Less than a month after America’s deadliest school shooting in five years, President Donald Trump on Thursday met with members of Congress, video game executives, conservative media watchdog Brent Bozell and a mother from the Parents Television Council to “discuss violent video-game exposure and the correlation to aggression and desensitization in children,” according to a statement from White House Deputy Press Secretary Lindsay Walters.

The meeting was closed to the press, and the White House has not released any statement, but Trump has blamed video games in recent weeks for making kids more violent, as have some of the guests he invited to the meeting.

“I’m hearing more and more people say the level of violence on video games is really shaping young people’s thoughts,” he told Florida’s attorney general after the Parkland school shooting — a view he expressed even more strongly in a 2012 tweet.

Similar rhetoric has been part of the conversation about mass violence for years. In 2007, for example, an Oregon psychiatrist published a study saying the perpetrators of the 1999 Columbine High School shooting may have been compelled to kill after they were abruptly forbidden from playing the computer games to which they had become addicted. Since then, video games have consistently been blamed for, or at least implicated in, school shootings and other acts of mass violence.

But blaming violence on video games over-simplifies a deeply complicated issue.

“The short answer is there is virtually no research” on whether or not video games cause violent acts like school shootings, says Mark Appelbaum, an emeritus professor of psychology at the University of California San Diego and the chair of a 2015 American Psychological Association Task Force on Violent Media. “There is no scientific evidence that confirms or disconfirms that speculation.”

That’s because ethical standards prevent researchers from carrying out the kind of experiments that could, in theory, show cause-and-effect relationships between video games and violence, explains Brad Bushman, a professor of communication and psychology at the Ohio State University and one of the authors of a 2017 Pediatrics paper that analyzed 60 years’ worth of research around screen violence and aggressive behavior.

“You can’t randomly assign people to play a violent or non-violent game in the lab, give them a gun and see what they do with the gun,” Bushman says. As a result, “you can’t say it’s the cause, or the most important cause,” of criminal actions.

What researchers have done is look for links between violent media, like video games, and traits connected to violent behavior. Here, they’ve found plenty. Decades of research has found that playing violent video games can cause an increase in aggressive thoughts and actions, a decrease in empathy and desensitization to violence. Some research, like that from a widely cited 2010 meta-analysis published in Psychological Bulletin, has also suggested there may be a link between violent video games and heightened physiological arousal, which may also be a precursor to aggressive behavior.

Part of the explanation for those links, Bushman says, may be the visceral experience of playing modern video games, especially those in which the user takes on the perspective of the shooter.

“Think about if you wanted to fly an airplane. What would be the best thing to do: read a book about it, watch a TV program about it or play a flight simulator video game?” Bushman says. “We know that when people are actively involved, they learn much better than when they’re just passive. The best way to learn something is to actually do it. And in the video game, what you learn how to do is kill others.” Video games also often reward the player for doing so, through points, game advancement or verbal affirmations.

“There are fairly consistent effects that show an increase in aggressive thoughts and ideations, as a function of having played these first-person shooter games,” Appelbaum says. “There is also a decrease in empathy.” But what we don’t know is whether those changes are permanent, cumulative or different for people who play more or less frequently, he says.

There’s also a big leap between having aggressive thoughts and committing criminally violent acts. Many things can qualify as aggressive thoughts for the purposes of psychological studies, Appelbaum says, including something as simple as writing an “I” instead of an “A” when presented with a blank between the letters “H” and “T” — thus forming the word “hit” instead of “hat.”

Researchers also don’t know much about how video game exposure affects young children, specifically. Appelbaum says most research on the topic has been conducted among college-age people, in part because it’s practical — researchers are often professors with easy access to undergraduates — and in part because it’s ethically dubious to expose small kids to something potentially damaging.

With so many ethical hurdles in the way, researchers may never be able to say for sure whether video games cause violent acts, even as evidence supporting associations between violent media and violent behavior piles up. But that likely won’t stop lawmakers and armchair psychologists from making that claim.

“People would be really happy if we knew the cause of these actions,” Appelbaum says. The problem, of course, is there is no simple explanation for something like a mass shooting. Many factors — societal, individual and environmental — are at play in each incident.

“If I could create a parallel world, and in that parallel world no one had thought up the idea of a first-person shooter game…would I expect there to be any major difference in the number of these kind of shooting incidents?” Appelbaum asks. The answer, he says, is “no.”

But even without definitively proving cause and effect, Bushman says the amount and quality of research on the impact of violent video games is too vast to ignore.

“I can think of no theory that would say it’s harmful for children to be exposed to violence in their home, in their neighborhood or in their school,” Bushman says, “but that it doesn’t really matter if they’re exposed to violence in the media.”

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Write to Jamie Ducharme at jamie.ducharme@time.com.