With 43 attacks claiming over 200 victims, mass killings reached a forty-year high in 2019 (AP/USA TODAY/Northeastern University, 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.

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. 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).

In terms of victim-offender relationship, nearly half of all homicide incidents involve acquaintances, with another 25% related by blood or marriage (Decker, 1993; Federal Bureau of Investigation, 2019; Riedel & Zahn, 1985). Cases with stranger victims typically involve other felonies, such as robbery (Riedel & Zahn, 1985; Williams & Flewelling, 1988). Mass murderers also generally target people they know, as only one-quarter of their victims are strangers or random bystanders, a proportion similar to homicide generally (Duwe, 2007). However, mass murderers are more likely to target family members than are homicide offenders, with estimates of victims within the family over 40% (Duwe, 2007).

Although mass murder is traditionally studied separately from homicide generally, very 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. Results of the comparison showed that mass murderers are more likely to be male; to commit suicide; to kill young, white, and female victims; to use firearms; to co-offend; to operate in public places; and to 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.

(The expanded abstract is excerpted and adapted from the article cited above)