To know whether a gun policy is fair and effective, we need to determine how it affects outcomes, such as suicide rates and hunting participation. Scientific research could provide the most-reliable proof of these effects.

We reviewed thousands of studies to identify all available evidence for the effects of 18 gun policies on eight outcomes. After excluding studies that did not meet our criteria for establishing a law's effects, we found little persuasive evidence for the effects of most policies on most outcomes.

For eight of the 18 policies, either we found no studies examining the effects on any of the outcomes we considered or the evidence was inconclusive. However, we found some evidence that 10 policies affect one or more of four of the outcomes, as shown below. For example, evidence shows that waiting periods may decrease (brown lines) suicide rates and that concealed-carry laws may increase (teal lines) violent crime. The thicker the line, the stronger the evidence.

For four of the outcomes we studied— defensive gun use, hunting and recreation, mass shootings, and police shootings—we found inconclusive evidence, at best, on the effects of any of the policies. However, these understudied outcomes are often central concerns in gun policy debates. For instance, even though we found that child-access prevention laws have measureable benefits in reducing accidental and intentional self-injuries and deaths, the effects of these laws on defensive gun use—one of the principal objections raised against child-access prevention laws—have not been evaluated rigorously.