There are more mass shootings in states with weaker gun laws, according to a new study published in The BMJ, a medical journal, on Wednesday.

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. It then tested each of these to see if there was a link.

The result: Where there are more guns, there are more mass shootings. And where gun laws are weaker, there are more mass shootings.

“A 10 unit increase in the permissiveness of state gun laws was associated with an approximately 9 percent higher rate of mass shootings after adjusting for key factors,” the researchers concluded. “A 10 percent increase in gun ownership was associated with an approximately 35 percent higher rate of mass shootings after adjusting for key factors.”

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.”

To find this, the researchers drew on data for mass shootings from the FBI’s Uniform Crime Reporting System, defining a mass shooting as an event in which four or more people, not including the shooter, were shot and 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, which is compiled, they explained, “by legal professionals as a reference guide for gun owners traveling between states.”

To estimate levels of gun ownership, the researchers used the percentage of suicides by firearm — a common proxy for gun ownership that’s well established in other research. (A household survey of gun ownership, which would provide better data, is unfortunately not available for all of 1998 to 2015.)

There are some caveats to the study. The biggest one: It found correlation, not causation. It’s possible other factors not accounted for in the study, besides levels of gun ownership or the permissiveness of state gun laws, are driving higher mass shooting rates.

The researchers also raised concerns that the FBI’s mass shooting data could be incomplete, since states sometimes fail to consistently report to the FBI system. But, the researchers note, the evidence suggests that it’s the states with more permissive gun laws that are less likely to fully report — which would make them look worse, not better, if their data were more complete.

Daniel Webster, a gun policy researcher at Johns Hopkins who wasn’t involved in the BMJ study, also raised concerns about using broad indices of gun laws, like the Traveler’s Guide to the Firearms Laws of the Fifty States, in these sorts of studies. His worry, he told me, is that these indices can treat different gun laws similarly in value — even if one type of law, like gun licensing, has more evidence for effectiveness than other policies, like an assault weapons ban. That makes it hard to draw concrete policy lessons from findings attached to such indices.

This is also just one study. As the authors acknowledge, there isn’t much research into how levels of gun ownership and weaker gun laws influence mass shootings. It’s possible that as more studies come out with different or more rigorous methodologies, the results could differ.

But David Hemenway, a gun researcher at Harvard who was 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.”

The study’s findings are also consistent with other research. Studies have repeatedly found that where gun laws are weaker, and where there are more guns, there are more gun deaths (including homicides and suicides).

For the US, this is especially pertinent. America suffers higher levels of gun violence than any other developed nation, and it has the highest levels of civilian gun ownership in the world and the weakest gun laws in the developed world. The research, from the BMJ study to other evidence, indicates that these weak gun laws and high levels of firearm ownership are helping drive America’s greater levels of gun violence.

America’s gun problem, briefly explained

America’s gun problem comes down to two basic issues.

First, America has uniquely weak gun laws. Other developed nations at the very least require one or more background checks and almost always something more rigorous beyond that to get a gun, from specific training courses to rules for locking up firearms to more arduous licensing requirements to specific justifications, besides self-defense, for owning a gun.

In the US, even a background check isn’t a total requirement; the current federal law is riddled with loopholes and stymied by poor enforcement, so there are many ways around even a basic background check. And if a state enacts stricter measures than federal laws, someone can simply cross state lines to buy guns in a jurisdiction with looser rules. There are simply very few barriers, if any, to getting a gun in the US.

Second, the US has a ton of guns. It has far more than not just other developed nations but any other country, period. Estimated for 2017, the number of civilian-owned firearms in the US was 120.5 guns per 100 residents, meaning there were more firearms than people. The world’s second-ranked country was Yemen, a quasi-failed state torn by civil war, where there were 52.8 guns per 100 residents, according to an analysis from the Small Arms Survey.

Both of these factors come together to make it uniquely easy for someone with any violent intent to find a firearm, allowing them to carry out a horrific shooting.

This is borne out in the statistics. The US has nearly six times the gun homicide rate of Canada, more than seven times that of Sweden, and nearly 16 times that of Germany, according to United Nations data for 2012 compiled by the Guardian. (These gun deaths are a big reason America has a much higher overall homicide rate, which includes non-gun deaths, than other developed nations.)

The research, compiled by the Harvard School of Public Health’s Injury Control Research Center, is also pretty clear: After controlling for variables such as socioeconomic factors and other crime, places with more guns have more gun deaths. Researchers have found this to be true not just with homicides but also with suicides (which in recent years were around 60 percent of US gun deaths), domestic violence, violence against police, and, in the BMJ study, mass shootings.

As a breakthrough analysis by UC Berkeley’s Franklin Zimring and Gordon Hawkins in the 1990s found, it’s not even that the US has more crime than other developed countries. This chart, based on data from Jeffrey Swanson at Duke University, shows that the US is not an outlier when it comes to overall crime:

Instead, the US appears to have more lethal violence — and that’s driven in large part by the prevalence of guns.

“A series of specific comparisons of the death rates from property crime and assault in New York City and London show how enormous differences in death risk can be explained even while general patterns are similar,” Zimring and Hawkins wrote. “A preference for crimes of personal force and the willingness and ability to use guns in robbery make similar levels of property crime 54 times as deadly in New York City as in London.”

This is in many ways intuitive: People of every country get into arguments and fights with friends, family, and peers. But in the US, it’s much more likely that someone will get angry during an argument and be able to pull out a gun and kill someone.

To put it another way, America does not have a monopoly on crime, mental health issues, bigots, extremists, or other factors commonly blamed for gun violence and mass shootings; what is unique about the US is that it makes it so easy for people with all sorts of motives to obtain a gun.

Researchers have found that stricter gun laws could help. A 2016 review of 130 studies in 10 countries, published in Epidemiologic Reviews, found that new legal restrictions on owning and purchasing guns tended to be followed by a drop in gun violence — a strong indicator that restricting access to guns can save lives. A review of the US evidence by RAND also linked some gun control measures, including background checks, to reduced injuries and deaths. A growing body of evidence, from Johns Hopkins researchers, also supports laws that require a license to buy and own guns.

That doesn’t mean that criminals, bigots, and extremists will never be able to carry out a shooting in places with strict gun laws. Even the strictest gun laws can’t prevent every shooting.

And guns are not the only contributor to violence. Other factors include, for example, poverty, urbanization, alcohol consumption, and the strength of criminal justice systems. But when researchers control for other confounding variables, they have found time and time again that America’s loose access to guns is a major reason the US is so much worse in terms of gun violence than its developed peers.

So America, with its lax laws and abundance of firearms, makes it uniquely easy for people to commit mass shootings. Until the US confronts that issue, it will continue to see more gun deaths than the rest of the developed world.

For more on America’s gun problem, read Vox’s explainer.