After a bystander helped stop the shooter in Sutherland Springs, Texas on Sunday, there is more talk about “good guys with guns.” NPR’s Robert Siegel talks with Stanford University law professor John Donohue about his research on whether or not increased gun ownership helps decrease crime.

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SIEGEL: We’ll never know whether Willeford’s shots prevented the gunman from harming more people, but Stanford law professor John Donohue’s research offers some clues as to the impact of so-called good guys with guns on violent crime rates. He’s looked at more than 30 years of data from states with right-to-carry laws.

And professor Donohue, welcome to the program.

JOHN DONOHUE: Good to be with you.

SIEGEL: First, describe what statistics you actually looked at.

DONOHUE: Yes, we were looking at data across all 50 states and the District of Columbia over the period from 1977 through 2014 to evaluate what the impact on violent crime would be when a state adopted a right-to-carry law that allowed citizens to carry concealed weapons outside the home.

SIEGEL: And what did you find?

DONOHUE: The basic finding was that the net effect of allowing citizens to carry concealed handguns was an increase in violent crime, which essentially rose to about a 15 percent increase after 10 years of existence of the right-to-carry law.