Gunfire from assaults, suicides, and unintentional shootings exacts an enormous burden on public health globally. The epidemiologic reviews in this special issue enhance our understanding of various forms of gun violence, inform interventions, and help chart directions for future research. The available science, however, is limited to answer many important questions necessary for mounting successful efforts to reduce gun violence. Certain data are lacking, and there are numerous analytical challenges to deriving unbiased estimates of policy impacts. Significant investments in research over the long term are warranted to answer questions central to successful prevention of gun violence.

Firearms violence exacts an enormous burden on the public's health. Annual rates of firearm homicide in 2013 were staggeringly high in certain countries in Central America (e.g., El Salvador and Guatemala, 52.4 and 21.82 per 100,000 population, respectively); South America (e.g., Colombia and Venezuela, 38.1 and 32.7 per 100,000 population, respectively); and Sub-Saharan Africa (e.g., Lesotho and Swaziland, 13.5 and, 8.46 per 100,000 population, respectively). The high levels of violence involving firearms in these regions often have destabilizing effects on entire countries, threatening civil society as well as public safety. Most high-income Western democracies have comparatively low rates of firearm homicide. The United States is the exception; its firearm homicide rate of 3.55 per 100,000 population in 2013 is many times higher than that of any other such nation and is comparable to those of Nicaragua (4.86 per 100,000 population), Palestine (3.09 per 100,000 population), South Africa (3.65 per 100,000 population), and Uganda (3.71 per 100,000 population) (1).

Gun ownership is positively associated with suicide risks at both the household level and the state level, which is likely related to differences in the lethality of means of intentional self-harm (2). In the United States and many other countries, firearm suicides outnumber firearm homicides by a large margin (3). Although more challenging to measure and track than firearm-related deaths, mental and physical nonlethal trauma from gun violence is no less important than is the death toll from the use of firearms.

The role of firearms in violence is debated passionately across political, ideological, and cultural divides as well as among researchers. Synthesizing the epidemiologic evidence will not settle these debates but can enhance our ability to answer important questions and inform efforts to prevent firearm violence. This special issue of Epidemiologic Reviews presents 9 articles that examine the evidence on some of these important questions and critiques the methods by which that evidence was obtained.

One important question is whether the relationship between firearm access and suicide risk is causal or a function of confounders, measured or unmeasured. In their paper, Miller et al. (4) assess whether unmeasured confounding might account for the repeatedly observed positive association between household access to firearms and risk for firearm suicide. In essence, they conduct a global test of the “it's not the gun” hypothesis. Relying on data from 4 case-control studies that met stringent selection criteria, Miller et al. determine the properties such a confounder would need to have to explain an apparent association between firearms and suicide if no independent association existed. Such a confounder, they find, would need to be at least as strong a risk factor for suicide as are major depression and substance use disorders. It would also need to be implausibly more prevalent in case (suicide) households than in control households; the imbalance would need to be larger, by an order of magnitude, than has been observed for any other risk factor. The possibility that such a confounder could exist undetected is vanishingly small.

Three articles touch on the topic of substance use and risks connected to firearms. In the broadest of all reviews, Chen and Wu (5) present a narrative review on the relationship between different types of substance use (i.e., alcohol, tobacco, marijuana, cocaine/crack, drugs in general) and 4 types of behaviors or exposures involving firearms (access, unsafe handling, carrying, and violence). Branas et al. (6) conducted a review and meta-analysis on the relationship between alcohol use and firearms, while McGinty et al. (7) examined the relationship between use of controlled substances and violence.

Most studies report positive bivariate associations between substance abuse and firearm-related risks. Some may assume that there is a causal relationship; however, confounding arose as an important concern across the 3 reviews. Researchers have commonly considered youth gun carrying to be a part of a set of risky behaviors that include illicit drug and alcohol use but also include other deviant behaviors (e.g., stealing, fighting) and criminal social networks. Substance use, illegal gun carrying, and violence may result from other common personality traits or environmental conditions. Indeed, in many cases, associations disappeared or showed a marked decrease with adjustment for psychiatric disorders, especially antisocial personality disorder as noted in the reviews by Chen and Wu (5) and McGinty et al. (7).

Important concerns about causal inference pervade research in this area. In addition to inadequate controls for potential confounders, there was a limited number of prospective studies that appropriately considered the temporal relationship between substance use and gun-related behaviors. Those studies that were prospective included long lag periods between measures of substance use and violence, making it difficult to address intervening sources of time-dependent confounding and to assess potential psychopharmacological effects of alcohol and controlled substances. A more robust and potentially causal association may exist between use of controlled substances and suicide, as well as between selling illicit drugs and perpetration of gun violence.

Proving cause and effect, in the case of substance abuse and gun violence, may be less important for some interventions than others. If substance abuse and selling illicit drugs are highly correlated with gun violence, policies to prevent access to firearms by substance abusers and drug sellers may reduce gun violence even if the underlying risk is more tied to antisocial personality disorders that cannot be practically identified when screening individuals applying to purchase or carry concealed firearms.

Unfortunately, few data exist on the impact that interventions directed at substance abuse have on firearm-related activities. Only 1 study across the 3 reviews evaluated this question. A clinical trial found that a collaborative care intervention that targeted substance abuse along with post-traumatic stress syndrome and depression among adolescents admitted to a US level I trauma center for injuries significantly lowered risk of weapon carrying at 12 months after injury (8). There have been no studies of the impact that firearm disqualifications based on indicators of substance abuse, such as prior driving convictions or substance use-related misdemeanor convictions, might have had on firearm violence. These reviews suggest that studies designed to assess the specific relationship between substance use and firearm violence, using a prospective design and accounting for key confounders, as well as evaluations of policies that disqualify individuals from purchasing or possessing firearms based on substance use history, are sorely needed.

Most violence involves friends or associates, intimate partners, or family members, which is to say members of the same social network. There are a variety of mechanisms by which one's social network may influence involvement in serious violence: Growing up in a violent home may provide the children with a model for their own subsequent behavior. Violence in an intimate partnership may be chronic and in some cases escalating. Gangs may engender serious violence by providing members with weapons and social norms that promote violence. Although there is an extensive literature on the “cycle of violence” and other social contagion processes, there have been very few studies that explore the particular mechanisms associated with gun violence. Tracy et al. (9) review the peer review literature since 1996. Of these reports, only 3 focus specifically on gun violence, and all of those concern co-offending networks. Those studies, which utilize formal social-network-analysis methods, find that the likelihood of gun victimization or perpetration is predicted by network distance from individuals who use guns. This interesting result suggests the potential of social network analysis to predict gun violence and, hence, guide prevention efforts. The review article serves to identify a vital opportunity for future research, which has largely neglected to focus on the dynamics of social transmission of gun violence despite its predominant role in homicide.

Intimate partner violence is another context in which firearms access can determine the odds of lethal outcomes. Zeoli et al. (10) conducted a narrative review on the risks that firearm access poses for intimate partner violence (IPV) and the impact of interventions designed to reduce firearm violence in intimate relationships. In 2013, 50% of IPV homicides in the United States were committed with firearms (11). Studies on the risk of IPV posed by firearm access are mostly cross-sectional, making it impossible to conclude whether firearm access led to abuse. Measurement of firearm access by the perpetrator also poses an important challenge. Among those in abusive relationships, perpetrator access to firearms is associated with IPV severity and lethality. Intervention studies show evidence that firearm prohibitions for individuals with domestic violence restraining orders are associated with reductions in intimate partner homicide. In contrast, firearm prohibitions for convictions for domestic violence misdemeanor crimes and laws that authorize law enforcement officers to confiscate firearms from the IPV scene were not associated with intimate partner homicide risks. There have been few data collected on the enforcement of these laws or research on the effects of enhanced enforcement on the risk of IPV involving firearms.

Another article in this special issue takes on the ambitious task of reviewing research on the effects of gun laws globally, presenting a systematic review of data from 130 studies of gun laws in 10 countries published from 1950 to 2014 (12). Unfortunately, the bulk of these studies had serious limitations that make it difficult to draw unequivocal conclusions. Deriving unbiased estimates of the effects of gun laws is a particularly challenging undertaking because of potential biases from selection and historical confounds. Decisions about whether or not to enact gun laws may depend on levels and trends in gun violence as well as on levels of gun ownership. Nearly all of the studies reviewed are ecological and typically do not isolate the target of the firearm policy with the outcome being measured. For example, background check and licensing requirements for purchasing or carrying firearms should have different effects on individuals' capacity for violence depending on whether they meet legal requirements for purchasing or carrying firearms, but studies examine only aggregate levels of homicide or other violent events.

Santaella-Tenorio et al. (12) note these challenges and others pertinent to evaluations of gun laws. Many are illustrated in the evolution of studies of so-called “shall issue” laws that loosened or eliminated impediments to legal carrying of concealed firearms. Initial studies suggested that shall issue laws reduced homicides, while subsequent studies identified a number of errors in the data and in the analyses. Once those errors were corrected, no effects of shall issue laws on homicides were evident. A few areas showed some consistency with reasonably strong methods. In addition to the protective effects of domestic violence restraining order laws, child access prevention laws requiring gun owners to make guns inaccessible to unsupervised youth are associated with reduced rates of firearm suicides and unintentional shooting deaths. Some gun laws may be too weak or have important gaps that prevent their effectiveness. For example, the US federal law requiring background checks for firearm purchases that applied only to sales by federally licensed gun dealers did not reduce homicide rates, but a study of the repeal of a Missouri law requiring background checks and purchaser licensing for all handgun sales suggests that the law had suppressed firearm homicide rates before its repeal. More comprehensive firearms regulations adopted at the national level also showed promise in lowering homicide rates, although forecasting a counterfactual for what would have occurred absent the new regulations is particularly challenging with national studies.

Two systematic reviews will facilitate efforts by clinicians to prevent firearm violence as part of their patient care. Roszko et al. (13) provide an overview of such prevention efforts. They review 53 articles on clinician attitudes toward and actual adoption of practices that encourage firearm safety among patients during health-care delivery. They find that such practices are uncommon, even when clearly indicated. Several specific and modifiable factors are associated with failure to adopt such practices, and the available evidence suggests that a variety of training and educational approaches can improve clinician performance. Their particular focus is on 12 additional studies, including 6 randomized trials, of interventions to increase safe firearm behavior among patients. Interventions that focused only on firearms, rather than including firearms among other topics, showed evidence of effectiveness.

Rowhani-Rahbar et al. (14) address safe storage interventions, based on results for 4 randomized trials and 3 quasi-experimental studies. Three interventions provided a free storage device. All 3 yielded beneficial results, even though they differed substantially in design and implementation. Of the 4 interventions that did not provide free storage devices—including those that offered discounts or other incentives to purchase such devices—only 1 showed positive effects. This article includes an excellent discussion of the theoretical considerations that should inform future efforts to encourage safe practices related to firearm storage and use.

Both papers emphasize the need for additional, and more rigorous, research on the best ways to incorporate firearm violence prevention efforts into many aspects of health care. Given increasing support for such efforts by medical professional organizations and others (15), that recommendation is very timely.

We believe that the research reviewed and critiqued in this special issue will be valuable to researchers, practitioners, and policymakers as they seek to better understand and prevent gun violence and injury. Given the magnitude of this public health problem, interventions should be informed by the best available research. Importantly, we should strive to advance the science by addressing the gaps and limitations described above and in these reviews. As with any area with the scope and complexity of gun violence, this will require a greater investment of resources over the long term than have been devoted to date.

ACKNOWLEDGMENTS

Author affiliations: Center for Gun Policy and Research, Department of Health Policy and Management, Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland (Daniel Webster); Department of Epidemiology, Mailman School of Public Health, Columbia University, New York, New York (Magdalena Cerdá); Sanford School of Public Policy, Durham, North Carolina (Philip Cook); Violence Prevention Research Program, School of Medicine, University of California at Davis, Sacramento, California (Garen Wintemute).

Conflict of interest: none declared.

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Author notes

Abbreviation: IPV, intimate partner violence.