How Often Do Mass Killings Inspire Similar Acts?

Sandy Hook
New research suggests that 30 percent of mass killings and 22 percent of school shootings were inspired by earlier similar events. Carlo Allegri/REUTERS

Investigators who searched the home of Adam Lanza following his attack on Sandy Hook Elementary School made a dark discovery—the troubled teen who killed himself, his mother and 26 elementary school students in 2012 had been obsessed with the Columbine High School massacre. An official report said investigators uncovered hundreds of documents, images and videos that Lanza had collected about the event. Though Lanza's motive to kill may never become clear, the Columbine connection suggests that perhaps the idea to commit a mass killing and school shooting does not arise on its own.

Researchers at Arizona State University and Northeastern Illinois University published findings in the journal PLOS ONE on Thursday that they say is the first study to quantify how mass killings and school shootings inspire similar acts. They applied a mathematical contagion model, typically used to predict earthquake aftershocks or the spread of disease, to data on shootings and killings.

Sherry Towers, a research professor at Arizona State University and lead author on the study, says she became interested in the subject last year, when she was scheduled to attend a meeting at Purdue University the same day a student there killed another student. "It occurred to me that day that there had been about three other school shootings the week prior and it seemed excessive," she says. "It made me wonder if these things were clustering in some way."

The researchers used data on mass killings between 2006 and 2013 from USA Today and school shootings from 1998 and 2013 from the Brady Campaign to Prevent Gun Violence.

Towers and her team found that 30 percent of mass killings appeared to have been inspired by a past mass killing. And 22 percent of school shootings appeared to have been inspired by other school shootings. The period of contagion was on average 13 days. That is, on average, the later events took place within 13 days of the earlier events. That unlucky 13 was the figure for both the separate mass killings and school shootings formulas was just a coincidence, Towers says.

Towers acknowledges that her team did not look at the reasons for contagion. Media coverage could be one possible factor, she says. As Psychology Today has noted, for decades, social scientists have explored the issue of "copycat" crimes and suicides. The PT article points out that in the six weeks following a junior high school shooting in 1996, for example, three similar shootings happened. Since then, reports show, the frequency of such tragedies has only increased.

The more information the media provides on a suicide or a significant crime, the more likely someone else will try to emulate it. Groups such as the World Health Organization and the American Foundation for Suicide Prevention issue recommendations for journalists to help prevent copycat acts.

Still, after those 13 days, Towers says, as was the case with Lanza, an event remains contagious. "It doesn't go away ever really entirely, it just gets less and less and less the further you get away from an event," she says.