Are mass shootings contagious? Recent data says no.

Andrey Kamenov

Andrey Kamenov, Ph.D. Probability and Statistics

A study published in 2015 suggests that mass shootings in the U.S. are contagious. Not in a sense of spreading like a disease, however — just that each shooting slightly increasing the chance of another one happening shortly afterward.

The original article used data from the Brady Campaign to Prevent Gun Violence. It appears that we now have much more complete data on shootings, thanks mostly to the Gun Violence Archive project. Additionally, it should be interesting to see if the findings still hold true.

Here’s the chart showing how the average number of mass shooting per day changed in the past two years. The graph below has been smoothed using a rolling average with gaussian weights and a 60-day window.

Average number of mass shootings per day

We could reproduce the findings of the original article (using the same data from Brady’s campaign). The results did change, however, when we used the more recent (and seemingly more complete) data from the Gun Violence Archive. Using the same model, we have arrived at the conclusion that there is no significant evidence of any positive “contagion” effects of mass shootings.

We have also tried fitting the same model using the data from a single state (we chose California). The results were the same: no “contagion.”

One possible explanation would be that Brady’s campaign data was incomplete, containing only the shootings that received significant media coverage, thus influencing the results.

Let’s see if the effect can at least be connected to the most violent incidents. In 14 days after the infamous Las Vegas shooting, there were only 10 other shootings. On average, one could expect 13.9 incidents during the same timeframe (we used a 14-day window to reduce the day-of-week variation).

For the 20 incidents with the most total victims, the average incident count in the following two weeks is slightly higher than average at 14.9. At the same time, if we take 14 days before each incident, the number gets even higher: 15.7. Most likely, there’s some other factor influencing sudden violence spikes.

Finally, here’s a map showing all gun violence outbreaks during the last two years. Each marker corresponds to two (or more) mass shootings that happened within a week of each other and less than 50 miles apart. For some cities, there was more than one outbreak (this is also mentioned).

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About Andrey Kamenov

Andrey Kamenov

Andrey Kamenov, Ph.D. Probability and Statistics

Andrey Kamenov is a data scientist working for Advameg Inc. His background includes teaching statistics, stochastic processes and financial mathematics in Moscow State University and working for a hedge fund. His academic interests range from statistical data analysis to optimal stopping theory. Andrey also enjoys his hobbies of photography, reading and powerlifting.

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