In an era of deregulated gun laws, criminals are more likely to slip through the cracks, according to a study involving a Duke researcher.

States with less stringent concealed carry permit laws were found to be correlated with higher overall firearm and handgun homicide rates in the study, which was published Thursday. Researchers at Boston University, Boston Children's Hospital and Duke analyzed data from 1991 to 2015 in two different types of states—"shall" states with less rigorous gun laws and “may” states with more regulated firearm access—to make the comparisons.

“According to our findings, states that have weaker concealed carry permit laws do have more risky people getting guns. Otherwise, they would be denied permits because of their history of violence,” said Michael Siegel, professor of community health sciences at the Boston University School of Public Health and lead author of the study.

Anyone who is legally permitted to own a firearm may carry it publicly—the practice of concealed carry—so long as there are no laws or private policies in place, said Kristin Goss, Kevin D. Gorter associate professor of public policy and a co-author of the study.

States with “shall issue" laws only require individuals to fulfill state-imposed criteria in order to obtain a permit. The study found that such states were “significantly associated” with a 6.5 percent increase in the rate of total homicides, as well as an 8.6 percent rise for firearm homicides and 10.6 percent increase in handgun homicides compared to “may” states.

Although “may issue" laws include set requirements, which Goss noted vary by state, they also allow for police discretion. The study showed a lower risk for violence in “may” states because the absence of discretion enabled “shall” state residents to have easier access to firearms.

“In some localities, you need to show you have some good reason that you have to carry the gun in public, and you have to get a permit,” Goss said.

Siegel—who has been working on firearm and crime research for the past five years—added that the study analyzed data from 1991 to 2002 and 2003 to 2015, and that the selected time spans appeared to yield “virtually identical results.”

However, in more recent years, handgun homicide rates in particular have become even more pronounced, he noted.

“Studies from the 1970s and 1980s did not find an effect that…states with 'shall' issue laws had lower rates of homicides,” Siegel said. “We are not arguing that those studies were wrong. We think that the relationship has actually changed and that it does make a difference what time period you are looking at.”

When asked if the homicides studied were committed by individuals with permits, Siegel questioned the feasibility of such a “level of specificity,” noting that the current study did not provide that information. However, the team will dive deeper into other policies—such as background checks and age restrictions—in future research.

Philip Cook, ITT/Terry Sanford professor emeritus of public policy studies, is at the forefront of crime prevention and firearm policies. With 40 years of experience conducting field research, he is a first-hand witness to the evolution of concealed carry laws.

Back then, most states, including North Carolina, banned concealed carry or made it possible only under restrictive licensing, Cook noted. Now, he said the National Rifle Association has “chipped away state by state”—all but nine states have a deregulated approach to permit licensing.

Some states—known as permitless states—do not have any restrictions, while other states have a small number of standards, so nearly everyone could carry concealed weapons.

Goss argued that over time, public carrying of firearms has become “more normalized.” In fact, the deregulation movement has gained so much momentum that it has permeated the federal government.

The Concealed Carry Reciprocity Act of 2017—which has not passed either house of Congress—would require states to accept concealed carry permits from other states. The legislation would necessitate states with stricter laws to accept permits of states with fewer requirements. Siegel argued that based on the study's findings, the act “imposes a public safety risk.”

“Someone who had committed violent crimes that did not arise to the level of a felony might have a weapon in a ‘shall’ issue state," Siegel said. "[When] that person travels to a ‘may’ issue state, that state would have to accept their carry concealed weapon permit and would be allowed to carry a concealed weapon, even though they have a history of violence. That represents a risk to public safety."

But in Washington, research data on concealed carry legislation isn't enough to sway politicians, Cook said.

“It is hard for me to believe that the politicians are obsessed with some crazy notion of what the Second Amendment means,” he said. “I think it’s just a political survival strategy.”