Gun Control and Mass Shootings
Here is a pretty good illustration of why it matters:What Is a Mass Shooting?
In the 1980s, the Federal Bureau of Investigation (FBI) defined mass murderer as someone who “kills four or more people in a single incident (not including himself), typically in a single location” (Krouse and Richardson, 2015). However, the government has never defined mass shooting as a separate category, and there is not yet a universally accepted definition of the term. Thus, media outlets, academic researchers, and law enforcement agencies frequently use different definitions when discussing mass shootings, which can complicate our understanding of mass shooting trends and their relationship to gun policy. The table below provides examples of the variation in the criteria set by five of the most commonly referenced data sources on mass shootings in the United States.
Although there is no official standard for the casualty threshold that distinguishes a mass shooting from other violent crimes involving a firearm, a common approach in the literature is to adopt the FBI’s criteria for a mass murderer and set a casualty threshold of four fatalities by firearm, excluding the offender or offenders (Duwe, *Kovandzic, and Moody, 2002; Krouse and Richardson, 2015; Gius, 2015c; Fox and Fridel, 2016). However, this categorization is not without controversy. It does not capture incidents in which fewer than four victims were killed but additional victims were injured, and it does not include multiple-victim homicides in which fewer than four fatalities resulted from gunshots but additional fatalities occurred by other means. Additionally, the FBI classification of mass murderer was established primarily with the aim of clarifying criminal profiling procedures, not for the purpose of data collection or statistical *analysis (Ressler, Burgess, and Douglas, 1988). Thus, many have chosen alternative definitions of casualty thresholds for mass shootings. For instance, Lott and Landes (2000) adopted the definition of two or more injured victims, the Gun Violence Archive (undated-a) defined mass shooting as an incident in which four or more victims (excluding the shooter) are injured or killed, and Mass Shooting Tracker (undated) set a criterion of four or more people injured or killed (including the shooter).
Another definitional disagreement is whether to include multiple-victim shooting incidents that occur in connection with some other crime or domestic dispute. Because mass shootings that stem from domestic and gang violence are contextually distinct from high-fatality indiscriminate killings in public venues, some have argued that they should be treated separately. In their analyses of “mass public shootings,” Lott and Landes (2000) excluded any felony-related shooting, and Duwe, Kovandzic, and Moody (2002) excluded incidents where “both the victims and offender(s) were involved in unlawful activities, such as organized crime, gang activity, and drug deals” (p. 276). Similarly, Gius (2015c) restricted analysis to events that occurred in a relatively public area and in which victims appeared to have been selected randomly. However, others have claimed that this narrow definition ignores a substantial proportion of gun-related violence from family- or felony-related murder (Fox and Levin, 2015). Data collection efforts by Mass Shooting Tracker and the Gun Violence Archive thus counted all incidents that met their designated casualty threshold as mass shootings, regardless of the circumstances that led to the event.
These definitions matter. Depending on which data source is referenced, there were seven, 65, 332, or 371 mass shootings in the United States in 2015 (see table below), and those are just some examples. More-restrictive definitions (e.g., Mother Jones) focus on the prevalence of higher-profile events motivated by mass murder, but they omit more-common incidents occurring in connection with domestic violence or criminal activity, which make up about 80 percent of mass shooting incidents with four or more fatally injured victims (Krouse and Richardson, 2015). Broader definitions (e.g., Mass Shooting Tracker) provide a more comprehensive depiction of the prevalence of gun violence, but they obscure the variety of circumstances in which these incidents take place and their associated policy implications. Furthermore, if the effects of a firearm policy are expected to affect only public mass shooting incidents, then analysis that includes domestic violence mass shootings in the outcome measure could obscure identification of significant effects that would be found in a more targeted analysis of public mass shootings alone. There is thus value in having multiple measurements of mass shootings—but only if their definitions are clearly and precisely explained and they are used by researchers in a manner appropriate to the analysis.