Are Mass Shootings Contagious?

A study finds that deadly attacks inspire copycat crimes for an average of 13 days after they occur.

A makeshift memorial at the San Bernardino shooting site. (Patrick Fallon / Reuters)

Eight days after the San Bernardino shooting came the scare at Arkansas State University: A 47-year-old man armed with a 12-gauge shotgun threw the campus into temporary lockdown.

The two incidents don’t seem to be connected: One was the deadly work of terrorists who’d been radicalized years before; the other ended with no casualties, the product of a suicidal man who was threatening only self-harm, police say.

But in the tense moments following initial reports from Arkansas, the timing seemed suspect for a country still tracking the news out of California, waiting for another shoe to drop. Were the two related? And if not, why did it seem so likely that they could be?

A recent study out of Arizona State University examined the notion that some shooting incidents have a contagious effect, resulting in clusters of shootings. High-profile assailants like Newtown’s Adam Lanza, who shot and killed 26 people three years ago this week, are known to have been inspired by past killers. But the ASU team was the first to quantify the effect, finding that school shootings and mass killings—where four or more people are killed—are contagious for an average of 13 days after they occur.

“When you look at these collective events over a long period of time,” said lead author Sherry Towers, “what you see as a hallmark of contagion is these things bunching together unusually in time.”

But Towers is quick to acknowledge that it’s impossible to know whether and which shootings have inspired others unless the shooters themselves tell us. Some shooters “might not be self-aware enough to know” what’s driven them to commit violence, said Towers, whose study examined incidents that got significant media attention. “They’re being subconsciously ideated by something they’ve just watched on the news.”

The contagious effect can last up to six weeks, her team found. Their data was based on media reports, and treated all incidents the same in terms of their impact.

The 13-day average “makes sense” when you consider the length of news coverage after a mass killing or school shooting, Towers said last week, noting how she was still seeing reports out of San Bernardino on her newsfeed. If you hypothesize that media exposure is what’s causing the ideation, the exposure dies off when media reports die off, and “that’s when the contagion starts to die off,” she said.

Towers’ study is based on the idea that contagious ideas can be modeled like contagious diseases. Just like a person’s political ideology can be “infected” by that of their family, a would-be shooter can be affected by the actions of others.

“You can be infected with these ideas about how you live your life and the choices you make based on exposure to other people’s ideas,” she said.

What’s still unclear—and practically unknowable—is how often shootings inspire unsuccessful imitators, who fail to kill many people or are thwarted before they can launch their attacks. Susan B. Sorenson, a firearms researcher at the University of Pennsylvania, said researchers don’t have access to data in “timely enough fashion” to be able to evaluate that.

But she does sense a shift in how people’s lives are touched by gun violence. Part of what’s frightened Americans lately in the wake of shooting incidents—perhaps the part that makes them search for connections—is the “broadening out” of where recent shootings have happened. These days, shootings are “covering so many spheres of people’s lives,” Sorenson said.

These incidents “used to be much more tightly focused on a disgruntled or mistreated worker, and then it was mistreated high-school students.” Now, “people who might be in those settings who are disgruntled or mistreated are … going places beyond the locale that caused the distress for them,” Sorenson said. “So they’re going to movie theaters, they’re going to elementary schools, they’re going to colleges and universities.”

And people can’t simply avoid those public spaces to decrease the threat of violence, like they’d avoid a dangerous neighborhood.

Towers said different shootings could have different impacts, too. Just as the slaying of 20 children at Sandy Hook Elementary continues to shock and sadden the American consciousness, it’s possible that particularly horrifying shootings could have a longer half-life in the imaginations of potential imitators.

“There may very well be a longer period of contagion” for those killings, Towers said.

This article is part of our With Great Power project, which is supported by a grant from the Joyce Foundation.​