Potential Determinants of Mass Shooting Perpetration and Casualties: A Systematic Review

1. Assistant Professor, Department of Epidemiology, Faculty of Medicine, Prince of Songkla University, Hat Yai, Songkhla, Thailand.

Abstract

Introduction: In the United States (US), mass shootings could be regarded as a public health issue due to their ubiquitous and public nature. Social-ecological models, commonly used to explain health behaviours, can contribute to the understanding of potential determinants of mass shootings based on current evidence.

Aim: To conduct a systematic review on factors associated with the perpetration of mass shooting events and casualties (injuries or fatalities) at mass shooting events.

Materials and Methods: The author searched the literature on PubMed using the term “mass shooting”. Inclusion criteria included titles containing the words “shoot*,” “firearm*,” or “gun*,” having an abstract or full text, and the study being either primary research or secondary data analysis on the perpetration or casualties of mass shootings.

Results: The author reviewed 36 articles, nearly all of which focused on mass shootings in US. For the perpetration of mass shootings, gun ownership and access were present at all levels of the social-ecological model. Other factors for perpetration included mental illness, relationship issues, and permissive firearm laws. Mass shooting casualties were associated with the use of high-powered firearms at the intrapersonal level and permissive firearm laws at the policy level.

Conclusion: Firearms-related factors were associated with both the perpetration and casualties of mass shootings at various levels of the social-ecological model. However, caveats, including a limited time frame of the review, limited sources of publications, and subjectivity in building the social-ecological model, should be considered in the interpretation of the study findings.

Aggression, Criminal behaviour, Firearms, Social-ecological model

On May 14, 2022, an 18-year-old male entered Tops Friendly Markets supermarket in the city of Buffalo, New York, USA, and opened fire with a Bushmaster AR-15 style semi-automatic rifle, killing 10 people. His apparent motivation was linked to white supremacy (1). Ten days later, another 18-year-old male entered Robb Elementary School in the city of Uvalde, Texas, USA, and opened fire with a Daniel Defense AR-15 style semi-automatic rifle, resulting in the deaths of 19 school children and two teachers (2). These incidents are categorised as mass shootings, which are typically defined as instances of gun violence unrelated to armed conflicts or domestic violence, resulting in three or more casualties (injuries or deaths), excluding the perpetrator(s) (2).

While mass shootings can occur in various countries, the US stands out as the only country in the Organisation for Economic Cooperation and Development (OECD) where public mass shootings occur every year, accounting for over 70% of such incidents in high-income countries (3). Additionally, mass shootings in the US often take place in public spaces. For instance, the deadliest mass shooting incident occurred in 2017 in Las Vegas, when a 64-year-old male, armed with 24 rifles, opened fire on attendees of the Route 91 Harvest Country music festival from his hotel room window. This incident resulted in the deaths of 58 people and injuries to more than 500 individuals (4).

Considering the significant increase in gun manufacturing and import in the US over the past decade (5), as well as the country’s highest civilian firearm ownership rate in the world at 120 firearms per 100 residents (6), it can be argued that no population subgroup is immune to gun violence and mass shootings. Thus, mass shootings (and by extension, gun violence) can be viewed as public health issues in the US. As public health issues, efforts to prevent mass shootings should entail both primary prevention (preventing the perpetration of mass shootings) and secondary prevention (minimising casualties during such events). Taking a public health perspective, the perpetration and casualties of mass shootings should not only be attributed to the characteristics of the perpetrators themselves, as portrayed in the media and certain literature, but also to social and environmental factors. Thus, the social-ecological model for health behaviours can be a valuable framework for examining the determinants of mass shootings, enabling relevant stakeholders to consider this phenomenon in a more comprehensive manner (7).

In response to the incidents in Uvalde and the increasing frequency of mass shootings, bipartisan agreement has been reached among US legislators regarding federal-level firearm safety laws (8). Conducting a systematic review on the determinants and casualties of mass shootings, along with interpretation using the social-ecological model, has the potential to inform relevant stakeholders and the public about the alignment between proposed firearm safety policy legislation and the body of empirical evidence. It can also shed light on whether there are additional determinants of mass shootings that should be considered in future agendas. The objective of the present review was to assess the factors associated with the perpetration of mass shooting events and the number of casualties (injuries or fatalities) at such events.

Material and Methods

Population, Intervention, Control, Outcomes (PICOs): Population= Mass shooting events;

Intervention=Characteristics at various levels of the social-ecological model;
Control=Not applicable;
Outcomes=1) perpetration of mass shootings; 2) number of casualties.

Inclusion criteria: Studies with the titles containing the words “shoot*,” “firearm*,” or “gun”, articles with abstracts or full text and primary research or secondary data analysis on the perpetration or casualties of mass shootings were included in the present review.

Exclusion criteria: Content unrelated to determinants or consequences of mass shootings, commentary or perspective articles, narrative reviews, systematic reviews, corrections or errata, book reviews, descriptions or treatments of direct casualties of mass shootings, descriptions or treatments of indirect casualties of mass shootings, studies pertaining to health system responses to mass shootings and those not conducted among victims, shooters, or healthcare workers were excluded from the present review.

Study Procedure

The present review sourced its information from PubMed Central (PMC).

Search strategy: On June 2, 2022, the author conducted a literature search on PubMed Central (PMC). The search was limited to articles published in English from January 1, 2017, to the date of the search. The search term used was “mass shooting.” The author chose 2017 as the starting year due to the deadliest mass shooting in US history occurring that year.

Selection process: The author excluded duplicate articles, screened the titles of the searched articles, excluded articles without abstracts or full text available, and then individually assessed each article for eligibility to be reviewed in full based on the eligibility criteria. The process and the number of publications excluded at each step can be found in (Table/Fig 1).

Data collection process: The author read the content of the eligible articles, summarised the findings, and assigned a unique ID to each article for ease of reference during interpretation and summary.

Synthesis methods: The author applied the social-ecological model (7) to identify factors associated with the perpetration and casualties of mass shooting events. Separate analyses were conducted for factors associated with the perpetration of mass shooting events and factors associated with the casualties of mass shooting events. The present systematic review did not involve the use of human subjects.

Results

Study flow and characteristics of included studies: Initially, the author found 336 articles through a search on PubMed, of which one was a duplicate and subsequently removed (Table/Fig 1). The author screened the titles of the remaining 335 articles and excluded 153 articles that did not contain the words “shoot*,” “firearm*,” or “gun*” in the title. The author then attempted to retrieve the remaining 182 articles, but 12 of them did not have an abstract or full text available. The author assessed the remaining 170 articles for eligibility and excluded 134 articles for various reasons, ultimately including 36 research articles in the review and assessment using the social-ecological model.

All but two of the reviewed articles focused on mass shootings in the US. One article analysed non war mass murders worldwide, including those not involving firearms, while another article examined Australian mass shooting incidents and offenders from 1964 to 2014 (Table/Fig 2) [9-44]. All articles were ecological, cross-sectional, or panel studies. One study, which described itself as a “cohort study,” did not follow a defined cohort but instead examined the location of mass shootings relative to schools and places frequented by children.

Factors associated with the perpetration of mass shootings: Intrapersonal factors associated with mass shootings included mental illness and experiences of acute life stressors (Table/Fig 3). Intrapersonal issues included mental illness, behavioural problems, and gun ownership were identified. Interpersonal issues related to behavioural problems, such as family dysfunction and estrangement, were also positively associated with the perpetration of mass shootings. Gun ownership and access to firearms were present at the community level. At the policy level, firearm safety measures had a negative (preventive) association with mass shooting perpetration.

The term “positive” refers to a factor being positively associated with the outcome (i.e., the higher the level of the factor, the higher the likelihood of mass shooting perpetration). The term “negative” refers to a factor being negatively associated with the outcome (i.e., the higher the level of the factor, the lower the likelihood of mass shooting perpetration). The term “neutral” indicates that a factor did not have any statistically significant association with the outcome (i.e., no association).

Factors associated with mass shooting casualties: At the intrapersonal and interpersonal levels, firearm characteristics were associated with casualties (Table/Fig 4). Shootings that involved semi-automatic or automatic firearms and the use of multiple firearms had higher mean fatalities compared to shootings that involved only handguns or a single weapon. Mass shooters who targeted women based on gender or were ideologically motivated tended to be more patient and lethal than disgruntled employees. At the organisational and community levels, most data pertained to school shootings. The presence of armed officers at schools did not appear to be preventive against casualties. At the policy level, firearm safety measures, including the FAWB and LCM ban, were associated with lower casualties at mass shootings.

The term “positive” refers to a factor being positively associated with the outcome (i.e., the higher the level of the factor, the higher the number of mass shooting casualties). The term “negative” refers to a factor being negatively associated with the outcome (i.e., the higher the level of the factor, the lower the number of mass shooting casualties). The term “neutral” indicates that a factor did not have any statistically significant association with the outcome (i.e., no association).

Discussion

In the present systematic review, the author systematically reviewed factors associated with the perpetration and casualties of mass shooting events. Intrapersonal, interpersonal, organisational, community, and policy factors were identified as being associated with mass shootings. These findings are particularly relevant given the increasing frequency of mass shootings in the US. As the author used the social-ecological model to describe the factors associated with the outcomes, the discussion of the study findings should also be disaggregated according to the levels of the model.

Intrapersonal factors: At the intrapersonal level, a history of mental illness and a lack of or discontinuation of treatment were commonly mentioned factors. Other reports indicated a progression of devolving behaviours in the shooter leading upto the mass shooting event, which ultimately involved the acquisition of firearms. The event would be more fatal if it involved semi-automatic firearms or shotguns. However, it is important to refrain from stigmatising individuals with mental illnesses or estrangement behaviours, as the vast majority of the nearly 53 million adults with mental illness in the US do not have plans to harm others, especially on a massive scale (45). The study findings should only be used as an attempt to understand the phenomenon in a complete manner.

Interpersonal level: Problems in interpersonal relationships seemed to be a recurring theme in the perpetration of mass shootings. Family problems, including domestic violence, separation, or divorce, and rejection from others, were common among mass shooters. Domestic violence was also associated with the lethality of mass shootings. Considering that mental health problems are associated with family/relationship issues (10) and that family/relationship issues can lead to mental health problems (46), it is not possible to ascertain whether a variable was a risk factor or a pathway variable between another risk factor and the outcomes.

Organisational level: Perpetration of mass shootings was associated with work or school problems. Further investigation is recommended to understand the extent to which management practices or policies of the workplace or school contribute to these problems, which in turn contribute to the perpetration of mass shootings. Concerningly, the presence of armed police officers at schools had the opposite effect of what was intended, with no reduction or even an actual increase in casualties at school shooting events. Data from another source (47) showed that weapons-related arrests accounted for only 0.1% to 0.2% of all arrests made by school resource officers (school police officers), and the majority were for disorderly conduct that unnecessarily funneled students into the juvenile justice system. Given the consistency of the evidence, relevant stakeholders should reconsider the presence of armed guards or police officers in schools.

Community level: Factors at the community level appeared to be coherent and complementary to the factors identified at other levels. There were variations in mass shootings with regards to time, place, and demographics of the shooting locations. Associations between the perpetration and casualties of mass shootings were also clear: states with higher gun ownership and access to assault weapons and ammunition had higher incidents of mass shootings compared to states that were less permissive of firearms. With such clear patterns of association, local government and community leaders should consider increasing firearm safety measures where possible.

Policy level: There were clear patterns of association between firearm safety measures and the perpetration and casualties of mass shootings. In particular, the FAWB and LCM ban had negative associations with both the perpetration and casualties of mass shootings. Considering that more lethal shootings tended to involve the use of assault weapons with a higher number of ammunitions (34),(39), these policies seemed to directly address these issues. Similarly, other preventive measures such as the emergency protection order and mental health expenditures were also negatively associated with the perpetration of mass shootings, suggesting that other measures should also be adopted concurrently with firearm safety laws.

A relatively new aspect of this review was the inclusion of the social-ecological model to consider mass shootings in a more holistic and systematic manner. Discussion remarks for organisational, community, and policy level factors may have direct policy implications.

Limitation(s)

A number of limitations should be considered in the findings of the present review. Firstly, the author included only manuscripts published since 2017, which is an arbitrary starting point. Systematic reviews conducted under a different approach could have yielded different results. Secondly, the level in the social-ecological model to which each potential determinant belonged was relatively subjective and based on the author’s own judgement. Findings from other authors could have differed from those presented by the author. In the present review, the risks of bias and reporting bias were not seen, and hence more studies should be conducted in the future considering this aspect.

Conclusion

In the present study, the author reviewed factors associated with the perpetration and casualties of mass shootings using the social-ecological model. Gun ownership and access to firearms were associated with the perpetration of mass shootings, whereas use of high-powered firearms and the number of firearms were associated with casualties at various levels of the social-ecological model. However, caveats including a limited time frame, limited sources of publications, and subjectivity in building the social-ecological model should be considered in the interpretation of the study findings.

DOI and Others

DOI: 10.7860/JCDR/2023/61802.18520

Date of Submission: Nov 23, 2022
Date of Peer Review: Mar 15, 2023
Date of Acceptance: Aug 08, 2023
Date of Publishing: Oct 01, 2023

AUTHOR DECLARATION:
• Financial or Other Competing Interests: None
• Was Ethics Committee Approval obtained for this study? NA
• Was informed consent obtained from the subjects involved in the study? NA
• For any images presented appropriate consent has been obtained from the subjects. NA

PLAGIARISM CHECKING METHODS:
• Plagiarism X-checker: Nov 26, 2023
• Manual Googling: Mar 22, 2023
• iThenticate Software: Aug 05, 2023 (16%)

ETYMOLOGY: Author Origin

EMENDATIONS: 7