Seasonal Influence on Mass Shootings

Mass shootings have become a terrible problem across the world, especially in the United States. In the aftermath of mass shootings, it has been observed that popular and political discourse frequently focuses on the causal impact of mental illness, conveying a message that people with serious mental illness (SMI) are “dangerous people” often reduced to “dangerous weapons,” thereby negatively influencing public attitudes about individuals with SMI.^1,2 Such widely held, if simplistic attitudes, have been challenged by Metzl and MacLeish, who recently proposed that notions of mental illness emerging in relation to mass shootings frequently reflect “larger cultural stereotypes and anxieties about matters such as race/ethnicity, social class, and politics.”^3(p240) They also showed that mass shootings result in a complex interplay between numerous factors from gun policy to risk factors, such as alcohol use or the male gender, which are more strongly correlated with gun violence than mental illness alone. We also believe that seasons might have an influence on these mass shootings. Indeed, seasons have a demonstrated affect on mood, hostility, anger, irritability, and anxiety in populations with and without SMI.^4,5

We therefore decided to analyze the freely available public data on all mass shootings, defined as when four or more people are shot in one event, that occurred in the United States since January 2013 (http://www.shootingtracker.com). Figure 1 shows a clear increase in the number of mass shootings as well as the numbers killed and injured during the summer, which is replicated in all of the three years that were investigated. The peak begins in May and finishes around September–October. We statistically examined this seasonal effect by using the cosinor model, which captures a seasonal pattern using a sinusoidal wave (analyses made with the “Season” R package, made by Barnett & Dobson, and used with R 3.2.2; http://www.R-project.org). Using an adjusted significance level of .025, we found a significant seasonality effect for the numbers of injured and killed in 2013, 2014, and 2015. We also found a significant seasonality for number of such events in 2013 and 2015, which did not achieve significance in 2014. We did not observe any particular increase in these events for 2015 compared with 2013 and 2014.

FIGURE 1—

Monthly Data From Mass Shootings in the United States by (a) Number of Mass Shootings and (b) Number of People killed or Injured: 2013–2015

Reproducing our results in two different countries would have strengthened our hypothesis. However, no reliable data exist on mass shootings besides in the United States. On the other hand, we replicated our findings for the three years investigated by using exactly the same definition of mass shootings in the only existing reliable public database. Such data indicate a seasonal influence, both on the frequency and the intensity, of mass shootings, with seasonal effects opening new avenues for the understanding and prevention of this major public and political issue.