Roughly a quarter of mass killings and school shootings occur because of contagion — with one event increasing the chances of another happening — even if the suspect doesn’t consciously realize it, according a study released today.
Researchers at Arizona State University and Northeastern Illinois University used USA TODAY’s mass killings data, which tracks killings of four or more people since 2006. USA TODAY’s data, unlike other sets, include many events that do not receive extensive media coverage.
The researchers also used 15 years’ of data on school shootings, plus data about public shootings where at least three people were shot but not necessarily killed, from the Brady Campaign to Prevent Gun Violence. They fit all three data sets into a contagion model to see if one event had a ripple effect.
They found that the biggest events – mass killings and school shootings, which tended to get more publicity – had an average window of 13 days of contagion.
“What we think might be happening is the very small cross section of the public that’s vulnerable, that message gets to them and causes unconscious ideation to perhaps do something similar,” says Sherry Towers, professor of statistics at Arizona State University’s Simon A. Levin Mathematical, Computational and Modeling Sciences Center.
In the case of the recent Charleston, S.C. church shooting, there haven’t been any mass killings in the two weeks since. But there have been a rash of church arsons, which investigators are probing.
“Even though there doesn’t seem to be a contagion in this certain case, there does seem to be related events that have occurred,” Tower says.
Mass shootings with fewer than four deaths created no such contagion effect, Towers said. The researchers only looked at similar events, so they did not measure whether a mass killing or school shooting lead to an increase in single shootings, for instance.
Towers said she started wondering if school shootings triggered similar events in January 2014, after she was visiting the Purdue University campus when a student entered a campus building and shot and killed another student.
“There had just been three other school shootings in the news in the past week,” she said. “I just thought, ‘Is there something that’s causing these things to bunch together?’”
Other researchers have linked increases in suicides to high-profile suicides in similar contagion studies.
Towers says she hopes their findings will serve as a starting point for other researchers to look into the contagion effect further.
“This is a huge problem in the United States,” she says. “The chances of a person being killed in a mass killing is very low, but much higher in the US than in any other industrialized country.”
The study was published in the peer-reviewed PLOS ONE. USA TODAY’s mass killings data showed they happen every two weeks on average in the U.S. , and nearly a quarter do not involve guns. Explore USA TODAY’s map of mass killings in America