Mass shootings are a tragic manifestation of the gun violence epidemic in the United States, resulting in multiple victims of firearm-related violence in a single incident, typically in a public location. These events are random in nature and unrelated to conventional crime, such as gang violence or armed robbery.

While there is no official definition of a mass shooting, multiple data sources indicate that they have become both more frequent and deadlier over time. In the past four decades, mass shootings have caused at least 1,000 deaths and 1,500 injuries in the United States. Although they account for less than 1 percent of all firearm deaths annually in the country, they evoke significant public interest and have detrimental effects that extend far beyond the harm to the direct victims and their families.

The detrimental effects of mass shootings on the health of the people who witness them, those who live in the communities surrounding them, and those who identify with the demographic groups targeted in them are well-documented. Emerging empirical literature shows that mass shootings increase depression and other mental health disorders among adolescents and adults, worsen infant health, and reduce overall community and emotional well-being.

Policy responses to mass shootings are designed to prevent future shootings (such as physical security measures and gun control policies) or to mitigate mass shootings’ impacts on victims and the general public (such as victims’ services). However, several factors complicate researchers’ abilities to study the efficacy of policy responses, as well as policymakers’ abilities to design policies that are based on evidence. These factors include the lack of an official definition of “mass shooting,” challenges in identifying people who were exposed to shootings, and the unpredictability and rarity of these incidents compared with broader firearm violence.

Despite the challenges, greater policy prioritization is needed to reduce the public health burden of mass shootings. Future research should study the efficacy of policies that may prevent such events. Moreover, there is an urgent need for additional research to understand which prevention and mitigation strategies are the most effective.

In the 1980s, the Federal Bureau of Investigation adopted the definition of a “mass murderer” as a person who kills four or more people, excluding themselves, in a single incident, and typically in a single location. After the 2012 Sandy Hook school shooting, Congress defined the related term “mass killing” to mean three or more killings during a single incident, excluding the perpetrator, without referring to a weapon. With the lack of a firm definition of a mass shooting, researchers rely on different organizations’ definitions and databases to analyze and track mass shootings. These inconsistencies lead to different assessments of the prevalence and lethality of mass shootings.

Despite the lack of a firm definition, most data sets show an increase in the number and lethality of mass shootings between 2000 and 2019. Moreover, there has been greater public awareness of these tragic events in recent years as a result of media coverage. According to a 2019 poll, nearly half of Americans live with the fear of becoming a victim of a mass shooting.

In conclusion, mass shootings are a tragic manifestation of the gun violence epidemic in the United States, resulting in significant public health harms that extend far beyond the direct victims and their families. Greater policy prioritization and future research are needed to reduce the public health burden of mass shootings and understand which prevention and mitigation strategies are the most effective.