Notes on The Role of Domestic Violence in Fatal Mass Shootings in the United States, 2014–2019

Background

• Fatal mass shootings, where four or more people are killed by gunfire (excluding the perpetrator), represent a small fraction (1%) of all firearm homicide fatalities in the United States.
• These events receive significant media attention and can heavily influence political discussions surrounding gun violence.
• Following mass shootings, there's a strong desire to understand the causes and prevent future occurrences.
• While risk factors for general gun violence (community gun violence, suicide) are well-known, the specific factors leading to mass shootings are less understood due to their rarity.
• Recent studies indicate that domestic violence (DV) is a significant factor in many mass shootings.
• An intimate partner is defined as someone with a close personal relationship, including current or former spouses, boyfriends/girlfriends, dating partners, or sexual partners. Relationships can be heterosexual or same-sex and do not require sexual intimacy, according to the CDC.
• The definition of DV includes intimate partners, cohabitants, those sharing a child, and family members, as defined by the United States Department of Justice.
• This study defines a fatal mass shooting as an event where four or more people are killed by gunfire, excluding the perpetrator.
• Federal law (Gun Control Act of 1968) prohibits firearm purchase and possession for individuals convicted of a misdemeanor crime of DV.
• State laws regarding misdemeanor crimes vary, and some states have weak measures to prevent DV perpetrators from acquiring firearms or removing guns from those legally prohibited.
• The link between DV and firearm violence is well-established.
  • Over half of all intimate partner homicides (IPH) involve firearms.
  • Firearms are used for threats and intimidation in intimate relationships.
  • Approximately 4.5 million women in the U.S. have been threatened with a firearm, and nearly 1 million have been shot or shot at by an intimate partner.
  • Access to firearms by abusers increases the risk of femicide by 400% (Campbell et al. 2003).
  • The risk of homicide is elevated when a woman attempts to leave an abusive partner.
• Limited research exists on the role of DV in mass shootings and multiple victim homicides.

Prior Research

• Zeoli and Paruk (2019) analyzed mass shooters from 2014 to 2017, assessing:
  • Whether offenders had known histories of DV or were suspected to have committed DV.
  • Whether they were legal firearm purchasers.
  • Whether they had prior involvement in the criminal justice system that should have restricted firearm purchase/possession.
  • They found that 31.5% of mass shooters had histories of DV perpetration.
  • They suggested that mass shootings could be prevented with better enforcement of DV-related firearm restrictions, such as domestic violence protective orders (DVPOs).
  • Zeoli and Paruk (2019) found more fatal victims when DV was mentioned (average 7.1 killed) versus when it was not (average 6.2 killed).
  • Their work highlights systemic gaps that allow potential mass shooters with DV histories to acquire firearms due to poorly implemented laws.
• Kivisto and Porter (2020) found that firearm use in domestic homicides increases the risk of multiple fatalities, unlike non-domestic homicides.
  • Male perpetrators using firearms in domestic homicides were almost twice as likely to kill at least one other person compared to those not using firearms.
  • 4.6% of domestic homicides had multiple victims, compared to 3.3% of non-domestic homicides.
• IPH events frequently result in multiple victims, including perpetrator suicide and the deaths of family, friends, new dating partners, coworkers, children, strangers, or police officers.
• About 40% of male-perpetrated IPHs result in multiple fatalities, either through perpetrator suicide or additional homicides (Kivisto 2015).
• A study of IPH events in 16 states (2003-2009) found that nearly 30% of IPV-related incidents resulted in multiple deaths, with a median of 2 deaths per incident (range 2-7 deaths) (Smith et al. 2014).
  • Almost 50% of the additional deaths were children or other family of the abused intimate partner, 27% were new intimate partners of the targeted partner, 20% were friends and acquaintances, 3% were strangers, and 1% were law enforcement officers (Smith et al. 2014).
• Reeping et al. (2019), using FBI Supplementary Homicide Report (SHR) data (1999-2014), found that 23.5% of mass shootings were DV-related.
  • Florida data was excluded due to nonparticipation in the FBI’s reporting system.
  • The FBI's definition is limited as it relies on the relationship between the offender and the first victim, which can misclassify shootings if the intimate partner was not the first victim.
• Limited research has specifically focused on the role of DV in mass shootings or on the differences in case fatality rates (CFR) between DV-related, history of DV, and non-DV-related mass shootings.

Current Study

• This study explores the correlation between DV and mass shootings and whether there are differences in the average number of injuries and fatalities for mass shootings that were DV, history of DV, or non-DV-related using data from the Gun Violence Archive (GVA).

Methods

Definition of Mass Shooting

• There is no legal definition of "mass shooting" in the United States, leading to disagreements on how to define it.
• Scholarly literature commonly defines mass shootings as those resulting in four or more deaths by gunfire, excluding the perpetrator (Booty et al. 2019; Zeoli and Paruk 2019).
• This study adopts this definition for fatal mass shootings.
• Different definitions across databases lead to varying numbers of mass shootings being captured, potentially affecting study results.
• A 2019 analysis of five mass shooting databases found little overlap in the number of shootings due to definitional differences (Booty et al. 2019).
• While recent work has called for an expanded definition to include both fatal and non-fatal injuries, this study focuses on mass shootings with four or more fatalities by gunfire, excluding the perpetrator.

Data and Measures

• This analysis uses GVA data on mass shootings from 2014 to 2019.
• The GVA tracks the date, city, state, address, number killed, and number injured for each incident.
• The GVA defines a mass shooting as “[Four] or more shot and/or killed in a single event [incident], at the same general time and location not including the shooter” (Gun Violence Archive n.d.-a, n.d.-b).
• This study applies its definition (four or more fatalities by gunfire, excluding the perpetrator) to the GVA data.
• This resulted in a sample size of 128 mass shootings across the study period, with an average of 21.5 mass shootings per year (Gun Violence Archive n.d.-a).
• Data was indexed by year and mass shooting, and the number of deaths and injuries was collected.
• Two authors independently reviewed news articles on each mass shooting and categorized them as:
  • DV-related: At least one victim was a dating partner or family member of the perpetrator.
  • History of DV: The perpetrator had a history of DV, but the mass shooting was not directed toward partners or family members.
  • Non-DV-related: The victims were not partners or family members, and there was no mention of the perpetrator having a history of DV.
• Discrepancies between coders were discussed with the PI until a consensus was reached.
• 94.0% of incidents were coded the same way by both coders.
• The 3/22/2017 shooting was coded as DV-related because the perpetrator specifically targeted his wife, even though the victims did not include family or partners.
• A similar methodology to Zeoli and Paruk’s (2019) paper was used.
• Definitions:
  • An "intimate partner" is a current or former spouse, dating partner, or someone whom the offender had a child in common or lived with.
  • A "family member" is someone related to the offender (either by blood or through the intimate partner) but who does not fall under the "intimate partner" category.
• If at least one news article mentioned that the offender had a known history of domestic violence (violence or threats of violence against a current or former intimate partner or family member), but the victims were not intimate partners or family members, the cases were coded as "history of DV" shootings.
• When neither DV nor a history of DV was mentioned in any news stories, the shooting was classified as “non-DV related.”
• Following Zeoli and Paruk (2019), if any victims of shootings with multiple perpetrators were family and/or intimate partners of the perpetrator, the mass shooting was classified as DV-related.
• If at least one of the perpetrators for shootings with multiple perpetrators had a history of DV, it was classified as a history of DV shooting.
• All other shootings were classified as non-DV related.
• Cases where the perpetrator was unknown (17 cases) were removed from the main analysis.
• The Las Vegas shooting (2017) was identified as an outlier and excluded from the main analysis because there were 471 total victims which was greater than three standard deviations from the mean (139 total victims).

Analytic Methods

• Descriptive analyses were conducted to summarize the percent of mass shootings that were DV-related, history of DV, or non-DV-related.
• One-way ANOVA was used to examine whether there were differences in the average number of injuries or fatalities or the CFR between DV, history of DV, and non-DV-related mass shootings.
• CFRs were calculated by category to reflect the total number killed over the total number injured and killed.
• 95% confidence intervals were calculated for each CFR; category CFRs were determined to be significantly different at the p = 0.05 level if the 95% confidence intervals did not overlap.
• The number of perpetrators who died during the mass shootings and whether they died by suicide or were killed by police was analyzed.
• A “hybrid” category combined DV-related shootings with history of DV shootings.
• A two-sample t-test was then conducted to determine whether this new hybrid DV-category had significantly different average victim fatalities and injuries from the non-DV-related shootings.
• Analyses were conducted using Stata version 16.1 (StataCorp 2019).
• Institutional Review Board approval was not required for this non-human subjects review of publicly available data.

Results

• There were 128 mass shootings between January 1, 2014 and December 31, 2019.
• After removing shootings where the perpetrator was unknown and excluding the Las Vegas shooting as an outlier, 110 mass shootings were included in the study.
• These shootings resulted in 651 deaths (excluding perpetrators) and 283 non-fatal injuries.
• 65 of the 110 shootings (59.1%) analyzed, at least one fatal or non-fatal victim was a partner or family member.
• 10 of the 110 shootings (9.1%), the perpetrator had a history of DV, but none of the victims were partners or family members.
• The remaining 35 mass shootings (31.8%) were non-DV-related.
• Twelve of the mass shootings had multiple perpetrators.
  • Of those 12 incidents, seven were non-DV-related, three were history of DV mass shootings, and two were DV-related mass shootings.
• Eight of the mass shootings involved female perpetrators, with one of the eight shootings having two female perpetrators.
  • Of those eight incidents, five were DV-related mass shootings, two were non-DV-related, and one was a history of DV mass shooting (although it involved two shooters and it was the male counterpart who had the history of DV, with the female having no known history of DV herself).
• Fifty-five perpetrators of 53 mass shootings died during the incident;
  • 39 (70.9%) died by firearm suicide,
  • 15 (27.3%) were killed by police,
  • one (1.8%) died from an intentional overdose.
• Of the 39 mass shooting perpetrators who died by firearm suicide, 36 (92.3%) were perpetrators of DV-related mass shootings and three (7.7%) were perpetrators of non-DV mass shootings.
• 42 of the 65 perpetrators of DV-related mass shootings (64.6%) died during the incident, with 36 of the 42 perpetrators (85.7%) dying by firearm suicide.
• Of the 15 perpetrators who were killed by police, five (33.3%) were perpetrators of DV-related mass shootings, four (26.7%) were mass shooting perpetrators with histories of DV, and six (40.0%) were perpetrators of non-DV mass shootings.
• The remaining perpetrator who intentionally overdosed in the aftermath of the mass shooting was a perpetrator of a DV-related mass shooting.
• On average:
  • DV-related shootings: 5.0 fatal injuries and 1.0 non-fatal injury
  • History of DV shootings: 10.5 individuals killed and 9.0 people non-fatally injured
  • Non-DV-related mass shootings: 6.3 fatalities and 3.7 non-fatal injuries
• There were statistically significant differences between the average number of fatalities, non-fatal firearm injuries, and total (fatal and non-fatal) injuries for DV-related and history of DV mass shootings.
• The difference between the average number of fatalities, non-fatal firearm injuries, and total injuries for history and non-DV related mass shootings approached significance.
• Case Fatality Rates:
  • DV mass shootings: 83.7%
  • History of DV: 53.8%
  • Non-DV-related mass shootings: 63.1%
• The CFR for DV-related mass shootings was significantly different from both history of DV and non-DV-related mass shootings.
• In 75 of the 110 (68.2%) shootings analyzed, at least one fatal or non-fatal victim was a partner or family member of the perpetrator or the perpetrator had a history of DV.

Hybrid DV Category Results

• Perpetrators of either DV or history of DV mass shootings killed an average of 5.7 people and non-fatally injured an average of 2.0 individuals.
• The CFR for this hybrid DV category was 73.7% compared to 63.1% for non-DV-related mass shootings.

Discussion

• Between 2014 and 2019, 68.2% of mass shootings involved a perpetrator who shot/killed a partner/family member or had a history of DV.
• DV-related mass shootings had a CFR of 83.7%, meaning only 16.3% of victims survived, compared to 46.2% for history of DV and 36.9% for non-DV related shootings.
• The hybrid DV category had a CFR of 73.7%.
• DV-related mass shootings saw a 32.6% increase in CFR compared to non-DV related.
• The hybrid DV category resulted in a 16.8% increase in CFR compared to non-DV related mass shootings.

Potential Explanations for Higher CFR in DV-Related Mass Shootings:

• The intent of the perpetrator may differ, with a greater intent to ensure all victims are killed in DV-related shootings (Zeoli 2018).
• Motives for DV-related mass shootings may include revenge, jealousy, power/controlAssertion, divorce, financial problems, or suicidality (Auchter 2010; Kelley 2009; Zeoli 2018).
• DV-related mass shootings may be more targeted than non-DV-related shootings.
• Non-DV-related mass shootings may have less clear motives and indiscriminate shooting.
• For the deadliest mass shootings, the driving motive was likely related to religion or race/ethnicity.
• Potentially unclear motive and/or indiscriminate shooting may be one explanation for why, on average, fewer victims of non-DV-related mass shootings died from their wounds.
• There are likely a number of factors that could explain this that were not controlled for in the current study, including type of firearm used, location and density of the mass shooting venue, location of wounds, and emergency services and law enforcement response time.

Implications

• This paper highlights the importance of including both “public” and “private” mass shootings in discussions around preventing these incidents.
• By only focusing on “public” mass shootings, many DV-related mass shootings may be left out of the discussion and missed opportunities for intervention.
• The results of this paper, that most mass shootings are related to domestic violence, highlights the need to focus on mass shootings more broadly.
• Restricting access to guns by perpetrators of DV reduces IPH.
• Civil domestic violence protective orders (DVPOs) that cover dating partners (13%), prohibit firearm possession for temporary orders (13%), or require firearm relinquishment (12%) are all associated with reductions in IPH (Zeoli et al. 2018).
• Effective enforcement of these laws is key to ensure that those prohibited because of a DVPO cannot obtain guns.
• Extreme risk protection orders (ERPOs), an evidence-based mechanism to temporarily remove firearms from individuals who are a threat to themselves or others.
• This study shows that most perpetrators of DV-related mass shootings died by suicide, highlighting that DV-related mass shooting perpetrators may be at an elevated risk for suicide.

Comparison to Prior Research

• Zeoli and Paruk (2019) found that the average number of fatal victims was higher for cases where there was a mention of DV, while this study found the opposite.
• Zeoli and Paruk (2019) found that 31.5% of the shooters in their study had histories of domestic violence. By creating a hybrid category that included both DV-related and history of DV cases, this study found that in 68.2% of mass shootings between 2014 and 2019, the perpetrator either killed a family member or intimate partner in the mass shooting incident or had a history of DV.
• The current paper’s findings show that the vast majority of mass shootings in the United States are related to domestic violence and while, on average, DV-related mass shootings result in fewer fatalities, fewer victims of DV-related mass shootings survive compared to victims of non-DV related mass shootings.

Limitations

• Cross-sectional study, which examines associations and cannot be used to assess causality.
• The GVA relies primarily on news reports to build its database, which is likely to result in an undercounting of the true incidence of mass shootings in the U.S.
• The relationship between the perpetrator of a mass shooting and the victims was not always known which could have introduced misclassification into our data.
• There is potential for measurement error that could have biased our findings.
• GVA updates data in real time and, as a result, there may be victims of mass shootings who did not die immediately and therefore were not recorded in the original death count of the shooting.
• Changes in the number of mass shooting deaths could affect how a mass shooting was classified for the purposes of this study.
• Future work should focus on differences in the location of shootings that are DV-related versus those that are not DV-related.
• The CFRs should be interpreted with caution because the definition of a mass shooting was restricted toward those where four or more people were killed, potentially inflating the CFRs.
• Future research should explore differences in CFRs across categories using an expanded definition of mass shootings.
• Future research should continue to examine the role that policies that disarm or otherwise restrict access to guns by perpetrators of intimate partner violence (IPV) or DV have in reducing or preventing mass shootings.

Conclusion

• DV, whether directly related or through a perpetrator’s history, plays an important role in mass shootings in the United States.
• DV-related mass shootings were associated with fewer casualties but a higher CFR, highlighting the lethality of these events.
• Increased focus should be placed on disarming and restricting access to guns by perpetrators of IPV and DV.