According to criminologist James Allen Fox:
[O]ur collective memories seem to forget or move past other anxious times when mass shootings have clustered in time, for the most part out of sheer coincidence. Although there have been cases in which mass gunmen have derived inspiration from others who preceded them, and perhaps wanted a share of the notoriety that follows, the impact of copycatting is often overstated.
Plumer compares Fox's statistics to other statistics that show an increase in mass shooting deaths:
Fox is looking at all mass shootings involving four or more victims — that’s the standard FBI definition. Mother Jones, by contrast, had a much more restrictive definition, excluding things like armed robbery or gang violence. They were trying to focus on spree killings that were similar in style to Virginia Tech or Aurora or Newtown. The definitions make a big difference: On Fox’s criteria, there’s no uptick. On Mother Jones’, there’s a clear increase.
Earlier Dish on this year's mass shooting numbers here.