A new analysis of public health and crime data has found that mass shootings and deaths by suicide in the United States follow a strikingly similar seasonal rhythm, both cresting in mid-summer and retreating in mid-winter. The findings, published in the Journal of Biological Rhythms, draw on national data and datasets from leading health and law enforcement sources.
In this context, mass shootings are defined as incidents in which four or more people, excluding the shooter, are killed or injured by a firearm. Public health experts have long been searching for patterns that might explain when these tragedies occur, with one line of inquiry suggesting that many mass shooters may 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.
Seasonality in mood and suicidality has long posed questions for clinicians. While depression tends to be worst in winter’s dark days, deaths by suicide often peak in the sunnier months. A 2019 study of psychiatric patients indicated that highly lethal suicide attempts tend to rise with greater sunlight, a contrast to less severe attempts and general depression.
“Seasonality has long been recognised in suicide, and deaths by suicide are lowest in winter and higher in late spring and summer,” Czeisler explained. “This raised the question: if a subset of mass shootings involves an element of suicidality, do they share the same seasonal rhythm?”
Demographic parallels were noted as well. A 2024 review of mass shooting epidemiology shows that most shooters are adult men—a group also experiencing the highest suicide mortality. Building on these connections, the researchers sought to determine whether mass shootings follow the same summer-peaking seasonal pattern as highly lethal suicide attempts.
Using data from January 2014 to October 2021, the team examined 3,305 mass shootings and 360,827 deaths by suicide. Mass shooting records were drawn from the Gun Violence Archive, while suicide data came from the Centers for Disease Control and Prevention mortality databases. The researchers standardised monthly lengths and employed statistical models to separate seasonal fluctuations from longer-term trends.
The results showed a highly synchronised seasonal rhythm: both mass shootings and deaths by suicide peak in mid-summer and fall to their nadirs in mid-winter. The researchers estimate the suicide peak occurs around June 23, with mass shootings peaking a few days later, around July 3. Across June, July and August, both phenomena are elevated compared with the rest of the year, while December, January and February see the lowest activity.
When measuring the magnitude of the summer rise, the increase for mass shootings is particularly pronounced. The relative summer peak versus winter low stands at about 613 percent, translating to an average rise from roughly 4.4 mass shootings per standardized month in winter to 31.3 per month at the summer peak. Suicide deaths rise by about 12 percent, from around 3,439 per month at the winter low to 3,845 per month at the summer high.
Looking year by year, the pattern remained largely stable from 2014 to 2019. In 2020, the summer spike for mass shootings intensified, while the summer peak for suicides was smaller than usual, a shift the authors attribute, in part, to the social disruptions and acute stress of the early COVID-19 pandemic.
“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 noted. “In other words, mass shootings and suicides tend to rise and fall together across seasons.”
Researchers stress that the synchronised timing does not prove that suicides cause mass shootings, nor that mass shootings cause suicides. Instead, the alignment is compatible with the idea that many mass shootings are externally directed manifestations of suicidality, without establishing a causal mechanism.
The study’s authors emphasise that identifying a shared seasonal peak could help public health officials and violence-prevention programmes allocate resources more effectively. The warmer months, they argue, may warrant expanded mental health services and preparedness for firearm-related injuries, alongside intensified fire-arms safety and suicide-prevention efforts in late spring ahead of the peak season.
As with any research, several caveats apply. The study notes that while the timeline is shared, it does not reveal exact causal pathways. There is no single national database for mass shootings, so the researchers relied on an independent repository whose definitions can influence counts. Additionally, suicide data for 2021 were provisional and may undercount due to reporting delays.
Future work could explore potential mechanisms across different demographic groups and environments, including comparisons between hemispheres where seasons diverge. The authors also recognise that mass shootings remain relatively rare outside the United States to support 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.
