Summary: There is no standard definition of what constitutes a mass shooting. Media outlets, academic researchers, and law enforcement agencies frequently use different definitions when discussing mass shootings, leading to different assessments of how frequently mass shootings occur and whether they are more common now than they were a decade or two ago.

In this essay, we provide an overview of mass shootings, one of the eight outcomes examined in our research syntheses. We first describe different approaches for defining a mass shooting and then discuss how using different definitions can influence estimates of mass shooting levels and trends. The information was collected from a targeted search of the literature separate from that outlined in the methodology description.

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.

Variation in How Mass Shootings Are Defined and Counted Source Casualty Threshold (for injuries or deaths by firearm) Location of Incident Motivation of Shooter Number of U.S. Mass Shootings in 2015 Mother Jones (see Follman, Aronsen, and Pan, 2017) Three fatal injuries (excluding shooter)* Public Indiscriminate (excludes crimes of armed robbery, gang violence, or domestic violence) 7 Gun Violence Archive (undated-a) Four fatal or nonfatal injuries (excluding shooter) Any Any 332 Mass Shooting Tracker (undated) Four fatal or nonfatal injuries (including shooter) Any Any 371 Mass Shootings in America database (Stanford Geospatial Center, undated) Three fatal or nonfatal injuries (excluding shooter) Any Not identifiably related to gangs, drugs, or organized crime 65 Supplementary Homicide Reports (FBI) (see Puzzanchera, Chamberlin, and Kang, 2017) The FBI's Supplementary Homicide Reports do not define mass shooting but do provide information on the number of victims, and the reports have been used by researchers in conjunction with news reports or other data sources. * Before January 2013, the casualty threshold for Mother Jones was four fatal injuries (excluding the shooter).

Are Mass Shootings on the Rise?

In 2014, the FBI released a study showing that “active shooting incidents” had increased at an average annual rate of 16 percent between 2000 and 2013 (Blair and Schweit, 2014). In contrast to the varied definitions for mass shootings, there is an agreed-upon definition among government agencies for active shooter: “an individual actively engaged in killing or attempting to kill people in a confined and populated area; in most cases, active shooters use firearm(s) and there is no pattern or method to their selection of victims” (U.S. Department of Homeland Security, 2008, p. 2). Using a modified version of this definition to include incidents that had multiple offenders or occurred in confined spaces, Blair and Schweit (2014) found that active shootings had increased from only one incident in 2000 to 17 in 2013.

The FBI study (Blair and Schweit, 2014) highlighted several key issues in determining trends in mass shootings. First, the absence of a systematic definition of mass shootings can lead to misinterpretation of reported evidence. While the study explicitly stated, “This is not a study of mass killings or mass shootings” (p. 5), extensive media coverage cited the study as evidence of a sharp rise in mass shootings and mass shooting fatalities (Lott, 2015). However, the definition of an active-shooter incident is broader than any of the commonly used criteria for mass shootings (see table above) because it does not set any casualty threshold. Of the 160 active-shooter incidents included in the FBI’s analysis, 7 percent resulted in zero casualties, 20 percent resulted in zero fatalities, and 22 percent resulted in a single fatality (Lott, 2015). Setting a threshold of zero victims increases the potential for measurement error, because shooting incidents with no casualties are more difficult to identify from police records and are less likely to receive media coverage (Duwe, Kovandzic, and Moody, 2002). Additionally, because it should be relatively easier to identify more-recent shootings with few fatalities, a low casualty threshold will tend to systematically bias estimates of the number of shootings upward over time. For example, the Stanford Mass Shootings in America database, which relies solely on online media sources to identify mass shooting events, cautions its users, “Data in the [database] spans a time period that includes the transition from traditional media to digital media in reporting. Numbers of incidents per year should at least in part be assumed to reflect this collection methodology and not just changes in incident frequency.” Thus, the more than threefold surge in mass shooting incidents from 2014 to 2015 shown in the Stanford data likely reflects increased online reporting and not necessarily a true increase in the rate of mass shootings.

Even when a more restrictive casualty threshold of four or more fatally injured victims (excluding the shooter) is imposed, empirical evidence on trends in these incidents varies depending on whether the motivation of the shooter is included as a criterion for considering an event a mass shooting. In their analysis of mass shooting trends from 1999 to 2013, Krouse and Richardson (2015) distinguished between mass shootings occurring in public locations that are indiscriminate in nature (“mass public shootings”), mass shootings in which the majority of victims are members of the offender’s family and that are not attributable to other criminal activity (“familicide mass shootings”), and mass shootings that occur in connection to some other criminal activity (“other felony mass shootings”). The two figures below show trends in these types of mass shooting incidents and fatalities, respectively, using the data provided in Krouse and Richardson (2015). Extending the data back to the 1970s, two studies found evidence of a slight increase in the frequency of mass public shootings over the past three decades (Cohen, Azrael, and Miller, 2014; Krouse and Richardson, 2015). However, using an expanded definition that includes domestic- or felony-related killings, there is little evidence to suggest that mass shooting incidents or fatalities have increased (Cohen, Azrael, and Miller, 2014; Krouse and Richardson, 2015; Fox and Fridel, 2016). Thus, different choices about how to define a mass shooting result in different findings for both the prevalence of these events at a given time and whether their frequency has changed over time.

Trends in Mass Shooting Incidents, by Type of Incident SOURCE: Adapted from data in Krouse and Richardson, 2015.

Trends in Mass Shooting Fatalities, by Type of Incident SOURCE: Adapted from data in Krouse and Richardson, 2015.

Definitional issues aside, the relative rarity of mass shooting events makes analysis of trends particularly difficult. Chance variability in the annual number of mass shooting incidents makes it challenging to discern a clear trend, and trend estimates will be sensitive to outliers and to the time frame chosen for analysis. For example, while Krouse and Richardson (2015) found evidence of an upward trend in mass public shootings from 1999 to 2013, they noted that the increase was driven largely by 2012, which had an unusually high number of mass public shooting incidents. Additionally, Lott (2015) showed that the FBI study’s estimate of a dramatic increase in active-shooter incidents was largely driven by the choice of 2000 as the starting date, because that year had an unusually low number of shooting incidents; extending the analysis to cover 1977 onward and adjusting the data to exclude events with fewer than two fatalities, Lott (2015) found a much smaller and statistically insignificant increase (less than 1 percent annually) in mass shooting fatalities over time.

Conclusions

While different choices about how to define a mass shooting and the period over which to calculate mass shooting trends have resulted in disagreement about whether the frequency of mass shootings has risen, there is clear evidence that the media’s use of the term mass shooting has increased significantly over recent decades (Roeder, 2016). Unfortunately, the ambiguity in how mass shootings are defined and counted may result in increased media coverage influencing public perception without better informing our understanding of the prevalence of mass shootings or their determinants, trends, social costs, or policy implications.

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