The media’s role as a vector of infection is becoming harder to ignore.

I have read of this hypothesis before and it echoes a similar one regarding suicide. From Mass Shootings May Be Contagious. Can We Contain Them? by Meredith Fore.

In the span of one week, three US cities suffered mass shootings: Gilroy, California; El Paso, Texas; and Dayton, Ohio. Occurring in such rapid succession, the incidents rocked the country. But it was far from the first time that a rash of mass shootings seemed to strike, a data trail that’s led some researchers to argue there’s something contagious about them.

"What you notice in a contagion model is that the events will cluster together, unusually closely in time, more so than you would expect from a model that just assumes that they kind of happen randomly," says Sherry Towers, a mathematician at Arizona State University.

Towers studies the spread of diseases as well as behavior or sentiment, such as the culture of fear that arose around Ebola in the US in 2014. And in 2015, she and her coauthors published one of the first papers demonstrating that mass shootings also acted like a contagion.

Because there is no federal database on mass shootings, Towers and her collaborators relied on databases from private groups, specifically USA Today and the Brady Campaign. They divided the data into three sets: mass killings where four or more people were killed (176 out of 232 incidents involved firearms), school shootings, and mass shootings where three or more people were shot but fewer than four people were killed (to avoid overlap with the first set). The researchers then compared this data, which included events from 1998 to 2013, to a mathematical model of a contagion.

For school shootings and mass killings, the contagion model explained the data better than simply assuming the events were random. The third set of data, events where fewer than four people were killed, showed no significant evidence of being contagious. There was evidence of contagion, however, among school shootings, regardless of death count.

Towers suggests that the three groups receive different amounts of media coverage, which could explain the discrepancy. Mass killings and school shootings tend to generate extensive coverage, while smaller-scale tragedies don’t always get as much attention. (Though it’s not an iron-clad distinction; the Gilroy shooting, which ended with three people dead plus the shooter, garnered significant media attention.)

"Even low-casualty-count school shootings can get national media, because I think it speaks to parents' fears about their children going off to school," Towers says. It was the lack of contagion among shooting events only reported in local news outlets that got her thinking. "That's what led us to hypothesize that media may be playing a role."

Economists Michael Jetter and Jay Walker, of the University of Western Australia and Old Dominion University, respectively, reached the same conclusion in a working paper published last year. Using statistics, they found that the amount of news coverage on mass shootings could predict the number of shootings in the week following.

They showed, among other things, that media coverage of mass shootings decreased when they overlapped with natural disasters, and that it was subsequently less likely for shootings to occur in the following week, even when considering different definitions of "mass shooting" (in terms of deaths or people shot). Jetter had previously studied the relationship between terrorists and news coverage of terrorism, and found the correlation to be similar.

Fore concludes.

The data on mass shootings is small and lacking compared to that on suicides, so news outlets have less research to lean on when weighing how to report on mass shootings. But with the evidence on contagiousness stacking up, the media’s role as a vector of infection is becoming harder to ignore.