Mass Shootings in the United States. Updated April 15, 2021

By Rosanna Smart, Terry L. Schell

There is no standard definition of what constitutes a mass shooting, and different data sources—such as media outlets, academic researchers, and law enforcement agencies—frequently use different definitions when discussing and analyzing mass shootings. For instance, when various organizations measure and report on mass shootings, the criteria they use in counting such events might differ by the minimum threshold for the number of victims, whether the victim count includes those who were not fatally injured, where the shooting occurred, whether the shooting occurred in connection to another crime, and the relationship between the shooter and the victims. These inconsistencies lead to different assessments of how frequently mass shootings occur and whether they are more common now than they were a decade or two ago. Data show that, regardless of how one defines mass shootings, perpetrators are likely to be men. But several other characteristics that are statistically predictive of perpetration are still uncommon among offenders on an absolute level. The rare nature of mass shootings creates challenges for accurately identifying salient predictors of risk and limits statistical power for detecting which policies may be effective in reducing mass shooting incidence or lethality. Implementing broader violence prevention strategies rather than focusing specifically on the most-extreme forms of such violence may be effective at reducing the occurrence and lethality of mass shootings.

Santa Monica, CA: RAND, 2021. 32p.