The study, titled "Contagion in Mass Killings and School Shootings", was carried out by researchers from Arizona State University, who looked into databases related to high-profile mass killings and school shootings between 1998 and 2013.
"The hallmark of contagion is observing patterns of many events that are bunched in time, rather than occurring randomly in time," Sherry Towers, a research professor at Arizona State University and lead author of the study, said.
Upon applying a mathematical "contagion model" to the data, scientists found out that mass killings and school shootings create "a period of contagion that lasts an average of 13 days," and that "roughly 20 to 30 percent of such tragedies appear to arise from contagion."
On average, killing sprees take place once every two weeks in the United States, while school shootings occur about once a month, according to the study.
Researchers believe that national media coverage of a mass shooting may play a role in the spread.
"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 on a larger scale," Towers said.
According to the armed violence prevention group GunPolicy, the United States has the world's highest gun ownership rate, and also has the highest rate of firearm-related murders per capita. According to statistics, the rate of private gun ownership is 101.05 firearms per 100 US citizens.
