Mass shootings and suicides share an unsettling seasonal rhythm, new study finds

A recent analysis of public health and crime data found that mass shootings and deaths by suicide in the United States share a nearly identical seasonal pattern, with both events peaking in mid-summer and reaching their lowest points in mid-winter. This parallel timing provides evidence for the idea that many mass shootings may be driven by similar underlying psychological forces as suicides, particularly severe and violent distress. The findings were published in Journal of Biological Rhythms.

Mass shootings (defined in this context as incidents where four or more people, not counting the shooter, are killed or injured by a firearm) have become increasingly frequent in the United States. To better prevent these tragedies, public health experts are looking for patterns that might explain when and why they happen. One emerging idea is that many mass shooters view their attacks as a final, suicidal act.

“The study was designed to explore whether epidemiological trends align with evidence from case series investigations that led to the hypothesis proposed by mass shootings researchers that many mass shootings are both homicidal and suicidal, designed as final acts,” said Mark É. Czeisler, a clinical fellow in medicine and resident physician at Brigham and Women’s Hospital and Harvard Medical School.

For centuries, the seasonality of mood and suicide has presented a paradox to medical professionals. While depression is known to be most severe during the shorter, darker days of winter, suicide deaths actually follow the opposite pattern. A 2019 study of psychiatric patients indicated that highly lethal suicide attempts tend to peak during the summer months when sunlight exposure is highest. In contrast, less severe suicide attempts and general depression do not follow this summer-peaking trend.

“Seasonality has long been recognized in suicide, and deaths by suicide are lowest in winter and higher in late spring and summer,” Czeisler told PsyPost. “This raised the question: if a subset of mass shootings involves an element of suicidality, do they share the same seasonal rhythm?”

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There are also demographic similarities between the two types of violence. According to a 2024 review of mass shooting epidemiology, the vast majority of mass shooters are adult men, a group that also experiences the highest rates of suicide mortality. Building on these connections, Czeisler and co-author Charles A. Czeisler wanted to see if mass shootings follow the same summer-peaking seasonal rhythm as highly lethal suicide attempts.

To conduct the analysis, the research team evaluated national data spanning from January 1, 2014, to October 31, 2021. They sourced mass shooting records from the Gun Violence Archive, a comprehensive database that aggregates information from law enforcement, government, and media reports. Information on suicide deaths was obtained from the Centers for Disease Control and Prevention mortality databases.

In total, the researchers evaluated 3,305 mass shootings and 360,827 deaths by suicide that occurred over the nearly eight-year study period. To ensure accuracy, they standardized the length of each month and used statistical models to separate seasonal fluctuations from long-term upward or downward trends in the data. This approach allowed them to isolate the natural rhythmic ebb and flow of these events across the calendar year.

The analysis found that both mass shootings and deaths by suicide exhibit a highly synchronized seasonal rhythm, with both consistently peaking in mid-summer and dropping to their lowest levels in mid-winter. The models estimated that suicide deaths reached their annual peak around June 23, while mass shootings peaked just days later, around July 3. During the months of June, July, and August, the average number of both mass shootings and suicides was elevated compared to the rest of the year, while December, January, and February saw the lowest numbers.

The magnitude of the summer increase was vast, particularly for mass shootings. Comparing the summer peak to the winter low, the researchers estimated a 613 percent relative increase in mass shootings. In absolute terms, this meant an increase from about 4.4 mass shootings per standardized month at the winter low to 31.3 mass shootings per month at the summer peak. Suicide deaths showed a 12 percent relative increase, rising from a winter low of roughly 3,439 deaths per month to a summer high of 3,845 deaths per month.

When looking at individual years, the researchers noted that this seasonal pattern remained largely stable from 2014 through 2019. In 2020, the seasonal spike in mass shootings became even more pronounced, while the summer peak in suicides was smaller than usual. The authors suggest the widespread social disruptions and acute stress of the early COVID-19 pandemic may have influenced these 2020 fluctuations, altering typical patterns of both self-directed and externally directed violence.

“Both mass shootings and suicide deaths exhibited clear seasonality, with closely aligned seasonal peaks and troughs that peaked in mid-summer and bottomed out in mid-winter,” Czeisler explained. “In other words, mass shootings and suicides tend to rise and fall together across seasons.”

“This finding does NOT demonstrate that suicides cause mass shootings, or that mass shootings cause suicides,” he noted. “The synchronization is consistent with the hypothesis that many mass shootings are externally directed manifestations of suicidality, but our study does not test or prove that mechanism.”

The synchronous timing of these events provides evidence for the theory that mass shootings and high-lethality suicides share underlying psychological and environmental triggers. The combination of summertime social demands, higher temperatures, and biological changes in impulsivity or irritability may collectively increase the risk of extreme violence in vulnerable individuals. By identifying this shared summer peak, the researchers hope to guide public health officials to expand mental health resources and violence prevention programs during the warmer months when the need is greatest.

“Even without fully understanding the underlying causes, recognizing that both mass shootings and suicide deaths consistently increase in mid-summer may help inform preparedness,” Czeisler said. “Health systems, crisis services, and trauma centers can anticipate times of increased demand for mental health and firearm-related injury care.”

“Similarly, public health agencies and community organizations could intensify firearm safety, safe storage, and suicide prevention efforts in late spring, ahead of the seasonal peak,” he added.

As with all research, there are a few things to keep in mind. The study highlights a shared timeline but cannot establish exactly why summer brings about this increase in violence. Because there is currently no official national database for mass shootings, the researchers relied on an independent repository, meaning the exact number of incidents could vary slightly depending on how a mass shooting is defined. Additionally, the suicide data for 2021 was provisional and might represent an undercount due to reporting lags.

“Studies designed to investigate potential mechanisms could further examine seasonality across demographic groups and environmental contexts, including comparisons between the Northern and Southern Hemispheres, where seasonal patterns are reversed,” Czeisler said of future research directions. “Fortunately, mass shootings remain sufficiently rare outside the United States to preclude robust regional comparisons at present.”

The study, “Seasonality of Mass Shooting Events and Deaths by Suicide in the United States, 2014 to 2021,” was authored by Mark É. Czeisler and Charles A. Czeisler.