Homicides are lower in states where firearms laws restrict who can get guns, not what guns people can buy.

That's the takeaway of two 2019 studies from Boston University that used data from the FBI and the Centers for Disease Control to compare the efficacy of different types of state firearms laws. The most recent study, published in the Journal of Rural Health, examined whether "state firearm laws impact homicide rates differently in suburban and rural areas compared to large cities" in the U.S. A second study, published in the Journal of General Internal Medicine on July 30, explored the relationship between firearm laws and gun-related deaths at the state level across all 50 U.S. states over 26 years.

Neither study showed that certain gun laws cause homicide rates to go down, but both revealed associations that point in the same direction: There are fewer gun-related deaths in places where it's harder for high-risk individuals to buy guns.

"Using completely different datasets, we've confirmed the same thing," Boston University (BU) School of Public Health researcher Michael Siegel told BU's The Brink. "The main lesson that comes out of this research is that we know which laws work. Despite the fact that opponents of gun regulation are saying, 'We don't know what's going on, it's mental health issues, it's these crazy people,' which doesn't lend itself to a solution — the truth is that we have a pretty good grasp at what's going on. People who shouldn't have access to guns are getting access."

The studies indicated that states with a combination of firearms laws see the fewest gun-related homicides.

"What surprised us the most was that in states that enacted a combination of universal background-check laws, laws prohibiting the sale of guns to people with violent misdemeanors, and concealed carry permit laws, the homicide rates were 35 percent lower than in states with none of those three kinds of laws," Siegel said. "The practice of keeping guns out of the hands of people who are at the greatest risk for violence — based on a history of violence — appears to be the most closely associated with decreased rates of firearm homicide."

Siegel's study also found that the efficacy of firearms laws seems to depend, in part, on location. For example, background checks seemed more effective at curbing gun-related deaths in urban areas, while misdemeanor laws seemed comparatively more effective in rural and suburban communities. But requiring gun owners to obtain a permit was associated with fewer homicides no matter the area.

"This is suggestive that applying a cluster of different types of state laws is necessary, because not every law will work the same for each local population," Siegel said.

Siegel suggested a general combination of firearms laws that he thinks might help reduce gun-related deaths in the U.S.

"I believe that the three most important things that lawmakers can do to reduce gun violence in their home states are to pass laws that: one, require universal background checks; two, prohibit gun purchase or possession by anyone with a history of violence, whether it be a felony or a misdemeanor; and three, provide a mechanism, called red flag laws, to address people who are at an extreme risk of committing violence, not only to other people but to themselves."

After the mass shootings in El Paso, Texas, and Dayton, Ohio, President Donald Trump expressed support for expanding gun background checks and implementing so-called "red flag" laws, which can temporarily prevent individuals from buying guns if they're reported to authorities as dangerous.

In February, the House of Representatives passed a bill that would extend federal background check laws to apply to private gun sales across all 50 states. Currently, some jurisdictions allow individuals to sell or gift guns to on another without conducting any type of background check. The new bill could close that gap, but it remains unclear whether the Republican-controlled Senate will pass it.

Meanwhile, Democratic presidential hopefuls are generally united in their desire to expand federal background checks and ban so-called assault weapons. Only one of these would likely have a meaningful impact on gun violence in the U.S., according to Siegel.

"Although I completely understand the desire to ban assault weapons, I just don't see empirical evidence that such bans have any substantial impact on homicide rates," he said. "These bans are most often based on characteristics of guns that are not directly tied to their lethality. In contrast, requiring universal background checks in all 50 states could have a substantial impact on gun violence because it would essentially set a minimum standard across the nation — that standard being very simply that people purchasing a gun need to be checked to see if they have a history that puts them at high risk for violence."