THE SCORPION AND THE FROG:

When Illinois passed a law in 2014 permitting the concealed carrying of firearms--becoming the last of the 50 states to do so--Sam Rannochio opened Check Your 6, Inc. in the Chicago suburb of Arlington Heights. The store sells handguns and rifles, and also offers concealed-carry classes. "The two kind of go hand-in-hand together," Rannochio says.

Check Your 6 was one of hundreds of gun dealerships that opened across the United States between 2010 and 2017, notes a preprint study that was published last month on social science research website SSRN and has not yet been peer-reviewed. According to the study, which looked at county-level data nationwide over a 17-year period, when the number of gun dealerships within 100 miles of a given area went up, the number of gun homicides in that area also increased in subsequent years--even as nongun killings declined overall (see graphic). Majority-Black communities bore the brunt of that violence, the study found.

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