This column by ACRU Policy Board Member Ken Blackwell was published October 7, 2015 by The Washington Times.

Would you put a sign outside your house saying, “Doors unlocked,” or “We’re not home”?

An underreported study released a year ago by the Crime Prevention Research Center (CPRC), the organization led by America’s leading gun policy expert John Lott, found that 92 percent of mass shootings since 2009 have taken place in designated gun-free zones.

Or, as CPRC puts it, “Since 2009, only 8 percent of mass public shootings have occurred where civilians are allowed to defend themselves.”

“Gun-free” zones are purported to forestall gun violence, but in practice they attract deranged gunmen like porch lights attract moths. Why? Because deranged gunmen are crazy, not stupid. An identified gun-free zone literally advertises the ideal environment for committing mass murder.

Mass shootings are committed by mentally ill people to achieve a form of immortality, or by political zealots to terrorize the public. They know the more they kill, the more publicity they’ll get. They also know the only way they will be stopped is by a good guy with a gun, so they choose to attack in places where they know nobody else will be armed.

Most mass shootings don’t end until the police arrive. Killers typically have several minutes to slaughter as many victims as they can without fear of interference.

At least one mass murderer explained in his “manifesto” that he took armed resistance into account when choosing his target. According to CPRC, California killer Eliot Rodger, who murdered six University of California-Santa Barbara students in 2014, wrote, “I figured this would be the perfect day to attack Isla Vista, but after watching YouTube videos of previous Deltopia parties, I saw that there were way too many cops walking around on such an event. It would be impossible to kill enough of my enemies before being dispatched by those damnable cops.”

Then there’s Canadian killer Justin Bourque, who murdered three Royal Canadian Mounted Police officers in his attempt to start a revolution in New Brunswick in 2014. According to CPRC, “On his Facebook page, Bourque posted comics poking fun at how gun-free zones make these crimes possible.”

CPRC also notes that the 2012 Aurora, Colo., “Batman” gunman, James Holmes, drove past seven more-nearby theaters playing the movie to launch his attack at the one theater identified as a “gun-free zone.”

Holmes was able to murder 12 people and wound another 70 before he walked out of the theater and was arrested in the parking lot.

In contrast, Colorado Springs New Life Church gunman Matthew Murray was shot by an armed volunteer security guard after killing just two people in a crowded church service in 2007.

Many mass shootings have been ended quickly, with reduced loss of life, by people with concealed carry permits who acted before police could reach the scene. CPRC points to incidents at malls in Portland, Ore., and Salt Lake City, the Appalachian Law School in Virginia, and shootings in Pearl, Miss., and Edinboro, Pa.

The best thing school administrators can do to protect their charges is to take those gun-free zone signs down, and start allowing faculty members and staff, vigorously trained in gun safety, to carry concealed weapons. The possibility of confronting an armed gym teacher or secretary — who knows who might be carrying? — would dissuade many a would-be mass murderer.

It’s time to stop the slogans and the feel-good symbolism. We are blessed to live in a free society, but we’ll remain one only as long as the good guys have guns.