States with permissive gun control laws and higher rates of gun ownership tend to have more mass shootings, according to a study published this week in the BMJ.

The study also found that the gap in mass shootings between states with weak gun laws and those with tough ones has widened in recent years.

“Our analyses show that U.S. state gun laws have become more permissive in recent decades, and that a growing divide in rates of mass shootings appears to be emerging between restrictive and permissive states,” write the authors of the study.

Previous research has shown that states with lax gun laws tend to have higher rates of homicides and suicides involving firearms. Studies have also have linked higher rates of gun ownership with greater numbers of gun assaults and homicides.

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But it hasn’t been clear how state gun laws and gun ownership influence mass shootings.

The authors of current study, a team of researchers at Columbia University’s Mailman School of Public Health, set out to explore that issue.

Digging through the data

For the study, the researchers used 1998 through 2015 editions of “Traveler’s Guide to the Firearms Laws of the Fifty States,” which gun owners refer to when traveling across state lines. The guide rates the gun law strictness of each state on a scale from 0 (completely restrictive) to 100 (completely permissive).

Scores are based on 13 factors, including whether state residents have the right to carry guns in the open, whether the ownership of semi-automatic rifles and machine guns is limited, and whether out-of-state gun permits are recognized.

To determine the number of mass shootings in each state, the study’s researchers also used data from the FBI’s Uniform Crime Reporting program. The FBI defines a mass shooting as an event in which four or more people were killed by a firearm.

For their analysis, the researchers also divided the mass shootings into two categories: domestic (incidents in which the perpetrator shot an immediate family member or partner) and non-domestic (all other incidents).

Florida, where the mass shooting at Marjory Stoneman Douglas High School occurred last year, was the only state not included in the analysis because it does not participate in the Uniform Crime Reporting program, according to the study’s authors.

Gun ownership data is not available for all 50 states, so for that statistic the researchers relied on a commonly used state-level measure of how many residents own firearms in any particular year: the percent of suicides committed with a firearm.

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Key findings

The study found that gun laws became more permissive overall during the period from 1998 through 2014 — by an average of 0.16 points per year on the “Traveler’s Guide” scale.

Massachusetts was found to have the most restrictive gun laws, while Vermont had the most permissive. (Individual state rankings are not listed in the published study, however.)

The study also identified 344 reported mass shootings between 1998 and 2014. Of those 81 were domestic and 263 were nondomestic.

On average, states with more permissive laws and greater gun ownership tended to have more mass shootings — trends that gained strength from 2010 onward.

Specifically, each 10-point relaxation in a state’s gun laws was associated with a 9 percent higher rate, on average, of mass shootings in that state. And each 10 percent increase in state gun ownership correlated with a 35 percent higher rate of mass shootings.

“This means that a state like California, which has approximately two mass shootings per year, will have an extra mass shooting for every 10 unit increase in permissiveness over five years,” the study’s authors write.

“It will also have three to five more mass shootings per five years for every 10 percent increase in gun ownership,” they add.

The results were the same whether or not someone in a close relationship with the victims committed the shootings.

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Limitations and implications

This study was observational, so it can’t prove a direct relationship between state gun laws, gun ownership and mass shootings.

In addition, the restrictiveness-permissiveness score used in the study hasn’t been scientifically validated.

As Paul Reeping, the study’s lead author and a graduate student at Columbia University, told Newsweek reporter Kashmira Gander, “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.”

Still, the study’s findings can’t be dismissed, particularly as they support previous research that has linked gun-law permissiveness and gun ownership to other types of gun violence.

Just last year, researchers reported that gun deaths among children and teens are twice as common in states with the most lax gun laws.

Reeping and his co-authors say there is an urgent need for better data collection and more research into the effect of gun laws on gun deaths in general and mass shootings in particular.

“Domestic violence and suicide are commonly connected to mass shooting events, so state gun laws involving restraining orders and extreme risk protection orders may be valuable first opportunities for scientific evaluation,” they point out.

Meanwhile, such events continue to claim American lives. Since January 1, at least 39 people have died in eight separate mass shootings.

FMI: The new study can be read in full on the BMJ website. (The BMJ was formerly known as the British Medical Journal.)