Semi-automatic rifles are seen for sale in a gun shop in Las Vegas, Nevada, on October 4th, 2017. (Photo: Robyn Beck/AFP/Getty Images)

Each time there is a mass shooting, activists call for stricter laws regulating firearms. But even legislatures amenable to such actions must decide among a variety of approaches. Which types of laws—or combinations of laws—actually reduce homicides?

New research that analyzes state-level homicide rates over a quarter-century provides some clear answers. Universal background checks and state laws prohibiting handgun possession for people who have committed a violent misdemeanor are associated with meaningful reductions in the homicide rate.

On the other hand, "shall issue" laws, which require local law enforcement to issue concealed-carry permits to anyone who qualifies, are associated with a significant increase in homicides.

Seven other varieties of gun-control laws, including assault weapons bans and bans of large-capacity ammunition magazines, had no effect on the homicide rate.

"It appears that laws that regulate the 'what' [what guns/products are allowed on the market] do not have much of an impact on overall population homicide," lead author Dr. Michael Siegel of the Boston University School of Public Health writes in a "roadmap for policymakers" that accompanies the study. "In contrast, laws that regulate the 'who' [who has legal access to firearms] may have an appreciable impact on firearm homicide, especially if access is restricted specifically to ... people who have a history of violence, or represent an imminent threat of violence."

The study, published in the Journal of General Internal Medicine, analyzed the relationship between state-level homicide and suicide rates and 10 types of firearms laws enacted in those states. The researchers looked at the years 1991 to 2016, and analyzed homicide and suicide rates beginning the year after each piece of legislation was passed.

Specifically, the researchers looked at the effects of laws that required background checks for all firearm purchases; prohibited possession of a handgun by anyone who has committed a violent misdemeanor; prohibited handgun sales to anyone under 21; mandated concealed-carry permits for anyone who qualifies, rather than leaving the decision to the discretion of local law enforcement; allowed people to carry concealed handguns without a permit; prohibited people from buying firearms and then selling them to someone who can't lawfully buy one; or prohibited junk guns, a.k.a. "Saturday night specials."

They also looked at the impact of "stand your ground" laws, assault weapons bans, and bans on large-capacity ammunition magazines.

After factoring in a dozen variables known to impact homicide rates, including a state's poverty rate and per-capita alcohol consumption, the researchers found three types of laws were related to homicide rates. (Their study cannot prove causality.)

"Universal background checks were associated with 14.9 percent lower overall homicide rates," they report. "Violent misdemeanor laws were associated with 18.1 percent lower homicide rates. 'Shall issue' laws [requiring police to issue concealed-carry permits] were associated with 9.0 percent higher homicide rates."

"None of the other seven laws was significantly associated with overall homicide rates," they add. "This does not necessarily mean these laws are ineffective. It may also be that the laws are not broad enough to affect overall population death rates, or that the laws are not being adequately enforced."

Nevertheless, Siegel and his colleague Claire Boine argue these findings have clear implications for the gun-control debate. "Public health advocates should prioritize universal background checks, strict permitting laws, and laws to keep guns out of the hands of people convicted of a violent crime [as opposed to fighting for] banning select types of weapons," they write.

In keeping the public safe, who is permitted to use a gun may be more important than the specific type of gun that person is able to obtain.