Are mass shootings contagious?


When a mass shooting happens, another often follows in close succession. That’s not just a feeling – it’s a fact.

The devastating shooting on May 24 in Uvalde, Tex., which killed 19 children, two teachers, and injured 17 others, occurred 10 days after a supermarket shooting in Buffalo, N.Y., which resulted in 10 deaths. In 2021, a shooting at a massage parlor in Atlanta, which left eight dead, came less than a week before a shooting at a supermarket in Boulder, Colo., that killed 10. And a 2019 shooting in Dayton, Ohio, on Aug. 4 that killed nine people took place only a day after a Walmart shooting in El Paso, Tex., which claimed 22 lives.

Contagion theory

Researchers argue that the clustering of mass shootings suggests that this type of violence spreads like a virus and should be treated as one.

This theory – called the “contagion effect” – has been examined at length in cases of suicide, especially among teens and young adults. Studies have demonstrated that the majority of adolescents who attempt suicide have previously been exposed to the suicidal behavior of a peer.

In many cases, mass shootings are also suicides, with shooters taking their own lives at the time of the shooting or not long after.

“They have literally and figuratively given up on their life as they know it.” said Joel Dvoskin, PhD, a clinical and forensic psychologist at the University of Arizona, Tucson, and former acting commissioner of mental health for New York state.

According to contagion theory, mass shootings – and the round-the-clock media coverage they generate – lead to even more killings.

A team of researchers at Arizona State University led by Sherry Towers, PhD, analyzed mass shooting data in 2015 to find out whether those events followed a similar pattern. Dr. Towers spent much of her career modeling the spread of infectious diseases, such as influenza, Ebola, and Zika.

Dr. Towers and colleagues discovered that a mass killing tended to give rise to more killings in its immediate aftermath. According to her evaluation of USA Today’s mass shooting database, a second incident was most likely to occur within 13 days of the initial event.

What defines a mass shooting?

The FBI defines a mass shooting as any incident in which four or more people die by gunfire. That definition, however, is not universally accepted. The lack of a standard definition complicates the work of researchers who study contagion theory.

Mother Jones magazine created an open-source database of mass killings that employs a similar definition but that includes only incidents that involve a person shooting indiscriminately in a public place.

With this narrower definition, shootings involving organized crime, robberies, and domestic violence – which make up the vast majority of shootings in which multiple fatalities occur in this country – are excluded. Events such as those that occurred in Sandy Hook or the killings in Highland Park, Ill., this past July would be included.

The Gun Violence Archive categorizes mass shootings as any incident in which four or more people are shot but not necessarily killed, while Everytown for Gun Safety tallies mass shootings that take at least four lives.

James Meindl, PhD, a professor of behavioral analysis at the University of Memphis who studies mass shootings, said parsing the differences between what happened in Uvalde and what happens during a shooting involving organized crime or domestic violence is crucial when thinking about intervention and prevention.

“If you want to intervene, you have to know why the person engaged in this behavior in the first place,” Dr. Meindl said. “The factors that led a person to commit gang violence, the factors in domestic violence, the factors in indiscriminate mass shootings – those are all very different factors that would call for very different interventions.”


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