Which gun-control laws work? This is a hard question to answer with research, not least because gun-control science is so politically unpopular that it often suffers from a lack of funding. A review published this month, however, promises some answers.

Daniel Webster of Johns Hopkins University and Garen Wintemute of the University of California-Davis reviewed 28 studies about the effects of laws that require firearms-buyers to apply for licenses and sellers to run background checks, in addition to other checks on who buys guns. The studies represent more than a decade of work by various researchers; they were published anywhere between 1999 and 2014. The review is freely available to read here, but below are some of the top takeaways.

Which laws have the best scientific evidence to support them?

Studies have found that a combination of laws seems have greater effects than individual laws working alone. For example, one study that Wintemute was a part of examined states whose laws included a three-part punch: Gun sellers had to run background checks on buyers; buyers had to apply for a permit; and owners had to register their firearms. Compared to states that were missing either a permitting or a registration law, states with all three were four times less likely to have guns coming in that were used in a crime soon after their purchase. (When guns move quickly from seller to crime scene, that's a clue there are criminal systems set up where guns that appear to be legally purchased are actually getting funneled to illegal channels.)

Of course, research finds that criminals tend to traffic arms into states with numerous gun-control laws, from states with fewer ones. Fortunately, there's a way to stem the flow. Two studies found that laws instructing gun-owners to immediately file a report when their guns are lost or stolen keep out-of-state weapons from showing up at crime scenes.

There's one line of research that's shown certain laws are associated with fewer homicides. Two studies found that after states pass laws keeping people who are under domestic violence restraining orders from obtaining guns, fewer state residents get killed by their spouses or other romantic partners.

"You put those things together and you have far fewer guns sold from the state that end up being recovered from criminals, and you have less violence," Webster says.

Which laws don't have much evidence saying they reduce violent crime rates?

Not all laws researchers examined turn out to be associated with less violence. For example, a study Webster led found laws that restrict people from buying more than one firearm a month to be pretty ineffective.

Webster and Wintemute also found problems with the Brady Handgun Violence Prevention Act, which requires licensed gun shops to run background checks on buyers. The act affects states that don't already have background check laws. One study compared crime in Brady-affected states with unaffected states and found no difference in their firearm-related suicide and homicide rates.

Webster thinks the problem is that the Brady Law, as it's often called, doesn't apply to sales by private owners. "We cannot expect that that's going to be the answer, to have half a system of accountability," he says.

How much does gun-law science really help?

Science is slow. Laws often pass long before researchers are able to check whether they work, as was the case with laws about domestic violence restraining orders.

Still, science can make a difference after the initial passage of laws. Webster has presented at a Congressional hearing and served as an expert in court cases in which folks have challenged gun-control laws.

He says lawmakers' staff contact him sometimes, to ask him about his work. "If I were a policymaker, knowing I was going to get heat whichever way I voted on some of these things, I would probably feel a lot better knowing I had some evidence that these policies work," Webster says.

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