Researchers fr­­om Stanford and Duke University examined recent studies on the causes of gun violence in the United States in an effort to find consensus in a body of research that often covers different states or different time periods, making conclusions difficult to draw.

The consensus of recent research shows that right-to-carry laws are linked with higher violent crime rates, according to a new Science paper co-authored by a Stanford professor. (Image credit: 400tmax / Getty Images)

The analysis by John Donohue, a professor of law at Stanford, and Philip J. Cook at Duke University published Dec. 7 in Science reports some emerging consensus in the studies. Among the findings was that lifting restrictions on concealed carry guns increases violent crime and that laws restricting gun ownership for people convicted of domestic violence reduced killings of female domestic partners.

The authors hope their work can help guide policymakers who are debating measures to reduce gun violence.

The level of gun violence in the United States places it as an outlier among developed countries. In 2015, over 36,000 people died from gunfire, according to the Centers for Disease Control and Prevention, with roughly two-thirds of those deaths being classified as suicide. America’s gun-murder rate is 25 times that of the other high-income nations, and the gun-suicide rate is eight times as high. Despite these numbers, the last extensive analysis of research into the origins of gun violence, conducted in 2004, was inconclusive.

“Fortunately, the flow of high-quality research has increased in recent years,” the experts wrote in the paper. “With journals in a variety of disciplines increasingly receptive to original research on gun violence and regulation, there has been a surge of publication in this area after a long plateau.”

Concealed carry increases violent crime

In the mid-1970s, all but five states had banned or severely limited concealed carry of firearms. But by 2014 all states except eight passed right-to-carry laws, which eased those restrictions. Understanding how that change affected crime has been challenging, however, because of the fluctuating nature of crime.

“If a gun regulation is most likely to be enacted in jurisdictions that have recently experienced a surge in gun violence and if that surge is temporary, the result will be that implementation of the new measure is followed by a drop in crime, giving the false appearance that it was effective,” the researchers wrote.

During the crack cocaine epidemic of the late 1980s and the early 1990s, right-to-carry laws were adopted more often in states that had less of a crack problem. This means that any analysis of right-to-carry laws during that period will show those laws as beneficial unless researchers can adequately control for the influence of crack cocaine, which has proved to be quite difficult, Donohue said.

“This problem has plagued every panel data analysis of RTC (right-to-carry) laws, except for those that started after the impact of crack had fully dissipated by the late 1990s and early 2000s,” the researchers wrote.

By analyzing studies from after that time period as well as recent research relying on new statistical techniques for assessing the impact of legal changes, the pair found an emerging consensus that deregulating concealed carry restrictions increases violent crime. This finding comes out of recent research published by Donohue and his team in June, as well as research from Duke and the University of Pennsylvania and from Boston University.

“The dilemma for science is that you’re always working with imperfect data and imperfect statistical models,” Donohue said. “What’s appealing about the current growing body of evidence on right-to-carry laws is that different researchers using different methodologies and different data sets are coming to similar conclusions. … We are all coalescing around the same message, and that’s the best that science can do: Look at the imperfect data in different ways and see if a consistent story emerges.”

Domestic violence, harsher sentencing

The researchers also analyzed studies exploring the effects of an amendment of the Gun Control Act of 1968 and the add-on sentencing laws adopted by a number of states during the 1970s and 1980s, which added prison time to crimes committed with a gun rather than a less lethal weapon.

They found that the 1996 Gun Control Act amendment that prevented people with a misdemeanor conviction for domestic violence from owning guns reduced killings of female intimate partners. Gun murders for that group were reduced by 17 percent.

Another research study analyzing a nearly 40-year time interval shows that harsher sentencing for crimes that involved firearms had the greatest effect on reducing aggravated robberies, decreasing them by 5 percent.

Challenges of measuring reform

Congress is currently considering a bill that would expand the ability of Americans to carry concealed weapons across state lines. The House of Representatives passed the measure Wednesday, and it is now headed to the Senate for approval.

“If Congress moves on that proposal, suddenly everybody from, say, Louisiana or Arizona can come to California and start carrying concealed guns, which is quite a violation of the traditional notion of a state’s right to decide who carries weapons within its borders,” Donohue said. “The November 2016 referendum results showed that the very large majority of Californians wanted more gun control. Yet we may be in a position where Congress is undermining the desire of California voters on an issue that is ordinarily left up to local control.”

The Supreme Court is also likely to consider one of several cases involving conceal carry rights in the future because of conflicting rulings from intermediate courts of appeal, Donohue said.

“Right now, more than ever, it seems pretty important for the Supreme Court justices to know what the best research says about right-to-carry laws,” Donohue said.

Despite advances in statistics that helped researchers tease out findings in the latest gun violence research, Donohue said that statistical research will never yield perfect answers.

“These are complicated issues,” Donohue said. “Nothing in statistics is 100 percent certain. Science advances our knowledge when sufficient evidence is amassed that points in a certain direction.”