The Science of Gun Policy: A Critical Synthesis of Research Evidence on the Effects of Gun Policies in the United States, Fourth Edition

Short abstract

This study, now in its fourth edition, synthesizes available scientific evidence for the effects of various firearm policies on firearm injuries and deaths, violent crime, suicides, the gun industry, defensive gun use, and other outcomes. The authors highlight policies whose effects are more rigorously and reliably demonstrated by existing research, as well as areas where better information could contribute to establishing fair and effective gun policies.

Abstract

In this study, part of RAND's Gun Policy in America initiative, researchers seek objective information about what scientific literature reveals about the likely effects of various gun laws. In the fourth edition of this study, the authors incorporate more-recent research in their synthesis of the available scientific data regarding the effects of 18 state firearm policies on firearm injuries and deaths, violent crime, suicides, the gun industry, defensive gun use, and other outcomes. By highlighting where scientific evidence is accumulating, the authors hope to build consensus around a shared set of facts that have been established through a transparent, nonpartisan, and impartial review process. In so doing, they also illuminate areas in which more and better information could make important contributions to establishing fair and effective gun policies.

In 2016, RAND launched the Gun Policy in America initiative, a unique attempt to systematically and transparently assess available scientific evidence on the real effects of gun laws and policies. Our goal has been to create resources where policymakers and the general public can access unbiased information that informs and enables the development of fair and effective policies. Through this initiative, we released the first edition of this study (RAND Corporation, 2018), which synthesized the available scientific data on the effects of 13 classes of firearm policies on firearm deaths, violent crime, the gun industry, participation in hunting and sport shooting, and other outcomes.¹ In 2020, a second edition was released in which we expanded the classes of policies considered to 18, and in which we updated evidence for all laws with newly published research through 2018 (Smart et al., 2020). A third edition updated our findings and conclusions with research published through October 2020 (Smart et al., 2023). This fourth edition updates our findings and conclusions with research published through February 2023.

Methodology

We followed similar systematic review procedures used for the third edition of this study (Smart et al., 2023). We focused on the empirical literature assessing the effects of 18 classes of firearm policies on any of eight outcomes, which include both public health outcomes and outcomes of concern to many gun owners. We reviewed scientific studies that have been published since 1995, a date chosen to correspond to when such econometric tools as cluster adjustment for serial correlation in panel data—which are now understood to be critically important for estimating the significance of the effects of gun policies—became available in standard statistical packages (Aneja, Donohue, and Zhang, 2014; Helland and Tabarrok, 2004; Schell, Griffin, and Morral, 2018). Our results for this fourth edition now cover the literature published between January 1995 and February 2023.

The 18 classes of gun policies considered in this research are as follows:

Policies regulating who may legally own, purchase, or possess firearms

• minimum age requirements

• prohibitions associated with mental illness

• prohibitions associated with domestic violence

• surrender of firearms by prohibited possessors

• extreme-risk protection orders

Policies regulating firearm sales and transfers

• background checks

• licensing and permitting requirements

• waiting periods

• firearm safety training requirements

• lost or stolen firearm reporting requirements

• firearm sales reporting, recording, and registration requirements

• bans on the sale of assault weapons and high-capacity magazines

• bans on low-quality handguns

Policies regulating the legal use, storage, or carrying of firearms

• stand-your-ground laws

• child-access prevention (CAP) laws

• concealed-carry laws

• gun-free zones

• laws allowing armed staff in kindergarten through grade 12 (K–12) schools.

The eight outcomes considered in this research are as follows:

• suicide

• violent crime

• unintentional injuries and deaths

• mass shootings

• police shootings

• defensive gun use

• hunting and recreation

• gun industry.

Policy Analyses, by Outcome

Building on our earlier review (Smart et al., 2023) and using standardized, explicit, and pre-registered criteria for determining the strength of evidence that individual studies provide for the effects of gun policies, we produced research syntheses that describe the quality and findings of the best available scientific evidence.² Each synthesis defines the class of policies being considered; presents and rates the available evidence; and describes what conclusions, if any, can be drawn about the policy's effects on outcomes.

In many cases, we were unable to identify any research that met our criteria for considering a study as providing minimally persuasive evidence for a policy's effects. Studies were excluded from this review if they offered only weak correlational evidence for a possible causal effect of the law, such as showing that states with a specific law had lower firearm suicides at a single point in time than states without such a law. Correlations like these can occur for many reasons other than the effects of a single law, so this kind of evidence provides little information about the effects attributable to specific laws. We did not exclude studies on the basis of their findings, only on the basis of their methods for isolating causal effects. Specifically, our methodological inclusion criteria required, at a minimum, that studies include time-series data and use such data to establish that policies preceded their apparent effects (a requirement for a causal effect) and that studies include a control or comparison group (to demonstrate that the purported causal effect was not found among those who were not exposed to the policy).

For studies that met our inclusion criteria, we summarized key findings and methodological weaknesses, when present, and provide our consensus judgment on the overall strength of the available scientific evidence. We did this by establishing the following relativistic scale describing the strength of available evidence:

1. No studies. This designation was made when no studies meeting our inclusion criteria evaluated the policy's effect on the outcome.

2. Inconclusive evidence. This designation was made when studies with comparable methodological rigor identified inconsistent evidence for the policy's effect on an outcome or when a single study found only uncertain or suggestive effects.

3. Limited evidence. This designation was made when at least one study meeting our inclusion criteria and not otherwise compromised by serious methodological problems reported a significant effect of the policy on the outcome, and no studies with equivalent or stronger methods provided contradictory evidence.

4. Moderate evidence. This designation was made when two or more studies—at least one of which was not compromised by serious methodological weaknesses—found significant effects in the same direction, and contradictory evidence was not found in other studies with equivalent or stronger methods.

5. Supportive evidence. This designation was made when at least three studies not compromised by serious methodological weaknesses found suggestive or significant effects in the same direction using at least two independent data sets.

These ratings are meant to describe the relative strengths of evidence available across gun policy research domains and should not be interpreted as ratings of our absolute confidence in the reported effects. For instance, when we find supportive evidence for the conclusion that CAP laws reduce self-inflicted injuries and deaths, we do not mean to suggest that it is comparable to the evidence available in more-developed fields of social science. That is, in comparison with the evidence that smoking causes cancer, for example, the evidence base in gun policy research is very limited. Additionally, our ratings reflect the relative strength of evidence within a particular outcome domain. For example, a designation of limited evidence of the effects of a given policy on suicide may represent a stronger body of evidence than a designation of limited evidence of the effects of that same policy on mass shootings because of the particular challenges that arise in evaluating policy effects for sparse outcomes (such as mass shootings; Smart and Schell, 2021). Nevertheless, we believe that such research may be valuable to the public and policymakers to understand which existing laws have more or less persuasive evidence concerning the effects the laws are likely to produce.

Table 1 summarizes our evidence ratings for all policy and outcome pairings. Several outcomes show multiple ratings, and these correspond to different characterizations of the specific policy-outcome association. For instance, we identified limited evidence that waiting periods reduce total suicides and moderate evidence that they reduce firearm suicides.

Table 1.

Strength of Evidence Across Gun Policies and Outcomes

Outcome	Minimum Age Requirements		Prohibitions Associated with Mental Illness	Prohibitions Associated with Domestic Violence	Surrender of Firearms by Prohibited Possessors	Extreme-Risk Protection Orders	Back-ground Checks		Licensing and Permitting Requirements	Waiting Periods	Firearm Safety Training Requirements	Lost or Stolen Firearm Reporting Requirements	Firearm Sales Reporting, Recording, and Registration Requirements	Bans on the Sale of Assault Weapons and High-Capacity Magazines	Bans on Low-Quality Handguns	Stand-Your-Ground Laws	Child-Access Prevention Laws	Concealed-Carry Laws		Gun-Free Zones	Laws Allowing Armed Staff in K–12 Schools
	Purchasing	Possessing					Dealer	Dealer and Private										Shall Issue	Permitless Carry
Suicide
Total suicides	↓La	I	I	I		↓L	I	I	I	↓L				I	I	I	↓M, Ib	I	I
Firearm suicides	↓Sc	↓Lc	I	I	I	↓L	I	I	I	↓M			I	I	I	I	↓S, Ib	I	I
Violent crime			↓L		I		I									↑L	I	↑S		I
Total homicides	I	I	I	↓L	I	I	I	↓M	I	↓M	I			I	I	↑S	I	↑S	I
Firearm homicides	I	I	I	↓L	I	I	↓M	↓L	I	↓L			I	I	I	↑S	↓Sd	↑S	I
Intimate partner homicides				↓M, ↑L, Ie	I, ↓Lf					I
Robberies		I		I	I											I	I	↑L
Assaults		I		I	I											I	Id	↑M
Rapes		I		I	I												I	I
Unintentional injuries and deaths		I																I
Unintentional firearm injuries and deaths		I				I											↓L, ↓Sg	I
Mass shootings	I			I	I	I	I	I	↓L	I	I		I	I, ↓Lh		I	I	I	I
Police shootings								I						I					↑L
Defensive gun use																I
Hunting and recreation
Gun industry
Firearm ownership										I						I		I
Firearm manufacturers or retailers							I		I	I				I			I
Firearm purchases									I	I	I		I		I	I		Ii	I
Prices or sales of banned firearms in the short term														↑L

NOTE: I = inconclusive; L = limited; M = moderate; S = supportive. When we identified no studies that met eligibility criteria, cells are blank. ↑ = the policy increases the outcome; ↓ = the policy decreases the outcome.

^aWe concluded that there is limited evidence that higher minimum age requirements for purchasing a handgun may reduce suicides among young adults.

^bWe found moderate evidence for reductions in total suicides among youths, and we found supportive evidence for reductions in firearm suicides and firearm self-injuries among youths. Evidence for effects on outcomes among the full population was inconclusive.

^cWe concluded that there is moderate evidence that minimum age requirements for purchasing a firearm decrease firearm suicides among young people and limited evidence that minimum age requirements for possessing a firearm decrease firearm suicides among young people.

^dWe found supportive evidence that child-access prevention laws reduce firearm homicides or firearm assault injuries among young people. Evidence for effects on outcomes among the overall population was inconclusive.

^eWe concluded that there is moderate evidence that laws establishing firearm prohibitions for individuals subject to domestic violence restraining orders decrease total and firearm-related intimate partner homicides, there is limited evidence that prohibitions associated with stalking misdemeanors increase total intimate partner homicides, and there is inconclusive evidence for how firearm prohibitions for those convicted of stalking or misdemeanor domestic violence affect total and firearm-related intimate partner homicides.

^fWe found inconclusive evidence for the effects of firearm-surrender laws alone but moderate evidence that firearm-surrender laws, when paired with expansions of prohibited-possessor classes, reduce firearm-involved intimate partner homicides. We found limited evidence that firearm-surrender laws, when paired with expansions of prohibited-possessor classes, reduce all intimate partner homicides.

^gWe concluded that there is supportive evidence that child-access prevention laws decrease unintentional firearm injuries and deaths among children and limited evidence that they decrease such injuries and deaths among adults.

^hWe found inconclusive evidence for the effects of assault weapon bans on mass shootings and their fatalities and limited evidence that bans on high-capacity magazines reduce mass shootings and fatalities.

ⁱWe found inconclusive evidence for the effects of may-issue, shall-issue, and permitless-carry laws on firearm purchases.

Rather than concerning how strong a policy's effects are, our findings concern the strength of the available scientific evidence that examines those effects. Thus, even when the available evidence is limited, the actual effect of the policy may be strong. Presumably, every policy has some effect on a range of outcomes, however small or unintended. Until researchers are able to design studies that can detect these effects—studies that may require data that are not currently collected in a reliable way—available evidence is likely to remain inconclusive or limited. Furthermore, unless multiple jurisdictions have implemented a policy and sufficient time has elapsed for outcome data to become available, rigorous and generalizable evidence on the policy's effects is likely to be limited. But concluding that there is limited evidence on the effects of some policies should not be confused with the conclusion that the policies themselves have limited effects. They may or may not have the effects they were designed to produce; available scientific research cannot yet answer that question. Moreover, even a policy with a small effect may nevertheless be beneficial to society or worth its costs. For instance, a policy that reduces firearm deaths by just a few percentage points could save more than 1,000 lives per year. This kind of “small” effect might be very difficult to detect with existing study methods but could represent an important contribution to public health and safety.

Conclusions and Recommendations

Of more than 200 combinations of policies and outcomes that we considered, we found that there is growing evidence to support the effects of certain policies on firearm deaths and crime, but many areas have received little methodologically rigorous investigation. Looking across the columns in Table 1, it is apparent that research into five outcomes is either unavailable or almost entirely inconclusive. It is noteworthy that three of these five outcomes—defensive gun use, hunting and recreation, and the gun industry—are issues of particular concern to gun owners or gun industry stakeholders, such as firearm manufacturers, firearm dealers, hunting outfitters, and firing ranges. The lack of research on a wide range of outcomes makes it difficult or impossible to conduct a comprehensive cost-benefit analysis of the gun policies. For instance, some of the strongest evidence we found suggests that CAP laws could reduce firearm injuries or deaths among children. But restricting access to guns could also prevent gun owners from accessing their weapons in an emergency. The lack of research on defensive gun use means that we do not have a way of directly estimating how the benefits of these laws (in terms of the number of child lives saved) compare with the possible costs (in terms of forgone opportunities for self-defense).

In the section that follows, we summarize the key conclusions and recommendations that can be drawn from the policy-outcome combinations with the strongest available evidence (conclusions 1 through 11). Thereafter, we draw conclusions and recommendations concerning how to improve evidence on the effects of gun policies (conclusions 12 through 16).

Conclusions and Recommendations Based on the Existing Evidence

Our first set of conclusions and recommendations describes the policy-outcome combinations with the strongest available evidence as identified through our review of the existing literature and recommendations for policy based on this evidence.

Conclusion 1. We find supportive evidence, our highest evidence rating, that CAP laws, or safe-storage laws, reduce self-inflicted fatal or nonfatal firearm injuries, unintentional firearm injuries and deaths, and firearm homicides among youth. There is also moderate evidence that CAP laws reduce firearm suicides among young people, and limited evidence that such laws reduce unintentional firearm injuries among adults. The evidence is stronger for negligent-storage laws than for reckless endangerment laws; reckless endangerment laws are sometimes considered a weaker form of CAP law.

• Recommendation 1. States without negligent-storage CAP laws should consider adopting them or other safe-storage laws as a strategy to reduce total and firearm suicides, unintentional firearm injuries and deaths, and firearm homicides among youth.

• Recommendation 2. States implementing CAP or other safe-storage laws should support data collection and investigation of the mechanisms by which CAP laws affect injury outcomes, for example, by investigating effects on gun ownership, storage, and thefts.

Conclusion 2. There is supportive evidence that stand-your-ground laws are associated with increases in firearm homicides and in total homicides.

• Recommendation 3. States with stand-your-ground laws should consider repealing them as a strategy for reducing homicides.

Conclusion 3. There is supportive evidence that shall-issue laws increase total homicides, firearm homicides, and overall violent crime. There is also moderate evidence that shall-issue laws lead to an increase in rates of assault and limited evidence that they increase rates of robbery. Finally, there is limited evidence that permitless-carry laws—which are arguably a form of shall-issue law and are often implemented while maintaining a shall-issue licensing system to ensure license reciprocity with other states—lead to increases in fatal police shootings.

• Recommendation 4. States with shall-issue (including permitless-carry) laws should consider whether other regulations, either through requirements implemented as part of the permitting process or through other aspects of law (e.g., laws that may prevent firearm theft), might ensure that the effects of concealed-carry laws are aligned with public safety.

Conclusion 4. There is supportive evidence that increasing the minimum age required to purchase a firearm above the threshold set by federal law can reduce firearm suicides among young people. There is also limited evidence that higher minimum age requirements for purchasing or possessing a firearm may reduce total suicides among young adults and firearm suicides among young people.

• Recommendation 5: States should consider raising the minimum age to purchase firearms and ammunition above the level of federal requirements as a strategy to reduce firearm suicides among youths.

Conclusion 5. There is moderate evidence that state laws establishing firearm prohibitions for individuals subject to domestic violence restraining orders decrease total and firearm-related intimate partner homicides, and there is limited evidence that laws prescribing a mechanism for ensuring the surrender of firearms by prohibited possessors, when combined with laws expanding the class of prohibited possessors, reduce intimate partner homicides.

• Recommendation 6. States without laws prohibiting gun ownership while individuals are subject to domestic violence restraining orders should consider passing such laws as a strategy for reducing total and firearm-related intimate partner homicides. States should consider the possibility that these laws may be most effective when they can be applied to a wide range of domestic violence cases, when the law ensures that information about the cases is included in databases used to conduct background checks, and when the prohibitions are accompanied by firearm surrender requirements.

Conclusion 6. There is moderate evidence that dealer background checks reduce firearm homicides, moderate evidence that background checks on both dealer and private sales reduce total homicides, and limited evidence that background checks on private sales reduce firearm homicides.

• Recommendation 7. States that do not require background checks for the private sale or transfer of firearms should consider mandating such checks as a strategy for reducing total and firearm homicide rates.

Conclusion 7. There is moderate evidence that waiting periods reduce firearm suicides and total homicides and limited evidence that they reduce total suicides and firearm homicides.

• Recommendation 8. States without waiting-period laws should consider adopting them as a strategy for reducing suicides and homicides.

• Recommendation 9. States should consider regulations that delay access to firearms by those who may be in crisis. Examples of laws that may delay such access and for which there is at least moderate evidence that they reduce total or firearm suicides include minimum age of purchase and possession laws, waiting-period laws, and child-access prevention laws.

Conclusion 8. There is limited evidence that extreme-risk protection order (ERPO) laws may reduce total and firearm suicides.

• Recommendation 10. States without ERPO laws should consider adopting such laws as a potential strategy for reducing total or firearm suicides.

Conclusion 9. There is limited evidence that laws prohibiting the purchase or possession of guns by individuals with histories of adjudicated mental illness or incapacity reduce violent crime.

• Recommendation 11. States that do not require a background check investigating all types of adjudicated mental health histories that lead to federal prohibitions on firearm purchase or possession should consider implementing robust mental health checks, which may reduce rates of gun violence. The most robust procedures involve sharing data on all prohibited possessors with the National Instant Criminal Background Check System.

Conclusion 10. The sparseness of mass shooting data creates challenges for drawing strong inferences about policy effects because it is difficult to detect signal from noise, and low statistical power means that real policy effects are unlikely to be detected as statistically significant and significant effects are likely to be exaggerated in magnitude and may even be wrong-signed. With these caveats, we find limited evidence that permit-to-purchase requirements and bans on high-capacity magazines may reduce mass shootings and their associated fatalities.

Conclusion 11. No studies meeting our inclusion criteria have examined the effects of lost or stolen firearm reporting requirements or laws allowing armed staff in K–12 schools. Only inconclusive evidence exists for the effects of gun-free zones; firearm safety training requirements; firearm sales reporting, recording, and registration requirements; and bans on low-quality handguns.

Conclusions and Recommendations for Improving Gun Policy Research

Considering the findings from our review of the existing literature on the effects of firearm policy changes, we offer the following conclusions and recommendations for improving the evidence base on the effects of gun laws.

Conclusion 12. The modest growth in knowledge about the effects of gun policy since 1995 reflects, in part, the past reluctance of the U.S. government to sponsor work in this area at levels comparable to its investment in other areas of public safety and health, such as transportation safety or opioid overdoses. Particularly over the past decade, researchers have advanced our understanding of gun violence and policy by adopting more-rigorous methodological approaches in several areas, but sustained support of high-quality research is needed to fill gaps in the evidence base.

• Recommendation 12. To improve the development of firearm violence prevention strategies, the federal government should commit to an ongoing program of research funding. This could include investments in firearm research portfolios not only at the Centers for Disease Control and Prevention and the National Institutes of Health but also at the National Institute of Justice and the National Science Foundation at levels comparable with the government's existing investments in research on other threats to public safety and health.

• Recommendation 13. Given uncertainty about whether federal investments in gun policy research will be sustained in future years so as to support a large-scale program of research, private foundations should take further steps to ensure the development of improved data collection and research on gun violence prevention.

Conclusion 13. Rigorous research examining the effects of many state gun policies on defensive gun use, hunting and recreation, and gun industry outcomes is limited or nonexistent.

• Recommendation 14. To improve understanding of outcomes of critical concern to many in gun policy debates, the U.S. government and private research sponsors should support research on how to best measure and evaluate a wider set of outcomes, including defensive gun use, hunting and sport shooting, and gun industry outcomes (e.g., sales, prices, purchasing).

Conclusion 14. The lack of data on (1) gun ownership and availability and (2) guns in legal and illegal markets severely limits researchers' ability to determine the mechanisms by which policies have their effects, understand policy implementation and enforcement, and answer basic scientific questions about the relationships between policies, firearm availability or access, and public health and safety.

• Recommendation 15. To make important advances in understanding the effects of gun laws, the Centers for Disease Control and Prevention or another federal agency should resume collecting voluntarily provided survey data on gun ownership, acquisition, and use with sample sizes sufficient to support state- or substate-level analyses.

• Recommendation 16. To foster a more robust research program on gun policy, Congress should consider whether to eliminate or loosen the restrictions it has imposed on the use of detailed gun trace data for research purposes.

• Recommendation 17. Federal agencies and private foundations should consider substantial funding for a dedicated research program to improve data infrastructure to track illegal firearm markets and jurisdictional policies and practices focused on those markets.

Conclusion 15. Monitoring systems for crime, victimization, and nonfatal firearm injuries are incomplete and not yet fulfilling their promise of supporting high-quality gun policy research in the areas we investigated.

• Recommendation 18. Policing agencies not yet contributing crime data through the National Incident-Based Reporting System (NIBRS) should prioritize full participation in the program. States can support the transition by providing technical and financial support to local agencies, subsidizing updated record management systems, and establishing reporting standards or mandates.

• Recommendation 19. The Federal Bureau of Investigation's Criminal Justice Information Services Division should modify the NIBRS data collection system to allow agencies to report data on gunshot wounds sustained by crime victims, offenders, and law enforcement officers, as well as incidents during which a firearm was discharged.

• Recommendation 20. The Bureau of Justice Statistics' (BJS's) plan to generate crime victimization estimates for the 22 most populous states is a reasonable compromise between cost and the public's need for more-detailed information. BJS should work to facilitate access to and use of these estimates and the National Crime Victimization Survey microdata and consider whether a small grant program could support researcher use of these data. In addition, BJS should continue to expand its development of model-based victimization rates for all states and for a wider set of victimization experiences (including, for instance, crimes involving firearm use by an assailant or victim).

• Recommendation 21. The Agency for Healthcare Research and Quality at the U.S. Department of Health and Human Services should publish aggregated annual or quarterly state-level estimates of firearm-related injuries from its Healthcare Cost and Utilization Project data sets in an easy-to-use format, akin to what the agency does for opioid-related hospitalizations or emergency department visits.³ Ideally, data could be analyzed by whether an in-hospital death occurred.

• Recommendation 22. Agencies responsible for other ongoing surveillance efforts that measure nonfatal firearm-related injuries should develop mechanisms for sharing these data with researchers in a manner that could support evaluation of variation across jurisdictions and over time.

Conclusion 16. While a growing number of studies are using more-rigorous methods to evaluate the causal effects of gun policies, the overall methodological quality of research on firearms can be significantly improved.

• Recommendation 23. As part of the Gun Policy in America initiative, we have published a database containing a subset of state gun laws from 1979 to 2021 (Cherney et al., 2022). We ask that others with expertise on state gun laws help us improve the database by notifying us of its errors, proposing more-useful categorizations of laws, or submitting information on laws not yet incorporated into the database. With such help, we hope to make the database a resource beneficial to all analysts.

• Recommendation 24. Researchers, reviewers, academics, and science reporters should ensure that new analyses of the effects of gun policies improve on these studies by persuasively addressing the methodological limitations of earlier studies, such as inadequate attention to causal identification assumptions, problems with statistical power, model overfitting, poorly calibrated standard errors, multiple testing, undisclosed variation in law implementation, and unjustified assumptions about the time course of each policy's effects.

In conclusion, the evidence base has grown substantially since the publication of the first edition of this study, and there are now several policy areas for which we have more robust evidence of the effects—beneficial or harmful—on at least some outcomes. In light of this evidence, decisionmakers may want to reconsider their existing policy regimes and reflect on whether changes to policy are warranted. Political or contextual considerations, as well as ongoing Second Amendment litigation, may make some policy options more feasible or attractive than others. Additionally, because we have limited or nonexistent evidence for how these policies will affect many outcomes that are important to stakeholders, decisions will have to be made without a robust empirical understanding of the potential trade-offs. As evidence mounts in certain areas, however, decisionmakers should take that evidence into consideration.

Still, there is a surprisingly limited base of rigorous scientific evidence concerning the effects of many commonly discussed gun policies. This does not mean that these policies are ineffective; the policies themselves might well be quite effective. Instead, it reflects (1) the absence of scientific study of these policies, (2) insufficient variation in the policies across jurisdictions or over time so as to draw rigorous generalizable insights, (3) inadequate data infrastructure to evaluate effects on important outcomes, or (4) methodological shortcomings in the existing literature that limit rigorous understanding of policy effects. In addition, our review did not cover the full range of policy levers available but rather focused on a set of policies that have been implemented in the U.S. context and, therefore, have proven to be politically and legally feasible, at least in some places. This decision meant that none of the policies we examined would dramatically increase or decrease the stock of guns or gun ownership rates in ways that might produce more readily detectable effects on public safety, health, and industry outcomes. The United States has a large stock of privately owned guns in circulation, estimated to be somewhere between 265 million and 393 million firearms (Karp, 2018; Azrael et al., 2017; Cook and Goss, 2014). Laws designed to change who may buy new weapons, what weapons they may buy, or how gun sales occur will predictably have only a small effect on, for example, homicides or participation in sport shooting, which are affected much more by the existing stock of firearms. Although small effects are especially difficult to identify with the statistical methods common in this field, they may be important. Even a 1-percent reduction in homicides nationally would correspond to more than 2,000 fewer deaths over a decade.

By highlighting where scientific evidence is accumulating, we hope to build consensus around a shared set of facts that have been established through a transparent, nonpartisan, and impartial review process. In so doing, we also mean to highlight areas where more and better information could make important contributions to establishing fair and effective gun policies.

This research was supported by a grant from Arnold Ventures and conducted by the Justice Policy Program within RAND Social and Economic Well-Being.