MASS SHOOTINGAre Mass Shootings Contagious?

By Chip Brownlee

Published 8 February 2023

High-fatality shootings are becoming more frequent, raising the question of whether or not mass shootings are contagious, that is, whether or not one mass shooter copycats the actions of an earlier shooter. These questions arise especially when several mass shooting events are clustered in time. The evidence is not clear-cut either way.

On January 21, a man opened fire at a dance hall in Monterey Park, California, killing 11 people and injuring nine others. Two days later, on January 23, another man attacked two farms in Half Moon Bay, California, shooting and killing seven and injuring one.

Both shooters were seniors — 72 and 66 years old respectively — and both were Asian men. Both used semiautomatic handguns. Both tried to shoot multiple people in multiple places. And both incidents happened in public spaces, a few hundred miles from one another.

The similarities prompted a reader of The Trace to ask: “Is there any evidence that the mass shooting contagion effect is getting stronger? Has it become more common for a mass shooting that receives national media coverage to be followed by others?”

There’s no clear evidence that the Half Moon Bay shooter was inspired by the Monterey Park one, but the question hits upon some confusing evidence regarding the behavior of mass shooters.

First, let’s break down some basics.

What Is a Contagion Effect?
In social science, “contagion” generally refers to the spread of a phenomenon, often a behavior, through a population. When a person perceives a behavior as commonplace, they may be subconsciously or consciously inspired to act in a similar way. Researchers typically use statistical methods to identify a contagion effect by analyzing the rate at which a phenomenon normally occurs and whether specific events increase the probability of similar occurrences in the near future.

If a mass shooting contagion effect exists, one shooting would increase the probability of more shootings to come.

It’s not uncommon for mass shootings to cluster in time. In 2019, a mass shooting that targeted Latin American immigrants in El Paso, Texas, killed 23 and injured 23. Just a day later, a shooter in Dayton, Ohio, killed nine and injured 27. And the May 2022 shooting in Buffalo, New York, in which 10 people were killed and one wounded, was followed by the school shooting at Robb Elementary School in Uvalde, Texas, less than two weeks later, which killed 21 and injured 17.

While none of those shootings appeared to be directly linked, earlier examples of clustering led some researchers, advocates, and commentators to hypothesize that there is a contagion effect increasing the rate and frequency of mass shootings.