From Reason.com….
In the last month, the United States has seen four mass shootings in public places that killed at least four people aside from the perpetrator, including yesterday’s attack at a FedEx warehouse in Indianapolis. Prior to the Atlanta spa shootings on March 16, more than a year had elapsed without such a crime, which is the definition of public mass shootings used by the Congressional Research Service.
That pattern is consistent with data indicating that public mass shootings tend to happen in clusters, suggesting that one such crime makes others more likely. “One happens, and you see another few happen right after that,” Hamline University criminologist Jillian Peterson noted in a 2019 interview with NPR.
A 2015 PLOS One study seemed to confirm that impression. Statistician Sherry Towers and her collaborators looked at 232 “mass killings” (defined as “incidents with four or more people killed”) from 2006 through 2013, based on USA Today’s database. They also considered 188 school shootings—defined as incidents on campus (including college campuses) during school hours, on school buses, or at school-related events (such as football games) in which at least one person was shot—from 1998 through 2013.
“We find significant evidence that mass killings involving firearms are incented by similar events in the immediate past,” Towers and her co-authors reported. “On average, this temporary increase in probability lasts 13 days, and each incident incites at least 0.30 new incidents….We also find significant evidence of contagion in school shootings, for which an incident is contagious for an average of 13 days, and incites an average of at least 0.22 new incidents.” The researchers did not find such evidence when they looked at a broader category of shootings that included attacks in which at least three people were injured or killed.
