With 43 attacks claiming over 200 victims, mass killings reached a forty-year high in 2019. Defined as the killing of four or more individuals (excluding the offender) within 24 hours, mass murders are incredibly rare events that account for less than 1% of all homicides (Krouse & Richardson, 2015). Despite their rarity, mass killings disproportionately impact policy due to widespread public concern. According to the American Psychological Association (APA, 2019), nearly 80% of American adults experience stress related to mass shootings, and approximately one third avoid certain places and events due to their fear of victimization. Individual incidents have fueled moral panics, inspired social movements like March for Our Lives, and sparked calls for policy change on topics ranging from gun control to mental healthcare reform.

Despite this surge of public interest, little research has explored the correlates of mass murder. The extant literature has largely focused on estimating the prevalence of the crime (Duwe, 2004; Liem et al., 2013), identifying risk factors and developing a basic offender profile (Fox & Levin, 1998; Knoll, 2010a, 2010b; Knoll & Meloy, 2014; McPhedran, 2017), and classifying incidents into unique subcategories (Bowers et al., 2010; Petee et al., 1997). With a few notable exceptions (Fridel, 2021; Lankford, 2015, 2016), this work has almost exclusively been descriptive in nature and relies heavily on case studies. While these types of analyses provide rich detail on specific incidents, they overemphasize the most atypical and extreme cases, are not representative of or generalizable to the population as a whole and may propagate stereotypes. The dearth of empirical research on mass murder is a natural consequence of its low base rate and corresponding lack of reliable and/or official data (Huff-Corzine et al., 2014).

It is crucial to consider the role that suicide plays in distinguishing homicide from mass murder. While only 4% of homicides are followed by the perpetrator’s suicide (Liem et al., 2011), over one-third of mass murderers take their own lives (Lankford, 2015; Taylor, 2018), with some estimates as high as 50% for familicides and public massacres (Kelly, 2010; Liem & Reichelmann, 2014; Stone, 2015).

Lankford (2015, pp. 363) even goes so far as to argue that mass shooters “are virtually all suicidal prior to their attacks” as the killers do not expect to escape. Despite the high prevalence of suicide among mass killers, little prior work has focused specifically on mass murder-suicide or its relationship to homicide-suicide. By examining the shared and unique correlates of mass murder, homicide, and homicide-suicide, the current study seeks to integrate the literature on mass killing with the broader study of lethal violence. Using data from the National Violent Death Reporting System (NVDRS) and an expanded USA TODAY mass murder database in the United States from 2003 to 2015, logistic regression models are utilized to compare the incident, victim, and offender characteristics of: (1) all mass murderers (N = 457) to all homicide offenders (N = 48,398); and (2) mass murder-suicide offenders (N = 116) to homicide-suicide perpetrators (N = 2,505).

While most homicides are the result of arguments between two young male acquaintances of color, mass murders disproportionately involve middle-aged white males targeting family members and strangers. Although both homicide and mass murder are male-dominated crimes, with men constituting over 90% of perpetrators (Fridel & Fox, 2019), the two crimes significantly differ in terms of age and race/ethnicity. While nearly half of homicide offenders are under the age of 25, mass killers tend to be in their thirties—old enough to be married with multiple children and experience various stressors (Duwe, 2007; Fridel, 2021; Fox et al., 2019; McPhedran, 2017; Schildkraut & Elsass, 2016).

Homicides and mass murders also differ in terms of their victims. As with their killers, homicide victims are disproportionately young men of color: three-fourths of victims are male, half are under the age of 30, and half are black (Federal Bureau of Investigation, 2019). In contrast, mass murderers disproportionately target women and children, as nearly half of their victims are female and more than 35% are under the age of 18 (Fridel, 2021)

Although mass murder is traditionally examined as a separate construct from homicide generally, few studies have explored their similarities and differences. This study compares the incident, victim, and offender characteristics of: (1) mass murderers and homicide offenders; and (2) mass murder-suicide offenders and homicide-suicide perpetrators. Mass murderers are more likely to be male; commit suicide; kill young, white, and female victims; use firearms; co-offend; operate in public places; and kill as part of drug trafficking and/or gang warfare. The analysis demonstrates that mass murderers are distinct from both homicide and homicide-suicide perpetrators and represent a unique type of violent offender.