U.S. states with stricter gun controls have fewer mass shootings, according to a recent study. States with higher levels of gun ownership also see more mass shootings, the research published in the journalThe BMJ found.
Past studies have shown looser gun laws are tied to higher rates of homicides and suicides involving firearms, while gun ownership itself has been associated with more assaults and homicides. In the new study, researchers investigated gun control laws and firearm ownership in relation to the relatively unexplored topic of mass shootings per state.
To calculate the number of mass shootings that were carried out in each state, the team analyzed data from the Federal Bureau of Investigation's Uniform Crime Reporting System collected between 1998 to 2015. A mass shooting was defined as an incident in which four or more people were killed by a firearm. The events were separated into domestic incidents, in which the victims were a family member or partner of the shooter; and nondomestic.
The researchers also used a 0-100 scale of gun law strictness (with 100 indicating complete permissiveness) detailed annually in the Traveler’s Guide to the Firearms Laws of the Fifty States gun guide. Scores were based on 13 factors, including whether the ownership of semi-automatic rifles and machine guns was limited; whether citizens had the right to carry weapons in the open; and whether the right of self-defense was recognized by state law.
As gun ownership was not documented in all 50 states each year, the researchers also used data on suicides involving firearms taken from the Centers for Disease Control and Prevention to estimate the figure. For this reason, Florida was not included in the study.
Massachusetts was found to have the toughest gun laws, while Vermont’s were the most relaxed. Overall, the data revealed firearm laws in the U.S. had become more permissive by 0.16 points per year between 1998 to 2014. During this period, some 344 mass shootings were reported, according to data from the Uniform Crime Reports, with 263 of those incidents nondomestic and 81 domestic.
From 2010 onward, more mass shootings were carried out in states with lax gun laws, while the opposite trend emerged in states with stricter laws. A spike in permissiveness by 10 points was linked to a 9 percent higher rate of mass shootings on average, the authors found. And a 10 percent rise in gun ownership was linked to a 35 percent higher rate of mass shootings.
The authors concluded that states with looser gun laws and more firearm owners saw more mass shootings.
According to the data, a state like California—where two mass shootings occur annually—would see one more such event for every 10 points in which laws were relaxed in a five-year period. And a 10 point spike in gun ownership every half a decade was estimated to cause three to five more mass shootings.
Paul Reeping, co-author of the study at Columbia University Irving Medical Center, told Newsweek: “We were surprised most by the growing divergence in recent years of the rates of mass shootings in permissive and restrictive states.
Reeping acknowledged that the study was limited. “It’s hard to be 100 percent certain that what we found isn't possibly because states that experience more mass shootings in turn change their gun laws, or some other factors that we just couldn’t measure. However, we did include multiple state-level factors that we could measure—education, poverty, incarceration rate, etc.—and took into account a time lag to limit the reverse effect of mass shootings influencing state gun laws across a 15 year period.”
He added the team was concerned about potential under reporting in the FBI’s Uniform Crime Reporting System Supplemental Homicide data that could affect its conclusion, as some states did not fully report. Alabama, for example, only included reports from a handful of counties.
“Nevertheless, we believe that this under reporting would likely lead to an underestimate of the association that we found, as states with more permissive gun laws appear to be less consistent in their reporting of homicides than states with restrictive gun laws,” he said.
Reeping said he hoped policymakers would use the work to make informed decisions about firearm laws in their states, and called on fellow researchers to build on the work by testing the before-and-after effects of enactment or repeal of gun laws in specific states, or both, alongside appropriately matched control states.
Dr. Stephanie Chao of Stanford School of Medicine, who co-authored a recent study on gun laws and child mortality and was not involved in the research, told Newsweek that while mass shootings are "horrific and should never occur," they were difficult to study.
"They occur on a relatively infrequent basis. As the authors point out, studies examining the effect of legislation on mass shootings has been limited. This is a difficult association to study, and I think the authors tried to find an objective way to correlate legislation to mass shootings," she said.
She argued the research added to a growing body of evidence that legislative stringency appeared to correlate with deaths from firearms, and in this case, with fewer mass shootings.
"The authors do try to control for confounding factors, but it is nevertheless difficult to conclude causality. It is also difficult to evaluate to what extent legislation is being enforced at the local level," she said.
Last year, a separate paper showed double the number of children were killed by guns in states with more lenient gun laws than those with strict laws.
This article has been updated to include comment from Dr. Stephanie Chao.