After reviewing several thousand candidate studies, we identified 123 that met our inclusion criteria. These studies provided evidence for 47 of the 144 main policy effects we set out to examine (that is, the effects of each of the 18 policies on each of the eight main outcomes). We concluded that there was some evidence of an increase or decrease on an outcome for 13 of the policy effects, there was inconclusive evidence for 34 additional effects, and there were no qualifying studies that had evaluated any of the remaining effects (97). The table below summarizes the strength of evidence and direction (increase or decrease) of the effects that the scientific literature currently provides, with links to detailed syntheses of the available research.

We restricted our analyses to only those studies using methods designed to identify possible causal effects of the policies. For instance, studies that reported simple correlations between gun policies and various outcomes at a single point in time did not meet our inclusion criteria, because such studies provide no evidence that it is the gun policy itself that explains the outcome differences rather than other social, demographic, or historical differences between jurisdictions with and without those policies. After identifying research studies that used methods designed to establish the causal effects of gun policies, a team of RAND methodologists analyzed the studies by applying standardized and explicit criteria for determining the strength of the evidence provided by each. We categorized the scientific evidence on a relativistic scale, shown below.

As part of the RAND Gun Policy in America initiative, we conducted rigorous and transparent reviews of what current scientific knowledge could tell the public and policymakers about the true effects of many gun policies that are frequently discussed in state legislatures. Our first such review, released in 2018, synthesized the available scientific data from studies published between 2004 and 2016 examining how 13 classes of state-level gun policies affect firearm-related deaths, violent crime, the gun industry, participation in hunting and sport shooting, and other outcomes. In 2020, we released an expanded and updated review , which added five new classes of gun policies and extended the period over which we conducted our literature search to now span from 1995 to 2018. There has been a surge of new scientific publications on gun policy since our initial review, and we incorporate those studies in our updated analyses , sometimes drawing new or revised conclusions about the quality of evidence available to support claims about the effects of various policies.

Good public policies are based on facts and data, and the best laws—including gun laws—are written when policymakers understand the effects of the policy on a range of outcomes and can weigh the inherent trade-offs. For gun policies, relevant outcomes can include, among others, the health of the gun industry, individuals’ ability to defend themselves, and homicide and suicide rates. In other words, policymakers need to understand the costs and benefits that different policies are likely to produce for society as a whole, including gun owners, communities wracked by violence, and other affected groups. This is not to say that understanding the true effects of policies is the only information lawmakers need. There are many other considerations as well, such as whether policies are consistent with Second Amendment protections or might infringe on other rights. Nevertheless, understanding the true effects of policies on a variety of outcomes is essential to creating policies that are both fair and effective.

Note: In some cases, we found evidence for more than one sub-outcome for a given policy, and the strengths of the evidence differed across the sub-outcomes. For example, there is supportive evidence that child-access prevention laws decrease unintentional injuries and deaths among children, while there is limited evidence that these laws decrease the same outcome among adults. In this table, we show the highest strength of evidence for a policy and outcome relationship. To see details of all sub-outcomes, click on the associated box in the table.

Across all of the 18 policies that we examined, only two—child-access prevention laws and stand-your-ground laws—had evidence that we classified as supportive, our highest evidence rating, for an effect on a particular outcome. Specifically, there is supportive evidence that child-access prevention laws reduce firearm self-injuries (including suicides) and unintentional firearm injuries and deaths among children In addition, we found supportive evidence that stand-your-ground laws increase firearm homicides.

Child-access prevention laws differ from many of the other policies we considered in our analysis. Most of the others affect either the small proportion of guns that are newly acquired every year (e.g., background checks, waiting periods) or a relatively small proportion of gun owners (e.g., prohibitions that target the mentally ill or domestic violence offenders). Child-access prevention laws, in contrast, are designed to influence how all guns in a state are stored when children could be expected to encounter them. This likely represents a large proportion of all guns because, according to U.S. Census Bureau research published in 2013, one-third of all households in the country have children under age 18, and many more have children as occasional visitors. With such large numbers of guns potentially affected, child-access prevention laws (even with imperfect compliance) may have a greater chance of producing observable effects in population-level statistics than other types of laws do.

As for stand-your-ground laws, in our initial review of the research, we found only limited or moderate evidence for the effect of such laws on total and firearm homicides; however, four new studies meeting our inclusion criteria have since been published, and all of these suggest that stand-your-ground laws elevate homicide rates. Because these laws are designed to empower victims of crime to defend themselves more effectively, it might be suggested that the rise in homicide rates is an intended effect of the laws, if the increases were driven by a surge in justifiable homicides. Although more research is needed to draw definitive conclusions about how much of the increase in homicide rates is attributable to justifiable homicides, there is reason to doubt that justifiable homicides explain the increase that stand-your-ground laws seem to cause. Consider, for instance, that there were a combined 2,201 firearm homicides in 2017 in Florida and Texas, according to data compiled by the Centers for Disease Control and Prevention. Both states have stand-your-ground laws. If the effect size estimates for stand-your-ground laws are correct, then between 144 and 396 of these deaths could be attributable to the laws. But across the entire United States, there are only about 230 justifiable homicides recorded in the Federal Bureau of Investigation’s Supplementary Homicide Reports annually, according to records compiled by the Violence Policy Center. Therefore, many of the additional homicides attributable to the laws in Florida and Texas must be criminal homicides.

We found moderate evidence, our second-highest evidence rating, that dealer background checks reduce firearm homicides. Most of the studies in this area examined the effects of dealer background checks or the combined effects of dealer and private-seller background checks when both are required by a state. Therefore, the evidence base for universal background checks—that is, background checks for all sales, public and private—is quite limited compared with that for the dealer background checks currently required under federal law. Of course, if there is moderate evidence that dealer background checks reduce firearm homicides, it seems likely that extending background checks to private sales of firearms could further reduce those deaths. But we must emphasize that currently available research on this question is insufficient to prove that conclusion.

We also found moderate evidence that waiting periods reduce rates of firearm suicide and total homicide and that some gun possession prohibitions associated with domestic violence reduce intimate partner homicides.

We found evidence that several other policies increase or decrease one of the outcomes examined. To view detailed syntheses of the research that led to each of these findings, as well as the evidence that we categorized as inconclusive, click on the associated box in the table above.

Despite these findings, a large majority of the effects for which we sought scientific evidence have not been investigated with sufficient rigor to be included in our review. Indeed, we found no studies examining the effects of any of the 18 policy types on officer-involved shootings or on hunting and recreation outcomes, just two studies examining how the policies affect defensive gun use, and relatively few studies evaluating effects of the policies on gun industry outcomes. These are all outcomes that are frequently raised as concerns in gun policy debates. Because there is little empirical research examining these outcomes, policymakers have limited ability to use evidence to comprehensively consider how laws are likely to affect different interests.

Does Weak Evidence Mean Gun Laws Don’t Work?

With a few exceptions, 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; they might well be quite effective. Instead, it partly reflects shortcomings in the contributions that science has made to policy debates. It also partly reflects the policies we chose to investigate, all of which have been implemented in some U.S. states and so have proven to be politically and legally feasible (at least in some jurisdictions). 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 would produce more readily detectable effects on public safety, health, and industry outcomes.

Even a 1-percent reduction in homicides nationally would correspond to approximately 1,500 fewer deaths over a decade.

Furthermore, the United States has a large stock of privately owned guns in circulation—estimated by the Small Arms Survey to be more than 393 million firearms in 2017. Laws designed to change who may buy new weapons, which weapons they may buy, or where and how they can use guns will predictably have only a small effect on, for example, homicide rates or participation in sport shooting, which are affected much more by the existing stock of firearms. But 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 approximately 1,500 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.