States with more permissive laws around guns have more mass shootings on average, according to a new study from Columbia University’s Mailman School of Public Health .

On average, the researchers say, rates are 11% higher in more permissive states.

“Our analyses reveal that U.S. gun laws have become more permissive in past decades, and the divide between permissive states and those with more stringent laws seems to be on the uptick in concert with the growing tragedy of mass shootings in the U.S.,” said study co-author Charles Branas, chair of the school’s Department of Epidemiology, in a statement.

The researchers got data on mass shootings from the Federal Bureau of Investigation’s Uniform Crime Reporting System, looking at events where four or more people were killed with a gun. They also got data on gun law permissiveness using editions over time of a book called Traveler’s Guide to the Firearms Laws of the Fifty States.

In general, there has been a shift toward permissiveness in gun laws between 1998 and 2014, the time period studied, they reported.

Both domestic and non-domestic mass shootings appeared to vary depending on gun laws, the researchers found. Further work is needed to examine exactly which gun laws seem to impact mass shooting levels, they say.

“More studies that test the impact of specific state gun laws are warranted given our findings, the general increase in state gun law permissiveness, and the pressing need to stem the continued and increasing tragedy of mass shootings in the U.S.,” said Paul Reeping, a researcher in the Department of Epidemiology and the paper’s lead author.