There are more mass shootings in states with weaker gun laws, according to a new study published in The BMJ, a medical journal, reports Vox.
The study, from researchers at Columbia, New York University, Boston University, and the University of Pennsylvania, analyzed states’ mass shooting rates, the permissiveness of their firearm laws, and levels of gun ownership from 1998 to 2015.
The result: Where there are more guns, there are more mass shootings. Where gun laws are weaker, there are more mass shootings.
The researchers found that the difference between states with weaker laws and states with stronger laws is increasing, noting that there’s “a growing divergence in recent years as rates of mass shootings in restrictive states have decreased and those in permissive states have increased.”
Researchers drew on data for mass shootings from the FBI, which defines a mass shooting as an event in which four or more people, not including the shooter, were killed.
To measure permissiveness of gun laws, they drew on a 100-point scale from the Traveler’s Guide to the Firearms Laws of the Fifty States, a reference guide for gun owners traveling between states.
The biggest caveat is that the study found correlation, not causation. It’s possible other factors besides gun ownership levels or the permissiveness of laws are driving higher mass shooting rates.
Daniel Webster, a gun policy researcher at Johns Hopkins who wasn’t involved in the BMJ study, raised concerns about using indices like the Traveler’s Guide to the Firearms Laws of the Fifty States because such compilations can treat different gun laws similarly in value.
David Hemenway, a gun researcher at Harvard not involved in the BMJ study, said it was “an important study — another piece of evidence about the serious public health and safety problems caused by gun proliferation.”