Mass shootings have been on the rise in America in recent decades – and they are more prevalent in states with relaxed gun laws, according to a new study.

Researchers at Columbia University also found that the disparity is growing: states with permissive firearm laws have an 11 percent higher mass shooting rate than states that have cracked down on gun ownership.

'Gun laws, or lack thereof, have real potential to influence mass shootings,' lead author Paul Reeping said. 'Our study brings out a key disparity and sets the stage for better data collection on mass shootings and figuring out which specific gun laws could be most impactful in reducing mass shootings.'

In addition, a 10 percent increase in gun ownership was associated with a more than 35 percent higher rate of mass shootings in those states, according to the study published this week in the British Medical Journal.

This map, by the British Medical Journal, illustrates the level of permissiveness of each state's gun laws, along with the number of mass shootings per one million residents. The blue shades represent states with strict gun laws, while the pinker states have looser restrictions

Massachusetts was found to have the most restrictive gun laws, while Vermont's policies were the most permissive.

Vermont and South Carolina had higher rates of mass shootings than anywhere else in the country - both states with permissive laws and high gun ownership rates.

Researchers defined mass shootings as incidences in which four or more people were killed by a firearm.

They noted that some states have introduced tougher gun laws, while others have loosened their policies. Overall, there was a shift in the U.S. toward easing restrictions on gun owners from 1998-2004 – a period during which there were 344 mass shooting incidents.

'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 senior author Charles Branas, of Columbia University.

Researchers found a 'growing divergence' starting in 2010, with the rate of mass shootings declining in restrictive states and spiking in permissive ones.

A majority (69 percent) of Americans own no guns at all, while 30 percent say they have at least one firearm and 1 percent gave no response in a 2017 survey by Pew Research Center

They used data from the FBI's Crime Reporting System for their analysis and cross-referenced it with annual additions of the 'Traveler's Guide to the Firearms Laws of the Fifty States' to track changes in gun laws.

Their data was somewhat limited, with no information available from a handful of states.

America has more guns than people The U.S. is the only country in the world that has more guns than people, according to a report by Small Arms Survey, a Geneva-based think tank. America is home to 393.3 million civilian-owned guns and has a population of 316.5 million, amounting to 120.5 firearms per 100 people. It's significantly more than in Canada, where a population of 36.6 million people owns 12.7 million guns, equating to 34.7 firearms for every 100 people. And Mexico, home to 130.2 million people and 16.9 million guns, has a concentration of just 12.9 firearms per 100 people. While the U.S. has more than one gun per person, a majority (69 percent) of Americans do not currently own a gun, according to a 2017 survey by the Pew Research Center. In addition, two-thirds of gun owners have more than one firearm, including 29 percent who own five or more. Advertisement

'We need better nationwide data collection on mass shootings - for instance, the lack of data from Florida highlights this, and may make the differences we found appear even less stark than they really are,' Reeping said.

The research out of Columbia was released concurrently with a separate study by the University of Pennsylvania School of Medicine, which found that states with strict gun laws that border states with loose gun laws experienced an increase in firearm-related homicides.

That study, published this week in the Journal of Trauma and Acute Care Surgery, suggested that strict gun laws are being undermined by the transport of firearms across state lines.

'Strict state firearm legislation may be driving some (people) to (travel to) more lax neighboring states to retrieve guns, which in turn increases the number of guns and homicides back in the home state,' said senior author Dr. Mark J. Seamon.

With 393.3 million civilian-owned guns, the U.S. is the only country in the world that has more guns than people, according to a separate report by Small Arms Survey, a Geneva-based think tank.

America has a population of 316.5 million, amounting to 120.5 firearms per 100 people.