States "with more permissive gun laws and greater gun ownership" have higher rates of mass shootings, according to a new study published in the medical journal The BMJ. The study also found that states with more restrictive gun laws have fewer mass shootings, after adjusting for population.
"There's a relationship between the restrictiveness or permissiveness of state gun laws and the rate of mass shootings in those states," said Charles Branas, a professor of epidemiology at Columbia University and one of the study's co-authors.
Researchers looked at mass shooting rates within states, defining it as "independent events in which four or more people were killed by a firearm." They used data from the Federal Bureau of Investigation's Uniform Crime Reporting System (UCR) from 1998 to 2015.
The UCR is a system which compiles reported crime data from about 18,000 city, university and college, county, state, tribal and federal law enforcement agencies which voluntarily add data. Florida does not participate in this system, so the state's data on mass shootings was not included in the study.
The researchers then compared the number of each state's mass shootings to how permissive or restrictive a state's gun laws are, using the Traveler's Guide to the Firearms Laws of the Fifty States to rate how strict each state's gun laws are. The guide was first written by Scott Kappas in 1996. Kappas, a Kentucky lawyer, wanted to help gun owners traveling across the US navigate different state gun laws. The guide rates each state on a scale of 0 (restrictive) to 100 (permissive) and is updated each year.
Other studies have looked into how permissive gun laws and higher levels of gun ownership are associated with higher levels of gun homicide and gun suicide, but this new Columbia study is the first to look at how those laws impact mass shooting rates.
And more research is coming out about gun laws and their relationship to gun deaths. A recent study from Penn Medicine out of the University of Pennsylvania found that states with strict gun laws which border states with more permissive ones see an increase in homicides.
Branas, the Columbia study co-author, said this study shows that since 2008, states have become more polarized in their gun laws.
"If you look at the early Traveler's Guides, there were states that were in the middle, that were hovering in the 50s or 60s… and there's few if any of those left," Branas said.
He noted that this shift happened amid the economic recession of 2008, presidential elections, and the passing of the Heller Decision by the Supreme Court, which found that the Second Amendment protects an individual's right to possess a firearm.
Branas believes his study is only the beginning. He says further research into how specific laws affect homicide, suicide and mass shooting rates will help educate lawmakers on gun policy in the future.
"What we really want to do is then go on to find what specific state laws are carrying the lion's share of this relationship that we found," Branas said.
Branas wants to understand why the divide between permissive and restrictive state laws happened and whether it will increase in coming years.
KERA is part of Guns & America, a national reporting collaborative of 10 public media newsrooms focusing attention on the role of guns in American life. You can find more Guns & America coverage here, and learn more about the collaboration here.