Following the Marjory Stoneman Douglas High School shooting in Florida that killed 17, President Donald Trump and other lawmakers have landed on an old idea for preventing school shootings: putting more armed personnel — including teachers with guns — in schools.

This is essentially an extension of the “good guy with a gun” idea. As National Rifle Association CEO Wayne LaPierre put it following the mass shooting at Sandy Hook Elementary School in Newtown, Connecticut, in 2012, “The only way to stop a bad guy with a gun is with a good guy with a gun.” Basically, if more people are armed, they can stop violence before it gets worse or prevent it altogether.

There is no good research on the effect of arming teachers or the effect of putting more armed police or security in schools — which by itself should raise red flags, given that policy should be evidence-based. But based on the evidence we do have, there’s enough to suggest that putting more guns in schools could actually make gun violence worse.

The fundamental problem in the US is there are so many guns already in circulation. This makes it easier for any conflict to quickly escalate into a form of gun violence — and, as a result, the US has more shootings than its developed peers. So if more guns are added into circulation, it would very likely lead to more gun violence.

That’s not to say that no one has ever successfully defended themselves or others from an attack with a firearm. On Tuesday, for one, a school resource officer reportedly stopped a shooter at Great Mills High School in Maryland.

The question is whether these incidents of successful defense would outweigh the new incidents of gun violence that would crop up due to the addition of more firearms in schools. Based on the research, the presence of more guns typically translates to much more general gun violence, while justified uses of a gun for self-defense are few and far between.

That suggests that arming more people in schools would do more harm than good — making the latest proposal put forward by Trump and other lawmakers potentially very dangerous.

1) The research is clear: more guns, more gun deaths

The US is unique in two key — and related — ways when it comes to guns: It has way more gun deaths than other developed nations, and it has far more guns than any other country in the world.

The US has nearly six times the gun homicide rate of Canada, more than seven times that of Sweden, and nearly 16 times that of Germany, according to United Nations data compiled by the Guardian. (These gun deaths are a big reason America has a much higher overall homicide rate, which includes non-gun deaths, than other developed nations.)

Mass shootings actually make up a small fraction of America’s gun deaths, constituting less than 2 percent of such deaths in 2013. But America does see a lot of these horrific events: According to CNN, “The US makes up less than 5% of the world’s population, but holds 31% of global mass shooters.”

The US also has by far the highest number of privately owned guns in the world. Estimated in 2007, the number of civilian-owned firearms in the US was 88.8 guns per 100 people, meaning there was almost one privately owned gun per American and more than one per American adult. The world’s second-ranked country was Yemen, a quasi-failed state torn by civil war, where there were 54.8 guns per 100 people.

Another way of looking at that: Americans make up less than 5 percent of the world’s population yet own roughly 42 percent of all the world’s privately held firearms.

These two facts — on gun deaths and firearm ownership — are related. The research, compiled by the Harvard School of Public Health’s Injury Control Research Center, is pretty clear: After controlling for variables such as socioeconomic factors and other crime, places with more guns have more gun deaths.

“Within the United States, a wide array of empirical evidence indicates that more guns in a community leads to more homicide,” David Hemenway, the Injury Control Research Center’s director, wrote in Private Guns, Public Health.

For example, a 2013 study, led by a Boston University School of Public Health researcher, found that, after controlling for multiple variables, each percentage point increase in gun ownership correlated with a roughly 0.9 percent rise in the firearm homicide rate.

The correlation applies globally. This chart, based on data compiled by researcher Josh Tewksbury, shows the correlation between the number of guns and gun deaths among wealthier nations:

Guns are not the only contributor to violence. (Other factors include, for example, poverty, urbanization, and alcohol consumption.) But when researchers control for other confounding variables, they have found time and time again that America’s high levels of gun ownership are a major reason the US is so much worse in terms of gun violence than its developed peers.

As a breakthrough analysis by UC Berkeley’s Franklin Zimring and Gordon Hawkins in the 1990s found, it’s not even that the US has more crime than other developed countries. This chart, based on data from Jeffrey Swanson at Duke University, shows that the US is not an outlier when it comes to overall crime:

Instead, the US appears to have more lethal violence — and that’s driven in large part by the prevalence of guns.

”A series of specific comparisons of the death rates from property crime and assault in New York City and London show how enormous differences in death risk can be explained even while general patterns are similar,” Zimring and Hawkins wrote. “A preference for crimes of personal force and the willingness and ability to use guns in robbery make similar levels of property crime 54 times as deadly in New York City as in London.”

This is in many ways intuitive: People of every country get into arguments and fights with friends, family, and peers. But in the US, it’s much more likely that someone will get angry during an argument and be able to pull out a gun and kill someone.

Think about how this could apply to a school scenario: Some kids or teachers get into an argument. There’s a gun in the class. Someone reaches for that gun — and what may have otherwise been a feisty argument escalates into a fatal encounter.

This might seem ridiculous, but consider that there have been shootings over disputes about cheeseburgers and tacos. In the heat of the moment, people can do very stupid things.

America does not have a monopoly on these kinds of disputes. What it does have, again, is easy access to guns, making escalation much more likely.

Increasing the presence of guns in schools, then, could actually exacerbate gun violence.

2) For every criminal killed in self-defense, there are dozens more murders

There’s another set of statistics that throws cold water on the “good guy with a gun” theory: It’s way more likely in America that someone will shoot and kill another person in the course of committing a crime than will do so in self-defense.

Christopher Ingraham at the Washington Post ran through the statistics. He looked at how many gun homicides, suicides, and accidental shootings there were in comparison to “justifiable” homicides (“the killing of a felon, during the commission of a felony, by a private citizen”), based on the FBI’s 2012 data.

His findings: For every justifiable gun homicide, there were 34 criminal gun homicides, 78 gun suicides, and two accidental gun deaths.

Data on mass shootings tells a similar story: According to the FBI’s report on active shooter events between 2000 and 2013, only about 3 percent were stopped by a civilian with a gun. Unarmed civilians actually stopped more incidents — about 13 percent. Most of the incidents — more than 56 percent — ended on the shooter’s initiative, when the shooter either killed himself or herself, simply stopped shooting, or fled the scene.

Would more of these shootings be prevented if more people had guns? It’s hard to say — since, again, there’s no good research on that question.

But America already has a lot of guns. And as the other data shows, that’s likely making its overall gun violence problem worse, not better.

3) Stopping a mass shooting is hard, even with firearms training

In Trump’s comments about arming teachers, he’s suggested that this would be an easy way to end mass shootings quickly. He previously tweeted, “History shows that a school shooting lasts, on average, 3 minutes. It takes police & first responders approximately 5 to 8 minutes to get to site of crime. Highly trained, gun adept, teachers/coaches would solve the problem instantly, before police arrive. GREAT DETERRENT!”

Reality, however, is more complicated: Even when people are armed, that doesn’t mean they can properly respond to a mass shooting.

Multiple simulations have demonstrated that most people, if placed in an active shooter situation while armed, will not be able to stop the situation, and may in fact do little more than get themselves killed in the process.

This video from ABC News shows one such simulation, in which people repeatedly fail to shoot an active shooter before they’re shot:

As Chris Benton, a police investigator in Pennsylvania, told ABC News, “Video games and movies, they glorify gunfights. [People] get that warped sense that this is true — this video game is exactly what I can do in real life. That’s not reality.”

The Daily Show also put this theory to the test in another, more comedic, simulation segment. Jordan Klepper, who was a correspondent with the show at the time, trained on the basics of using a firearm and got a concealed carry permit that was valid in 30 states. Then he participated in mass shooting simulations to see how he would hold up in such a scenario.

He failed — miserably. In his final test, which simulated a school shooting, he shot an unarmed civilian, and he was shot multiple times by the active shooters and even law enforcement, who mistook him for the bad guy. He never took down the active shooters.

The fundamental problem is that mass shootings are traumatizing, terrifying events. Without potentially dozens or even hundreds of hours in training, most people are not going to be able to quickly and properly respond.

“There’s never enough training,” Coby Briehn, a senior instructor at Advanced Law Enforcement Rapid Response Training, told Klepper. “You can never get enough.”

The FBI’s analysis of active shooters between 2000 and 2013 has another relevant data point: “Law enforcement suffered casualties in 21 (46.7%) of the 45 incidents where they engaged the shooter to end the threat.” These are people trained to do this kind of thing full time, and nearly half of incidents resulted in at least one officer being wounded or killed. Teachers with limited training would very likely fare much worse.

That’s if armed personnel even respond. According to the local sheriff, the armed school resource officer for Marjory Stoneman Douglas High School heard gunfire during the shooting but didn’t go in. Even with proper training and a weapon, the officer stalled — and allowed the shooting to continue for about four to six minutes as he stood outside the building. (The officer, deputy Scot Peterson, has since quit.)

None of that is to say that a “good guy with a gun” wouldn’t ever be able to stop a shooter. We have seen some high-profile cases in which that happened. But the bulk of the findings, from news investigations to the FBI’s report to The Daily Show, suggest that this idea is often going to play out very differently than supporters like Trump envision — and sometimes could lead to more innocent people getting caught in the crossfire.

If America wants to confront its gun violence problem, then, the research suggests it should look to reducing the number of guns in circulation — not putting more armed people into schools.