Mass shooting refers to an incident involving multiple victims of gun violence. Mass shootings are a form of mass murder, which is commonly categorized as the murder of four or more people with no cooling off period.^[1][2] While the U.S. has 5% of the world's population, 31% of public mass shootings occur in the U.S.^[3][4]

In 2014, the FBI published a study on "active shootings". It defined a "mass killing" as three or more killed, not including the shooter if they are also killed.^[5] A U.S. congressional research service report excluded, from a study, mass shootings in which terrorist ideology was a motivation.^[6] Some have argued for the term to include domestic violence killings.^[7] The Washington Post states "the broader definition is nonetheless a useful one, because it captures many high-profile instances of violence — like the recent Lafayette theater shootings — that don't meet the FBI's criteria."^[8] According to CNN, a mass shooting is defined as having four or more fatalities, and do not include gang killings or slayings that involve the death of multiple family members.^[4]

In the United States, the majority of mass shooters are male.^[9][10] Notable American perpetrators include Nidal Malik Hasan, Charles Whitman, James Eagan Holmes, Seung-Hui Cho, and Aaron Alexis. Notable mass shooters from outside the United States include William Unek, Richard Komakech, Omar Abdul Razeq Abdullah Rifai, Martin Bryant, and Woo Bum-kon. Criminal histories and documented mental health problems did not prevent at least eight of the gunmen in fourteen mass shootings from obtaining firearms.^[11]

Survivors of mass shootings can suffer from posttraumatic stress disorder.^[12][13] There have been many shootings around the world where a large number of victims have died.^[14] Survivors have written about other mass shooting incidents.^[15] One paper studied Swedish police officers' reactions to a mass shooting.^[16] The father of a victim in a mass shooting at a movie theater in Aurora, Colorado, wrote about witnessing other mass shootings after the loss of his son.^[17] The survivors of the 2011 Norway attacks recounted their experience to GQ.^[18] According to the Huffington Post, 57 percent of mass shootings in the United States between 2009 and July 2015 involved a family member or intimate partner and 81 percent of the victims were women and children.^[7]

Mass shootings can be motivated by terrorism and caused by mental illness, among many other reasons.^[9] One criminologist claims that the individualistic culture in the United States puts the country at greater risk for mass shootings than other countries.^[19] Others believe that people with mental illness are scapegoated as the cause of mass shootings.^[20] Mass shooting expert and former FBI profiler Mary O'Toole characterizes some mass shooting perpetrators as "injustice collectors."^[21] Author Dave Cullen described killer Eric Harris as an injustice collector in his 2009 book Columbine.^[22] He expanded on the concept in a 2015 New Republic essay on injustice collectors,^[23] identifying several notorious killers as fitting the category, including Christopher Dorner, Elliot Rodger, Vester Flanagan, and Andrew Kehoe. The essay quoted O'Toole and Gary Noesner, who helped create and lead the FBI's hostage negotiation unit, and served as Chief Negotiator for ten years.^{[citation needed]}

Some people have considered whether media attention revolving around the perpetrators of mass shootings is a factor in sparking further incidents.^[24] The effects of messages used in the coverage of mass shootings has been studied. Researchers also studied the role the coverage plays in shaping attitudes toward persons with serious mental illness and public support for gun control policies.^[25]

Some in law enforcement have decided against naming mass shooting suspects in media-related events, in order to avoid giving them notoriety.^[26]

After the Port Arthur massacre in Australia, the government changed gun laws in Australia.^[27][28] Some in the U.S. believe that tightening gun laws would prevent future mass shootings.^[29] Others contend that mass shootings are a side effect of gun control itself, as they occur almost exclusively in locations where ordinary citizens cannot legally be armed (and therefore, typically are not). Some politicians in the U.S. introduced legislation to reform the background check system for purchasing a gun.^[30] After experiencing several deadly mass shootings, Great Britain enacted tough gun laws.^[31]

Others believe that mass shootings should not be the main focus in the gun law reform debate because these shootings account for less than one percent of the U.S. homicide rate and believe that these shootings are hard to stop. They often argue that civilians with concealed guns will be able to stop shootings.^[32]

As of October 2015, U.S. President Barack Obama had given speeches on eleven different mass shootings.^[33]

• Timeline: Deadliest U.S. mass shootings
• Public Mass Shootings in the United States: Selected Policy Implications Congressional Research Service
• Algoworld: Scientific Ways To Predict Mass Shootings
• Washington Case Revives Debate About 'Contagious' Mass Shootings
• Yes, Mass Shootings Are Occurring More Often. Mother Jones. October 21, 2014.