are a few more examples excluded by the Mother Jones methodology:

— Mayan Palace Theater, San Antonio, Texas, this week: Jesus Manuel Garcia shoots at a movie theater, a police car and bystanders from the nearby China Garden restaurant; as he enters the movie theater, guns blazing, an armed off-duty cop shoots Garcia four times, stopping the attack. Total dead: Zero.

— Winnemucca, Nev., 2008: Ernesto Villagomez opens fire in a crowded restaurant; concealed-carry permit-holder shoots him dead. Total dead: Two. (I’m excluding the shooters’ deaths in these examples.)

— Appalachian School of Law, 2002: Crazed immigrant shoots the dean and a professor, then begins shooting students; as he goes for more ammunition, two armed students point their guns at him, allowing a third to tackle him. Total dead: Three.

— Santee, Calif., 2001: Student begins shooting his classmates — as well as the “trained campus supervisor”; an off-duty cop who happened to be bringing his daughter to school that day points his gun at the shooter, holding him until more police arrive. Total dead: Two.

— Pearl High School, Mississippi, 1997: After shooting several people at his high school, student heads for the junior high school; assistant principal Joel Myrick retrieves a .45 pistol from his car and points it at the gunman’s head, ending the murder spree. Total dead: Two.

— Edinboro, Pa., 1998: A student shoots up a junior high school dance being held at a restaurant; restaurant owner pulls out his shotgun and stops the gunman. Total dead: One.

By contrast, the shootings in gun-free zones invariably result in far higher casualty figures — Sikh temple, Oak Creek, Wis. (six dead); Virginia Tech, Blacksburg, Va. (32 dead); Columbine High School, Columbine, Colo. (12 dead); Amish school, Lancaster County, Pa. (five little girls killed); public school, Craighead County, Ark. (five killed, including four little girls).

All these took place in gun-free zones, resulting in lots of people getting killed — and thereby warranting inclusion in the Mother Jones study.

If what we care about is saving the lives of innocent human beings by reducing the number of mass public shootings and the deaths they cause, only one policy has ever been shown to work: concealed-carry laws. On the other hand, if what we care about is self-indulgent grandstanding, and to hell with dozens of innocent children being murdered in cold blood, try the other policies.

Ann Coulter is an author and political commentator.