Recent measles outbreaks have sparked a national debate over vaccinations and herd immunity, but herd immunity applies to guns, too, Paul Hsieh argues:
Crime rates in Chicago dropped dramatically in 2014 after the state of Illinois allowed legal concealed carry. [...] According to AWR Hawkins, national FBI statistics also showed a significant decrease in crime in the first half 2014 even though gun sales soared in 2013. [...] This is a continuation of the longer-running trend described by Dylan Polk in Guns & Ammo: “Crime rates have dropped as gun ownership has risen, despite a population growth” over the 1993-2013 period.
On the other hand, the US prison population expanded from about 1 million persons in 1990 to almost 2.5 million persons in 2015. I’ll bet that has something to do with lower crime rates overall.
Also, one way that police departments improve their statistics is to “massage” them: