The notion stems from a paper published in 1997 by economists John Lott and David Mustard, who looked at county-level crime data from 1977 to 1992 and concluded that "allowing citizens to carry concealed weapons deters violent crimes and it appears to produce no increase in accidental deaths." Of course, the study of gun crime has advanced significantly since then (no thanks to Congress). Some researchers have gone so far as to call Lott and Mustard's original study "completely discredited."
One of the major critiques of the study came from the National Research Council, which in 2004 extended the data through the year 2000 and ultimately concluded that "with the current evidence it is not possible to determine that there is a causal link between the passage of right-to-carry laws and crime rates." Or in other words, "More guns, less crime? ¯\_(ツ)_/¯"
Now, Stanford law professor John Donohue and his colleagues have added another full decade to the analysis, extending it through 2010, and have concluded that the opposite of Lott and Mustard's original conclusion is true: more guns equal more crime.
"The totality of the evidence based on educated judgments about the best statistical models suggests that right-to-carry laws are associated with substantially higher rates" of aggravated assault, robbery, rape and murder, Donohue said in an interview with the Stanford Report. The evidence suggests that right-to-carry laws are associated with an 8 percent increase in the incidence of aggravated assault, according to Donohue. He says this number is likely a floor, and that some statistical methods show an increase of 33 percent in aggravated assaults involving a firearm after the passage of right-to-carry laws.
These findings build on and strengthen the conclusions of Donohue's earlier research, which only used data through 2006. In addition to having nearly two decades' worth of additional data to work with, Donohue's findings also improve upon Lott and Mustard's research by using a variety of different statistical models, as well as controlling for a number of confounding factors, like the crack epidemic of the early 1990s.
These new findings are strong. But there's rarely such a thing as a slam-dunk in social science research. Donohue notes that "different statistical models can yield different estimated effects, and our ability to ascertain the best model is imperfect." Teasing out cause from effect in social science research is often a fraught proposition.
But for this very reason it's important for policymakers on both sides of the gun control debate to exercise caution in interpreting the findings of any one study. Gun rights advocates have undoubtedly placed too much stock in Lott and Mustard's original study, which is now going on 20 years old. The best policy is often informed by good research. And as researchers revisit their data and assumptions, it makes sense for policymakers to do the same.