Seasonality of Mass Shooting Events and Deaths by Suicide in the United States, 2014 to 2021

It has been hypothesized that most mass shootings are suicidal acts, necessitating investigation of potentially common elements. We investigated whether US mass shootings and deaths by suicide during 1 January 2014 through 31 October 2021 exhibited significant seasonal variations. Outcomes included temporal patterns of mass shooting events and deaths by suicide and their relationship, as characterized by multivariable linear regression models detrended to isolate seasonality and estimate peak phases and amplitudes. Overall, 3,305 mass shootings and 360,827 deaths by suicide were analyzed. Detrended models revealed significant seasonal variation in mass shootings and deaths by suicide. Mass shootings and deaths by suicide displayed concurrent estimated annual peaks occurring on 3 July and 23 June, respectively. Detrended models estimated that at peak-versus-nadir, seasonality accounted for 613% increased mass shootings and 12% increased deaths by suicide. US mass shooting events and deaths by suicide exhibited strikingly synchronous seasonality, with concurrent mid-summer peaks and mid-winter nadirs. This finding calls for expanded implementation of summertime gun violence and suicide-prevention programs and for equipping treatment centers to respond to increased summertime demands. Further research is needed to investigate potential mechanisms underlying the shared seasonality and the hypothesis that many mass shootings are both homicidal and suicidal, intended as final acts.