Twenty years ago, in 1997, the Journal of Legal Studies published a paper by John Lott and David Mustard. In it, Lott and Mustard reported their findings that violent crime went down after states passed laws allowing their residents to carry concealed weapons. This counter-intuitive finding helped fuel the campaign for similar “Right-to-Carry” (RTC) laws in more states, including Wisconsin.

Lott and Mustard claimed these results came as a surprise, suggesting they were unbiased researchers just looking for empirical evidence. Since then, however, Lott has become a true believer. A glance at his website shows he has become an outspoken advocate for the “more guns, less crime” position. In fact, that is the title of one of his books.

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This June, a paper was published on the effect of RTC laws on crime, using the synthetic control approach. For each of the 33 states that passed an RTC law, the authors created a synthetic control state from the states that did not pass an RTC law. The control states were designed to match the actual states as closely as possible in their behavior prior to the RTC law. As the authors (John Donohue, Abhay Aneja, and Kyle Weber) explain, their approach:

uses a matching methodology to create a credible synthetic control based on a weighted average of other states that best matches the pre-passage pattern of crime for each “treated” state, which can then be used to estimate the likely path of crime if RTC-adopting states had not adopted a RTC law.

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Using this model, Donohue and his colleagues conclude:

the average impact of RTC laws across the 33 states that adopt between 1981 and 2004 indicate that violent crime is substantially higher after ten years than would have been the case had the RTC law not been adopted.