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There have been three high-profile shootings across the country over the past week and a half - shootings in El Paso, Texas, and Dayton, Ohio, this past weekend and the one in Gilroy, Calif., before that. This cluster is not an anomaly. Scientists who study this say acts of mass violence are contagious, as NPR's Rhitu Chatterjee reports.
RHITU CHATTERJEE, BYLINE: Sherry Towers is a scientist at Arizona State University. She spent most of her career studying the spread of infectious diseases, like Ebola and influenza. Back in January 2014, she was visiting Purdue University to meet with some collaborators.
SHERRY TOWERS: And the meeting was canceled that day because there was a shooting on campus. A student walked into a classroom and shot and killed another student dead.
CHATTERJEE: It was the third school shooting in the country within about a 10-day period. It got Towers wondering if such shootings were sort of like disease outbreaks.
TOWERS: And it occurred to me that is some kind of perhaps contagion going on?
CHATTERJEE: So she and her colleagues gathered data on school shootings and mass shootings where four or more people had died. Then she plugged the data into a mathematical model.
TOWERS: What we found was that for the mass killings - so these are high-profile mass killings where there's at least four people killed - there was significant evidence of contagion. And we also found significant evidence of contagion in the school shootings.
CHATTERJEE: Towers says the contagion spreads through heightened media coverage of these events. News of one mass shooting tends to spark more shootings.
JILLIAN PETERSON: So one happens, and you see another few happen right after that.
CHATTERJEE: That's Jillian Peterson, a criminologist at Hamlin University in Minnesota. Peterson says the mass shooting contagion is somewhat like a suicide contagion. That's when news reports of a high-profile suicide cause more people to take their own lives. In fact, she says a majority of mass shooters actually start out feeling suicidal.
PETERSON: We can show about 80% were actively suicidal prior to the shooting.
CHATTERJEE: She has interviewed shooters who are now in prison and people who knew past perpetrators well, and she's come to think of mass shootings as a form of suicide.
PETERSON: These really are suicides. They're angry, horrible suicides that take a lot of people with them. But the shooter never intends to live. There's never a getaway plan.
CHATTERJEE: Now, the majority of people thinking about suicide don't attack others. But Peterson says in very rare cases, individuals who are also angry and already considering violence, may read or watch the news of a mass shooting and identify with the shooter and be inspired by them.
PETERSON: There is this element of wanting notoriety in death that you don't have in life. So when one happens and it makes headlines and the names and pictures are everywhere and the whole world is talking about it, that becomes something that other people see as a possibility for themselves.
CHATTERJEE: Now, it's hard to know whether the shooter in Dayton was consciously influenced by the El Paso or the Gilroy shootings. But there's evidence that the suspect in El Paso, Texas, was inspired by the shooting in New Zealand just a few months ago.
Rhitu Chatterjee, NPR News.
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