In October, Stephen Paddock fired round after round from the window of his Las Vegas hotel room, spraying bullets on thousands of concertgoers below. He killed 58 people and injured more than 500. About a month later, in Sutherland Springs, Texas, gunman Devin Patrick Kelley burst into a church and massacred 26 of its congregants.

A month after that, on December 6, the House of Representatives passed its first major piece of gun legislation — a bill that requires each state to recognize permits for the concealed carry of a handgun issued in other states, making it easier for legal gun owners to travel across state lines.

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“What we can tease out from the empirical data is you do worse in terms of more violent crime if you allow anybody to carry concealed handguns outside the home,” said John Donohue, a professor of law at Stanford University who has studied right-to-carry laws. “There are a lot of factors at play. There are some good instances where someone with a gun actually can thwart a crime, so that tends to depress crime. On the other hand, lots of things go wrong when you start carrying guns outside the home.”

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Using this methodology, Donohue and his team found that over 10 years, right-to-carry laws have a 13 to 15 percent higher violent crime rate.

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Donohue said a few factors could explain his findings. Simply, things go wrong when you carry a deadly weapon outside the home. “Road rage is one small example of that, and we see that play out in many different scenarios,” he said.

“It also turns out when you take guns outside the home, they’re much, much more likely to be stolen,” Donohue continued, saying he estimated that about 100,000 guns are stolen from permit holders every year. “That’s a case where a good guy is essentially arming a criminal.”

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As Donohue points out, this could be a “nightmare” for police officers in California and New York, particularly in big urban areas where cops are constantly trying to take illegal guns off the street (and where, in New York City, for example, they’ve been remarkably successful at it).