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.​