Want to prevent the next mass shooting?

Give everyone a gun, allow them to carry it everywhere, and hope they fend off the killer.

That’s the idea from prominent gun-rights activists and politicians who insist that deadly mass shootings — including those at the Pulse nightclub in Orlando, Sandy Hook Elementary School in Connecticut, and Emanuel AME Church in Charleston, South Carolina — could have been prevented if only the victims had been allowed to bring firearms inside. The argument goes something like this: Places where guns are banned don’t just leave people defenseless, they attract shooters intent on killing as many people as possible. Restricting gun access isn’t just bad policy, it’s a danger to every American. (On the campaign trail, President Donald Trump went so far as to call gun-free zones “candy for bad people” and promised to do away with them “on my first day.”) The only way we can prevent attacks is by arming ordinary citizens in all the places ordinary citizens go, the theory holds.

It is, of course, possible that an armed citizen could fend off an attack in a gun-free zone. But the evidence shows that even trained law enforcement officers are vulnerable to an armed attacker who has the element of surprise. In 2016, five Dallas police officers were shot and killed by a sniper. The Pulse shooter evaded a uniformed, off-duty Orlando police officer as he entered the nightclub, and went on to kill 49 people.

Nevertheless, in the past decade, states have begun allowing firearms in more places where they were previously banned, including bars, universities, and government buildings. Last year, at least six states passed laws that chipped away at gun-free zones, making it easier to bring guns into schools, churches, and workplaces.

The movement is growing.

A Republican Congressman from Kentucky has introduced legislation that would roll back a federal ban on guns in schools. In Arizona, lawmakers are debating a bill that seeks to hold businesses liable if they post a no-guns-allowed sign, and someone is subsequently shot on the premises. In Wyoming, the state Senate approved a measure in February that would lift restrictions on carrying guns to government meetings.

The political battle isn’t just about safety, it’s also about opposing ideologies: pro-gun activists hope to normalize guns just about everywhere, while opponents are fighting to preserve existing boundaries. Here’s a quick primer to help you understand the facts about gun-free zones.

What is a gun-free zone?

There’s no legal definition of a gun-free zone. The term is often used by both sides in the gun debate to describe places where the average person cannot legally carry a firearm. Schools are typically gun-free zones, owing to a federal law which prohibits firearms in all K-12 schools: public, private, and parochial. But some schools make exceptions, for armed security guards or for hunting instruction, among other examples. At least nine states have extended the exception to teachers who have concealed-carry permits. In reality, few public spaces are truly gun-free, even gun-free zones. (For the purposes of this explainer, we will use the blanket term, understanding that there are often exceptions.)

Gun-free zones can also include courthouses, jails, airports, and sports arenas. Last year, the military eased restrictions on carrying private guns on bases, allowing some service members to holster their own weapons, despite opposition from the Army’s highest-ranking official.

Who decides where guns are allowed?

Gun laws are different in every state. In 2008, the U.S. Supreme Court ruled for the first time that the Second Amendment should be interpreted to permit private citizens to own guns and keep them in their homes. The court left it to states to decide who can carry firearms and where. Florida, for example, forbids guns in airport terminals — but dozens of other states, including Missouri and Oregon, permit them. And even within states, the rules get murky. In Texas, different cities had different interpretations of the state’s open-carry law, including whether firearms are permitted inside local zoos.

What about private businesses, like Starbucks or Target? Can they declare themselves gun-free zones?

In most cases, a business can decide if it wants to allow guns on its property. However, many companies are reluctant to officially ban guns, for fear of potentially alienating customers, and out of concern that doing so might make employees responsible for confronting armed customers. Some businesses, including Chipotle, Levi Strauss, Starbucks, Target, and Trader Joe’s, ask that people don’t bring firearms inside their locations, but stop short of explicitly prohibiting them. Walt Disney World bans weapons of all kinds, including toy guns. So do Costco, Ikea, California Pizza Kitchen, Whole Foods, AMC Theaters, and Waffle House. Others, like Kroger, have refused to take a side.

Many states have rolled back restrictions on where guns can be carried, forcing businesses large and small to set their own policies — meaning if they want to disallow guns, they must put up a notice. In Texas, any business wishing to ban firearms has to display two large “no guns allowed” signs, with strict rules about font size.

There’s also been a movement to force businesses to allow guns on their property. At least 23 states have laws making it illegal for employers to ban guns at workplace parking lots, according to the Law Center to Prevent Gun Violence.

Why would someone want to be armed while grocery shopping?

Fear of other people is now the number one reason why people buy guns, according to a survey from researchers at Harvard and Northeastern universities. A 2015 Gallup poll found that 56 percent of Americans believe the country would be safer if more people were allowed to carry concealed weapons. And gun rights organizations have not only stoked this fear, they’ve promoted the idea that everyday Americans with guns can stop attacks.

The only thing that stops a bad guy with a gun is a good guy with a gun,” said Wayne LaPierre, the executive vice president and CEO of the National Rifle Association, in the wake of the Sandy Hook Elementary School shooting.

For a contingent of people, who believe Americans have the right to own guns and bring them everywhere, gun-free zones aren’t just dangerous: they’re a violation of our liberty. About 5 percent of gun owners cite the Second Amendment as the reason they own firearms, according to a 2013 Gallup poll. (The same poll found that 60 percent cite safety and protection.)

For some of these people, “every gun control law is unconstitutional,” to quote a headline from one website about living off the grid. Another website, Ammoland, which bills itself as a news service for firearms enthusiasts, went as far as to publish a piece stating that the right to keep and bear arms is “God-given,” adding: “We don’t have the right to keep and bear arms because the Bill of Rights says so; rather, the Bill of Rights says so because the right to keep and bear arms is intrinsic to our very being: it is a right with which we were endowed by our Creator.”

What evidence is there that gun-free zones attract mass shooters?

The evidence is thin. It’s a belief popularized by John Lott Jr., an economist and gun rights activist who claims that, since 1950, almost every single mass public shooting in the United States has occurred in a place where guns were banned. Lott has published a large body of work claiming that crime goes down as states loosen gun laws and allow more people to carry concealed handguns.

But critics say Lott’s work is flawed because he cherry-picks data. Several experts and academics have tested his hypotheses and reached vastly different conclusions.

A committee of experts at the National Research Council, a non-governmental research arm of the National Academies of Science, Engineering and Medicine, found that there was not enough evidence to support Lott’s claim that more guns mean less crime. A team from Stanford and Johns Hopkins reached an opposite conclusion to Lott’s: more guns mean more violent crime. Another team of researchers studied two decades of mass shootings, defined as an episode that left four or more people dead. They concluded that allowing more people to carry concealed weapons had “no effect” on the frequency of such incidents.

And narrowing the scope to what could be called “rampage shootings” — events that result in at least six deaths and attract widespread media attention — the answer still appears to be that shooters don’t seek out gun-free zones. Louis Klarevas, whose book Rampage Nation: Securing America from Mass Shootings, examined 111 shooting attacks between 1966 and 2015 that left six or more victims dead in each instance. He divided the shootings into three categories based on where they took place: gun-free zones (no guns on the premises); gun-restricting zones (no guns allowed, but armed security is routinely present); and gun-allowing zones (civilians permitted to carry guns).

Klarevas found that only 18 rampage attacks occurred in spaces where civilians were not allowed to have guns. He also found that the overwhelming majority, or nearly 90 percent of gun massacres, happened in places where guns were allowed.

“Without question,” he concluded, “gun-allowing zones do not deter massacres.”

What the research does show is that mass shooters target places and people they know. A Mother Jones analysis of 62 mass shootings that took place in a 30-year span found that the majority of gunmen targeted places with which they had a personal connection, and “not a single case includes evidence that the killer chose to target a place because it banned guns.”

In fact, 20 of the attacks were workplace shootings, most of which involved a disgruntled employee. Using his dataset of 111 rampage attacks, Klarevas estimates that, in 86 percent of the cases, the attacker had a relationship to his victims.

How often have good guys with guns stopped mass shootings?

Not very often. A Federal Bureau of Investigation report analyzed 160 active shootings from 2000 to 2013, in an attempt to understand those attacks in which the killer was intent on committing mass murder. It found just one example of an armed civilian — a U.S. marine — stopping the rampage.

Is there a public safety risk to more people taking up arms to prevent the next mass shooting?

Almost certainly.

One study published last year found a relationship between a country’s rate of gun ownership and the rate of public mass shootings. Adam Lankford, a professor of criminal justice at the University of Alabama examined data from 171 countries, and found that countries with the highest rates of gun ownership had the highest rates of mass shootings, “even if they are relatively peaceful or mentally healthy according to other national indicators.”

Increased gun carrying has also been linked to increased rates of violent crime. A team from Stanford and Johns Hopkins universities found that laws allowing more citizens to carry concealed weapons were associated with an 8 percent uptick in aggravated assault.

A series of Harvard studies found that more guns mean more gun homicides. Increased gun ownership is associated with more accidental shootings and domestic violence, putting police officers, women, college students, children, even the elderly and their caretakers at risk. Another potential danger is gun theft: as many as 600,000 guns are stolen every year, and many of these are used in violent crimes.