One of the largest-ever studies of U.S. gun policy finds there is a shortage of evidence about the effects of most gun laws, although researchers from the RAND Corporation found there is some persuasive evidence about the effects of several common gun policies.

The findings are from RAND's sweeping Gun Policy in America initiative, which also evaluated the views of gun policy experts with opposing perspectives on the likely effects of gun laws to identify where compromise might be possible.

RAND researchers evaluated thousands of studies to assess the available evidence for the effect of 13 common gun policies on a range of outcomes, including injuries and deaths, mass shootings, defensive gun use, and participation in hunting and sport shooting. The strongest available evidence supports the conclusion that laws designed to keep guns out of the hands of children reduce firearm self-injuries, suicides and unintended injuries to children.

There is moderate evidence to support conclusions that background checks reduce firearm suicides and firearm homicides, and that laws prohibiting the purchase or possession of guns by individuals with some forms of mental illness reduce violent crime, according to the analysis. There also is moderate evidence that stand-your-ground laws, which allow people to use guns to defend themselves without requiring that they first attempt to retreat, if possible, may increase state homicide rates.

The RAND Gun Policy in America initiative is intended to provide new nonpartisan information to national and local discussions about gun policy.

"The goal of this project is to help build consensus around a shared set of facts about gun policy by demonstrating where scientific evidence is accumulating," said Andrew Morral, the project's leader and a behavioral scientist at RAND, a nonprofit research organization.

Out of the thousands of studies evaluated, the RAND analysis identified 62 that investigated the causal effects of gun polices on any of the outcomes RAND investigated, including those of concern to gun owners and the gun industry, as well as violence, suicide and injury. Most other studies demonstrate only an association between gun policies and outcomes, which offers less-persuasive evidence that the policies caused changes in the outcomes. In many cases, researchers found no studies linking gun policies to the many effects they examined.

"While science can teach us a lot about gun policy, research in this area is generally far behind where it is for most other causes of death that claim similar numbers of lives in the U.S. each year," Morral said. "This does not mean that gun polices have no effects. Most laws probably have some effect, however small or intended. Instead, the limited evidence base reflects shortcomings in the contributions that scientific study has made to the policy debate."

Many of the studies RAND reviewed used weak methods of establishing the effects of gun laws, often because historical information on variations in state gun laws is unavailable or difficult to collect. To encourage more high-quality research, the Gun Policy in America project created a large database of state-level U.S. gun policy laws, covering the period of 1979 to 2016. RAND is making this data set available to researchers and the public, and is using it to develop new estimates of the effects of gun laws that will be released later this year.

Because there is so little scientific evidence to draw on, RAND researchers also surveyed 95 gun policy experts from across the ideological spectrum to identify where there might be consensus or opportunities for compromise. Two groups with opposing views were identified among the experts. One group had views closely aligned with the National Rifle Association, and the other had views aligned with the Brady Campaign to Prevent Gun Violence.

Despite sharp disagreements on their ratings of the overall merits of different gun laws, the two groups of experts were often not far apart in their estimates of the likely effects of laws.

There was comparatively strong agreement between the groups of experts about the positive effects of expanded mental health-related prohibitions, the required reporting of lost or stolen firearms, media campaigns to prevent children from accessing guns, and the required surrender of firearms by those prohibited from possessing guns, such as people convicted of felonies.

The largest disagreements in opinion involved policies that allow the carrying of concealed weapons without permits and the elimination of gun-free zones. The project includes a website visualization tool that allows visitors to explore a wide range of scenarios to see how different combinations of gun policies would affect outcomes nationally and in individual U.S. states, according to the two groups of experts.

"Both groups overwhelmingly favored policies they believed would reduce firearm homicides and suicides, but there is disagreement about which laws would have these effects," Morral said. "Collecting more and stronger evidence about the true effects of laws is a necessary and promising step toward building greater consensus around effective gun policy."

Looking forward, the RAND team recommends that the federal government increase funding for gun research to levels comparable to federal research investments in other significant causes of death and injury, such as automobile accidents. In addition, the focus of research should expand to include the effects that policies have on defensive gun use, gun ownership, hunting and recreation activities, jobs in the gun industry and officer-involved shootings.

"Issues beyond gun violence are often central considerations in gun policy debates, but we identified no qualifying research examining most of them. If we had better information on the effects of gun laws on some of these issues, we would be in a better position to develop fair and effective gun laws," Morral said.

The United States has the highest gun ownership rate in the world, with estimates suggesting that Americans own as many as 300 million guns. Between 10 million and 20 million Americans actively participate in hunting or sport shooting annually, and the gun industry generates $16 billion in revenue and employs hundreds of thousands in gun manufacturing, distribution, sales and recreation.

At the same time, more than 36,000 people died of gunshot wounds in the U.S. in 2015, and Americans are 25 times more likely to die by gun homicide than residents of other wealthy countries.

About two-thirds of gunshot deaths in the U.S. are suicides, while mass shootings accounted for just 0.5 percent of all gun fatalities annually. Despite wide acknowledgement that gun violence levels are too high, little consensus has been reached about what gun policies should be adopted widely.

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RAND has funded the Gun Policy in America initiative -- underway for two years -- using income from its operations and unrestricted philanthropic gifts.