What Is a Mass Shooting?
The term mass shooting is more a term of convenience than a scientific concept. Both words that make up the term are problematic. How many victims qualify as a mass? The word mass means a large amount or number of something, but the lower bound for defining a mass in studies of mass shootings is typically no more than four (e.g., Wilson, 2014; see also Bjelopera, Bagalman, Caldwell, Finklea, & McCallion, 2013), which is not a mass in the conventional sense. The word shooting indicates that a firearm has been used to kill or injure victims. Common sense indicates that a shooting is experienced as disturbing or traumatic to victims and observers. However, this restriction is limiting if our interest is in events with fatalities and/or injuries that have serious psychological consequences. Similar acts using other means such as explosives, machete and knife attacks, and intentional vehicle homicides are also traumatic and disturbing (Fox & Levin, 2015). Thus, a focus on shootings may in some ways be too narrow. But without further qualification, it may also be too broad. Assuming that we mean that a mass shooting involves some number of people who have been killed or injured using firearms, do we mean any such incident (Fox & Levin, 2015)? Do we include gang-related violence, robberies, and homicide-suicides that occur in private residences? An additional issue relates to whether our assessment of the magnitude of an event should be based only on the numbers of victims shot fatally. Nekvasil, Cornell, and Huang (2015) reconceptualize the phenomenon as a multiple casualty homicide, and argue that single homicides with more than one victim (i.e., wounded or injured survivors) qualify for our attention as well.
There is no straightforward solution to determining what to include under the mass shooting umbrella. The underlying issue is that the way analysts define a mass shooting largely depends on the function that the concept serves in the project to which it is applied. For example, in their Congressional Research Service Report, Bjelopera and colleagues (2013) define public mass shootings as āincidents occurring in relatively public places, involving four or more deaths ā not including the shooter(s) ā and gunmen who select victims somewhat indiscriminately. The violence in these cases is not a means to an end such as robbery or terrorismā (p. 4). This definition is in line with the purpose of the report to provide the U.S. Congress with a basis for discussion and debate about a form of violence that may not be adequately addressed by current legislation and policy. The number of fatalities required in this definition of public mass shootings was based on a definition of mass murder that the FBI presented in a report on serial murder (Federal Bureau of Investigation, 2008).1 Arbitrariness in the number of fatalities in the definition of mass shootings is underscored by recent legislation passed by the U.S. Congress stating that āthe term āmass killingsā means 3 or more killings in a single incidentāā (Investigative Assistance for Violent Crimes Act of 2012, 2013, p. 126 STAT. 2435).
Researchers have also been inconsistent and have used several cutoffs from two to four shooting-caused casualties to define mass shootings (Nekvasil et al., 2015). In their study of nearly 19,000 homicide incidents from 2005 to 2010, Nekvasil and colleagues (2015) compare the effectiveness of cutoffs of two, three, four, and five or more victims, concluding: āIt seems likely that no specific cutoff for number of victims is sufficient to identify a meaningfully distinct form of homicidal violenceā (p. 8).
We can conclude that there is no fixed or universally accepted definition of a mass shooting. Definitions of mass shootings do not vary greatly, but all contain ad hoc and arbitrary elements that may affect research outcomes and thus our understanding of mass shootings prevention, prediction, and intervention innovation. This is also true of the definition used in the present volume: a gun violence incident that results in four or more victim deaths. Is there any rationale for settling, however provisionally, on this definition? We think that there is, and that the rational has two parts.
First, the focus on gun violence captures a large majority of multiple casualty homicides. Recent evidence demonstrates that the primary weapon used in more than four out of five such incidents is a firearm, and as the number of victims increases, the likelihood that a firearm was used increases monotonically (Nekvasil et al., 2015). A firearm was the primary weapon used in nearly 95% of multiple casualty homicides with six or more victims. Because shooting incidents are, by far, the most prevalent form of multiple homicide, they are more available for study than other incidents, and they provide evidence for understanding the vast majority of mass homicides that occur. Nonetheless, it is likely that as this tragic literature grows, studies will address an increasing diversity of research problems and theoretical issues, and researchers should be attentive to hypotheses about whether and how different forms of mass homicide ...
