According to this literature (Capellan et al., 2019; Duwe, 2020), the perpetrators of mass public shootings in the United States have been overwhelmingly male (98 percent), are most commonly non-Hispanic White (61 percent).
In addition, they are most commonly younger than age 45 (82 percent); more specifically, 26 percent of mass public shooters from 1976 to 2018 were younger than age 25, 27 percent were aged 25 to 34, and 29 percent were aged 35 to 44.
Compared to the overall U.S. population, mass public shooting offenders are much more likely to be male and are younger; compared to other homicide offenders, males and non-Hispanic Whites are overrepresented among mass public shooters, and mass public shooters are older.
For comparison, of the overall U.S. population in 2019, approximately 49 percent was male, 60 percent was younger than age 45, and 60 percent was non-Hispanic White (U.S. Census Bureau, 2020).
Of murderers in 2018 with known offender characteristics,
88 percent were men,
84 percent were younger than age 45
(38 percent younger than 25,
31 percent aged 25 to 34, and
16 percent aged 35 to 44), and
42 percent were White (Hispanic ethnicity information was not provided) (FBI, 2019f).
It is challenging to make broad generalizations about the individual-level motivations of mass shootings. When mass shootings are broadly defined to include familicides, felony-related killings, and mass public shootings, the events include heterogeneous incident types that vary in terms of victim, offender, and incident characteristics (Fridel, 2017; Taylor, 2018).
Felony-related killings show differences from familicides and mass public shootings. They are, by definition, criminally motivated (in contrast to familicides and mass public shootings, which are more commonly motivated by relationship problems, group grievances, or ideological extremist beliefs); result in significantly fewer deaths; and are significantly less likely to conclude with the death of the perpetrator (Fridel, 2017; Capellan et al., 2019).[13]
The etiology of felony-related mass shootings thus, unsurprisingly, bears a stronger resemblance to firearm homicides more broadly. In contrast, familicides and mass public shootings show stronger similarities in terms of offender characteristics and motivations (Fridel, 2017).
The past decade has seen an increase in the percentage of mass public shootings that are posited to relate to fame-seeking on behalf of the individual or on behalf of a broader ideology (Capellan et al., 2019; Lankford and Silver, 2020).
Some researchers have suggested that this rise in fame‐ and attention‐seeking motivations among mass public shooters has contributed to an escalation in the lethality of these incidents (Langman, 2018; Lankford and Silver, 2020).
Although there are noted differences across diverse types of mass public shootings (Capellan and Anisin, 2018; Capellan et al., 2019), an overarching commonality is that most incidents are preceded by some level of planning by the shooter.
Among active shooting cases from 2000 to 2013 for which sufficient information was available, 62 percent of offenders planned the attack for more than one month, and 9 percent planned for more than one year (Silver, Simons, and Craun, 2018).
Focusing on incidents involving eight or more fatally injured victims, Lankford, and Silver (2020) found that at least half of the eighteen high-fatality mass public shootings between 2010 and 2019 involved a planning period of one year or longer.
About 40 percent of mass public shooters make some form of verbal or written threat (e.g., threats made in front of family or friends or posted to social media) prior to the incident (Duwe, 2020).
The nature of mass shootings creates serious challenges for developing policies that will effectively prevent their occurrence.
For instance, their rarity makes it difficult to extract generalizable information to find useful predictors of risk.
The low base rates of these events also ensure that policies targeting individuals based on risk factors would result in an extremely high rate of false positives; even the best available risk factors can show only a subpopulation in which the risk of committing a mass shooting is about one in a million.
Finally, because individuals who perpetrate mass shootings often die by suicide (or expect to be killed by someone trying to stop the shooting), standard deterrence strategies used in crime prevention are unlikely to work; increasing the certainty or severity of punishments seems unlikely to be effective when the perpetrator already expects to die in the mass shooting.[17]
A comprehensive administrative data source that reliably captures mass shooting incidents with sufficient detail does not exist; relying on news reports alone is problematic because of well-established systemic bias in what gets reported.
Although these issues create problems for understanding the prevalence and patterns of mass shootings at a given point in time, they are worsened when trying to understand how mass shootings have evolved over time; this is because of temporal variation in the completeness of underlying data sources that could be used to name and classify incidents.
There may be fewer concerns about incomplete or biased data when adopting a narrower definition of mass shootings that includes only the highest-profile incidents with multiple fatalities, but movement toward a more restrictive definition results in identifying a set of incidents that are increasingly rare and idiosyncratic.
Thus, the researcher makes a trade-off that mitigates the serious problems with the underlying data but creates added statistical problems resulting from a much smaller sample size that will not support correct generalizations to a broader population of mass shooters.
Given statistical challenges with accurately estimating the causal effects of a policy on mass shootings, we may be able to learn more about the potential for effective prevention strategies through detailed analyses of the characteristics of mass shootings (ideally for both incidents that occurred and incidents that are believed to have been averted) or through detailed review of how specific policies are being implemented in an effort to prevent mass shootings.
Descriptive evidence that mass shootings involving firearms equipped with LCMs result in significantly higher injury and fatality rates may suggest potential benefits of restricting access to LCMs (Koper, 2020), although it may be that the choice to use LCMs reflects more-lethal intentions of the shooter (Kleck, 2016).[21]
Similarly, evidence that many mass shooters have a history of domestic violence has led some to suggest potential benefits of stronger implementation of firearm prohibitions related to domestic violence (Zeoli and Paruk, 2020).
Finally, although extreme risk protection orders are most commonly requested because of concerns about self-harm (Parker, 2015; Swanson et al., 2017, 2019), a detailed review of case records from 159 such orders issued in California found that 21 (13.2 percent) involved an individual who had access to or was planning to access firearms and expressed or exhibited behavior suggesting intent to perpetrate a mass shooting (Wintemute et al., 2019).
These analyses do not directly assess the causal effect of policies on mass shooting outcomes, but they can still supply important insights for crafting and implementing policies.
The nature of mass shootings creates serious challenges for developing policies that will effectively prevent their occurrence.
For instance, their rarity makes it difficult to extract generalizable information to name useful predictors of risk.
The low base rates of these events also ensure that policies targeting individuals based on risk factors would result in an extremely high rate of false positives; even the best available risk factors can find only a subpopulation in which the risk of committing a mass shooting is about one in a million. Finally, because individuals who perpetrate mass shootings often die by suicide (or expect to be killed by someone trying to stop the shooting), standard deterrence strategies used in crime prevention are unlikely to work; increasing the certainty or severity of punishments seems unlikely to be effective when the perpetrator already expects to die in the mass shooting.
For recent examples, see Klarevas, Conner, and Hemenway (2019) and Webster et al.
(2020).
Missingness by jurisdiction in the SHR is commonly handled through a weighting process using the ratio of homicide counts in the FBI’s Uniform Crime Reports to reported homicide counts in the SHR database in order to produce national or state-level estimates (Fox and Swatt, 2009).
Several methods for imputing missing item data in the SHR have been developed and are discussed in detail in Wadsworth and Roberts (2008) and Roberts, Roberts, and Wadsworth (2018).
The stated purpose of the FBI active shooter reports is to provide federal, state, and local law enforcement with data to better understand how to prevent, prepare for, respond to, and recover from active shooter incidents; data collection methods used to inform the reports are described in Blair and Schweit (2014). Data cover the United States and come from multiple sources, including FBI reporting, official law enforcement investigative data, publicly available sources (e.g., governmental agency reports, journal articles), a comprehensive list of incidents developed by the New York City Police Department, and a study of shooting incidents in the United States from 2000 to 2010 conducted by the Advanced Law Enforcement Rapid Response Training Center
As an example of this issue, the discontinued Stanford Mass Shootings in America database, which relied solely on online media sources to name mass shooting events, cautioned its users that information in the database spanned a period in which reporting shifted from traditional media to digital media, and thus annual incident counts partially reflect changes in data collection method (see Stanford Geospatial Center, undated).
Thus, the more than threefold surge in mass shooting incidents from 2014 to 2015 shown in the Stanford data reflects increased online reporting and not necessarily a true increase in the rate of mass shootings.
In comparison, general population studies tend to find the prevalence of diagnosable psychotic disorders (including schizophrenia, schizoaffective psychosis, bipolar disorder with accompanying delusion, substance-induced psychotic disorder, and delusional disorders) to be less than 1 percent, although the prevalence varies as a function of the method of measurement and the population studied (see Moreno-Küstner, Martín, and Pastor, 2018).
There are no benchmarks for prevalence that use the methods of post-incident diagnosis that are used with mass shooters.
The same study found romantic or family issues (e.g., divorce, child custody dispute) as a potential contributing stressor or triggering event in 46 percent of familicides, 23 percent of mass public killings, and 4 percent of felony-related mass killings (Fridel, 2017).
Different definitions of mass public shootings result in different temporal patterns. If mass public shootings are defined more narrowly as incidents with four or more victims killed, school shootings were rare prior to the 1990s and constituted 11 percent of all mass public shootings from 1976 to 2018 (Duwe, 2020).
Under the broader definition used by Capellan et al. (2019), 25 percent of mass public shootings from 1966 to 2017 occurred in schools.
The identification of “assault weapons” in these incidents is not straightforward, partly because the term assault weapon is controversial.
In state and federal gun laws, the term refers to specific semiautomatic firearm models that fire a high volume of ammunition in a controlled way or that have specified design features, such as folding stocks or pistol grips. For further discussion, see Koper (2020) and Klarevas (2019).