In much of America, the response to school shootings has been to put more guns in schools. In line with the National Rifle Association’s position that “the only thing that stops a bad guy with a gun is a good guy with a gun,” an increasing number of police — usually known as school resource officers — have been assigned to guard students. Florida is considering legislation to allow teachers and administrators to carry guns on the job, bringing to nine the number of states that allow school staffers to go to work armed. One Florida town hired veterans to keep watch with assault rifles.

The presence of guns in schools is a fact of life for millions of American children. Forty-three percent of public schools had an armed law enforcement officer during the 2015-2016 school year, according to a survey released last year by the National Center for Education Statistics. That doesn’t include the unknown number of schools that employ armed private security guards or arm teachers or administrators.

As part of our Ask the Trace series, a reader wondered how effective guns are in keeping kids safe at school.

In moments of extreme crisis like when an active shooter attacks, it’s true that an armed guard could potentially end the terror. But the presence of security hasn’t definitively deterred attacks in the first place.

“We don’t have any rigorous causal evidence that says armed guards reduce school shootings or school violence,” said Matthew Mayer, a professor at Rutgers University who studies violence in educational settings.

And in any case, active shooter incidents make up a very small part of overall gun violence: Despite the prominent media coverage of mass shootings, and school shootings in particular, only about 1 percent of all firearm fatalities each year occur during such incidents. Last year there were 24 shootings causing injury or death in K-12 schools throughout the entire country, whereas there are more than 100,000 K-12 schools. Meanwhile, the choice to put people with guns in schools comes with costs that are mostly borne by students.

Guns have stopped some mass shootings — but not usually in schools

There are a handful of documented cases in which an armed security guard or police officers have stopped a school shooting. Last year in Dixon, Illinois, a school resource officer chased a gunman off a high school campus, shooting and injuring the perpetrator, who survived. But it’s more likely for active shooters to stop firing on their own, either by leaving or taking their own lives.

The criminologists at Texas State University’s ALERRT program have studied responses to active shooter incidents more generally. According to ALERRT’s 2013 analysis of 160 incidents, lethal force from police or bystanders has brought an end to a little less than a third of active shooter rampages, but not usually in schools.

In the 25 shootings in the ALERRT study that targeted schools, none were brought to an end by armed staff, guards or police officers returning fire. These shootings most commonly ended when the shooters were restrained by unarmed staff.

In the broader group of gun attacks, ALERRT found that 52 ended after an exchange of gunfire with the perpetrator. Armed police responded in 45 of those incidents, while in seven cases civilians, security guards, or off-duty cops returned fire.

But advocates of guns in schools should not assume armed security would stop such attacks, said Peter Blair, the author of the ALERRT study. “It’s within the realm of possibility to think an armed guard could stop an attacker,” he said. “But they need to be trained to do that, including considerations of when to get involved or not, how to deal with crowded rooms or buildings.”

Moreover, the majority of the shootings Blair analyzed ended on the perpetrator’s initiative. That’s not surprising, since mass shooting events are rare, difficult to anticipate, and usually end within minutes. It’s unusual for armed bystanders or police to be in the right place at the right time.

Armed guards don’t deter gunmen

It’s an axiom of the pro-gun movement that mass shooters target “gun-free zones.” According to that logic, the presence of armed guards should deter shooters. President Donald Trump embraced that reasoning in a post-Parkland statement on school safety.

But the record usually doesn’t back up the deterrence theory — gunmen have often targeted schools with armed guards. In four high-profile 2018 school shootings — Kentucky’s Marshall County High School in January; Florida’s Marjory Stoneman Douglas High School in February; Maryland’s Great Mills High School in March, and Santa Fe High School in Texas in May — attackers stormed campuses despite the presence of armed guards. In all four of those cases, guards failed to stop the gunman from killing.

More generally, active shooters do not favor “gun-free zones.” Louis Klarevas, a professor at the University of Massachusetts, analyzed 111 shooting attacks between 1966 and 2015 for his book Rampage Nation, and found that only 18 took place in areas where firearms were banned.

Armed guards can have unintended consequences for schools

The current vogue for safeguarding schools with deadly force is a response to atrocities like Parkland. But school shootings remain relatively rare. Most of the guards hired as part of the recent trend will never encounter an active shooter. But they will carry guns around students every day on the job. And experts question whether this makes schools safer.

Instead, the push for armed security in schools is “perception based,” according to Mayer, the Rutgers professor. “The parents and the administrators feel good about having police officers in schools, and they perceive improved school safety.” However, the students themselves don’t necessarily feel comforted by police or other armed security. Instead, kids may just feel policed.

One 2013 paper published in the journal Justice Quarterly found that as schools increasingly rely on police for security, administrators refer more students to law enforcement for nonviolent infractions. Police in schools may also end up using force against students. An investigation by HuffPost found that, from January 2016 to September 2018, police in schools had pepper sprayed students at least 32 times and body slammed, tackled, or choked students at least 15 times.

A new report from the gun violence prevention group Giffords also found that police have mishandled guns or allowed students to get their hands on weapons more than 60 times. School security officers have left guns in bathrooms. One third-grader managed to pull the trigger of a gun still in an officer’s holster.

The National Association of School Psychologists has argued that armed guards increase student fear, rather than making them feel safer. This could harm the learning process, the NASP suggests.

“It creates a mindset of fear,” said Mayer, about the increasing reliance on armed security guards in schools.

Correction: an earlier version of this story erroneously stated that Florida had passed a law allowing teachers and administrators to carry guns in schools.