Are Mass Public Shootings Seasonal? (Corrected)

***CORRECTION: A FACEBOOK READER noticed something in the Mother Jones data I presented recently that I had missed. Beginning in 2013, MJ changed their definition of a mass public shooting from 4 or more victims to 3 or more victims in 2013 (see more below), but did not retroactively update their database. Although not deceptive (they said plainly they were doing this, I simply missed it), this is methodologically problematic. So I eliminated those cases, which reduces the total number in the database from 114 incidents to 95, and re-did the chart here.***

I was sitting at the park tonight, watching my wife play tennis and minding my own business, when Rob Pincus texted to ask me if I had any source for data on the seasonality of mass shooting activity. I.e., mass shootings by month.

I did not, but I was intrigued enough by the idea to do a little work when I got home tonight. The fruit of that labor is below. Important notes and interpretive points follow the chart.

Mass Shootings by Month Corrected


Average number of mass public shootings by season (corrected):

  • SPRING (March, April, May) = 7.0
  • SUMMER (June, July, August) = 7.7
  • FALL (September, October, November) = 8.3
  • WINTER (December, January, February) = 8.7

I did not statistically analyze the difference in number of shootings by season, but both eyeballing the chart above and looking at the average number of shootings by season suggest there is not a major seasonal effect on mass public shootings.

To the extent there is a seasonal effect, it is not what I expected. I had expected summer to have the highest number of mass public shootings, as I thought that homicides in general were higher in the summer time. (Turns out that may not be the case, either; it’s complex.) In these data, winter (corrected) is the mass public killing season, especially relative to spring.


I use data on mass public shootings compiled by Mother Jones magazine. Anyone can download, scrutinize, and analyze this data. Regarding the data: There is no commonly agreed upon or “official government” definition of a mass shooting. So, to be clear, Mother Jones has a particular definition of a mass shooting:

Our research focused on indiscriminate rampages in public places resulting in four or more victims killed by the attacker. We exclude shootings stemming from more conventionally motivated crimes such as armed robbery or gang violence.

I have no problem with this working definition. Everyday murders that claim a large number of victims or domestic violence murders (typically murder-suicides) in which someone wipes out a family do not generally make the entire public anxious in the same way that mass public shootings do.

***CORRECTION: As noted above, the working definition used by Mother Jones changed in the middle of their dataset. Mother Jones was not being sneaky in doing this, as hey make clear on their site:

In January 2013, a mandate for federal investigation of mass shootings authorized by President Barack Obama lowered that baseline to three or more victims killed. Accordingly, we include attacks dating from January 2013 in which three or more victims were killed. (Any analysis of the frequency of mass shootings should account for this.)

So, it was my fault for not making clear that they changed their inclusion criterion for 2013 forward. That said, I don’t think methodologically it is a good idea to make a change like this without retroactively updating the entire database using the new definition.***

* * *

FINAL NOTE: I didn’t bother to do a Google search on this, so if some other blogger or scholar out there has already addressed this issue, my apologies for not citing your work. This was just meant to be a quick and dirty look at the issue in response to Pincus’s question.


  1. Don’t really trust Mother Jones as a source of criminal activity, or any conclusions drawn from their data. On the other hand we have very good criminal data on hot spots of criminal activity in our cities. I don’t know how much money has been spent supporting the NICS back ground check system since it was mandated by the 1993 Brady Act. but had we used that money for intensive policing, in those counties where most crimes and murders take place, we would have had a much better outcomes.


    • you don’t have to trust them. They make their data publicly available, they are clear about how they define a mass public shooting, and you can scrutinize the data for yourself. Or you can just ignore the data if you want.

      Liked by 2 people

    • I imagine so but since the school year is 9 months long there would still be the question of if there is a time during the school year when most Mass public shootings in schools happen. I’m not sure if the mother Jones data categorizes each of the incidents by location. But the data is publicly available so you could see.

      Liked by 1 person

    • That’s almost tautological, though. The interesting thing would be when in the school year. Maybe if there is a Spring Columbine spike or not. Given the more immediate copycat/inspiration effect of clumping we see in mass killings and publicized suicides, it might not be noticeable as any given attack can trigger others. so it would only take one per season to even out the numbers. might have that data broken out.

      Liked by 1 person

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