What Are Mass Murderers’ Motives?

Predictably, when a certain type of mass shooting occurs, especially in a public setting with a high causality count and where the victims were unknown to the shooter, news anchors ask the onsite reporters and talking head experts, “What do we know about the shooter’s motive?” Given the regularity with which questions about mass murderers’ motives are asked, one might wonder: What have we learned, and does it offer any guidance?

Rarely, if ever, do we have a satisfactory explanation for a mass murderer’s actions. Typically, this void is filled with the generic prescription for more mental health services. Meanwhile, the media’s attention moves on to the next…and then to the next, next….and on to the next, next, next mass murder.

Research on Mass Murderers’ Motivations

There are, however, a few systematic studies of mass murderers’ motivations, although not as many as are warranted considering their incidence. Four or five databases are commonly used in investigations of mass murders and murderers. Some are official U.S. government projects (e.g., the Federal Bureau of Investigation's (FBI's) Supplemental Homicide Reports, the Centers for Disease Control and Prevention's National Violent Death Reporting System) and some are nonprofit efforts (e.g., Mother Jones, Everytown for Gun Safety, Gun Violence Archive). Information in these databases is usually supplemented with media accounts. Even when an incident does not receive national coverage, there is always local reporting, which averages about six stories per incident.

Virtually all mass murder researchers lament the paucity of good data and the large number of serious inconsistencies among the national data sets for the same time periods. Most surprising is the startling lack of agreement for the number of mass murders occurring during the same time period. In 2017, for example, one major database recorded its highest number of incidents (346), while another recorded its lowest number (11). For that same year, only two cases are included across four of the major databases.¹

The primary reason for this enormous variability is the difference in the definitions of what qualifies as an incident. Researchers often compensate for this heterogeneity by imposing a common definition across all of the data sets. Most frequently they use the FBI’s definition of a mass murder as one involving four or more victims (not including the perpetrator) killed, usually at the same location, and without an intervening cooling-off period.

When a common definition, such as the FBI’s, is imposed across the different databases, a number of distinct motivational and circumstantial subgroups emerge. These include domestic and relationship violence, murders occurring during the commission of crimes (especially gang violence and drug-related crimes), workplace violence, violence attributable to mental health problems, and violence associated with extremist political/religious/racial beliefs. It becomes apparent that the “motives” for these different categories differ significantly as do the circumstances, settings, and victims of their crimes.

It is also apparent that the incidents that receive the most national-level media coverage are actually a minority of the total cases that fit the FBI definition. Indeed, the most common mass murder scenario is family violence, typically occurring in the home, with few or no strangers among the victims. This accounts for 30 to 40 percent of all mass murders. In one study, the majority of domestic mass murders were triggered by a precipitating event such as a relationship breakup, a love triangle, or job loss. Domestic violence-associated mass murders also have the largest percentage of female and child victims.

The Contributions of Mental Illness

Across multiple studies, the percentage of mass murders associated with an offender’s mental health problems is about 30 percent, although one small, selective study found that 61 percent of cases had a potential mental health component. Schizophrenia was the most common diagnosis. In another study, 23 percent of juvenile mass murderers had a psychiatric history, but only 6 percent were judged to be psychotic at the time of their crime. On average, however, mentally ill mass murderers killed more victims.

One can argue that by virtue of the act of committing mass murder, offenders must have serious mental health problems. Forty percent of incidents end with the murderer’s suicide and another 5 percent are killed by police. While not diagnostic for any specific psychiatric disorder, these features suggest that mass murderers are often in extreme mental states at the time of the crime.

Mass Shootings Essential Reads

How to Help Kids Manage Anticipation of School Shootings

What Do We Really Know About Mass Shootings?

Common Factors Across Mass Murders

Two significant commonalities are found across the different motivational and circumstantial subgroupings of mass murderers. The first is that perpetrators primarily used semiautomatic handguns (70 percent), three-quarters of which were purchased legally. Secondly, the murderers were almost exclusively male (90-95 percent). The only category in which females accounted for a notable percentage of perpetrators (15%) involved financially motivated incidents. Female mass murderers were less likely to harm strangers and had high rates of suicide. They were also less likely to use firearms, but more likely to use arson.

What Have We Learned and Can It Help Us?

Once a common definition of mass murder is imposed across the relevant databases, several, largely distinct, subgroupings appear: domestic and workplace violence, crime (especially drug- and gang-related murders), mental health problems, and extremist antisocial belief systems. These subgroups differ in motivation, predisposing and precipitating circumstances, and public versus private settings that influence the likelihood of killing strangers or family and friends, or co-workers. If we wish to prevent mass murders in general, we must look at the motivations, circumstances, and settings for all of these categories.

It should be apparent that even if there existed a flawless mental health system of care with timely referrals to effective services, it would, at best, only address about a third of potential mass murderers. So while greatly improved mental health services can play an important role in reducing mass murders (as well as serious mental health problems in general), even in the very best case, they would only address a minority of potential offenders.

As the largest category (30-40 percent), a focus on family violence prevention and acute domestic and workplace crisis interventions is important. Given the finding that family violence-related mass murders are often precipitated by triggering events, frequently relationship problems, community investments in prevention and emergency domestic violence crisis intervention resources are warranted. School violence prevention programs would also be an important investment. Mass murders associated with crime or extremist beliefs are best left to law enforcement.