THURSDAY, July 2, 2015 (HealthDay News) -- Mass killings and school shootings in the United States may be "contagious," inspiring similar killing sprees, new research suggests.
"The hallmark of contagion is observing patterns of many events that are bunched in time, rather than occurring randomly in time," study author Sherry Towers, a research professor at Arizona State University's Simon A. Levin Mathematical, Computational and Modeling Sciences Center, said in a university news release.
Researchers from Arizona State and Northeastern Illinois University examined databases on widely publicized mass killings and school shootings in America. After applying a contagion model to the data, they found these events motivated similar tragedies. The "period of contagion" lasts an average of 13 days, according to the study, published July 2 in the journal PLOS ONE.
Up to 30 percent of these tragedies seem to arise from contagion, the researchers found.
"In January of 2014, I was due to have a meeting with a group of researchers at Purdue University," Towers said. "That morning, there was a tragic campus shooting and stabbing incident that left one student dead. I realized that there had been three other school shootings in the news in the week prior, and I wondered if it was just a statistical fluke, or if somehow through news media those events were sometimes planting unconscious [ideas] in vulnerable people for a short time after each event."
Previous studies have shown that suicide among young people can also be contagious. One suicide can prompt other vulnerable young people to also take their own life, the researchers pointed out.
"It occurred to us that mass killings and school shootings that attract attention in the national news media can potentially do the same thing, but at a larger scale," Towers said. "While we can never determine which particular shootings were inspired by unconscious [ideas], this analysis helps us understand aspects of the complex dynamics that can underlie these events."
The researchers said mass killings involving firearms occur in the United States about every two weeks, on average. School shootings occur an average of once a month. These tragedies happen far more often in states where many people own guns.
The U.S. Centers for Disease Control and Prevention provides data and statistics on school violence.