No one knows why mass murderers commit their appalling deeds. But new evidence reinforces the idea that mass shootings, publicized in the media, may have a contagious effect.

Researchers at Arizona State University analyzed news reports of gun-related incidents from 1997 to 2013. They hypothesized that the rampages did not occur randomly over time but instead were clustered in patterns. The investigators applied a mathematical model and found that shootings that resulted in at least four deaths launched a period of contagion, marked by a heightened likelihood of more bloodshed, lasting an average of 13 days. Roughly 20 to 30 percent of all such violence took place in these windows.

Previous studies have shown that suicide can be similarly contagious. In one recent example, researchers found a correlation between celebrity suicides, like that of Robin Williams, and an increase in suicidal thoughts in an online Reddit suicide watch group for people battling depression.

“People are susceptible to information about these events, but the mechanism is less clear,” says Andres Gomez-Lievano, a co-author of the mass-shooting study, published in July in PLOS ONE. Where and when the news reports were published could have an effect on incidence, says Dan Romer, director of the Adolescent Communication Institute at the University of Pennsylvania, who was not involved with the study. It is important to note, he says, that “suicides will trigger others, so it makes sense that people who want to commit suicide while killing others could be influenced in the same way.” —Kat Long

Psychological Contagions

Many types of thoughts and behaviors can be socially contagious, according to a growing body of work.