Mass shooting refers to an incident involving multiple victims of gun violence. Mass shootings can be a form of mass murder or massacre, which is commonly categorized as the murder of four or more people with no cooling off period.^[1][2]

Mass shootings can be a form of mass murder, which is commonly categorized as the murder of four or more people with no cooling off period.^[1] According to CNN, a mass shooting is defined as having four or more fatalities, not including gang killings or slayings that involve the death of multiple family members.^[3]

There is debate as to when to characterize a mass shooting as terrorism. Some have argued that certain mass shootings should not be characterized as terrorism.^[4] A U.S. congressional research service report excluded, from a study, mass shootings in which terrorist ideology was a motivation.^[5]

Some have argued that the term mass shooting should include domestic violence killings.^[6]

Different countries have considerably different rates of mass shootings. CNN reports that "[t]here are more public mass shootings in the United States than in any other country in the world, according to a new study."^[7] On the other end of the spectrum is Japan, which has as few as two gun-related homicides (including all homicides, not just mass shootings) per year.^[8]

The United States has an outsized number of mass shootings, with 5% of the world's population but 31% of its mass shootings.^[9]

In the years 2013, 2014, 2015 combined 1,264 people were killed in the United States during mass shootings and 3,611 wounded, for a total of 4,875 shot.^[10][11] In December 2015, the Washington Post reported that there had been 355 mass shootings in the United States so far that year.^[12] In August 2015, the Washington Post reported that the United States was averaging one mass shooting per day.^[13] An earlier report indicated that in 2015 alone, there had been 294 mass shootings that killed or injured 1,464 people.^[14][15][16]

After the Charleston church shooting, U.S. President Barack Obama said, "At some point, we as a country will have to reckon with the fact that this type of mass violence does not happen in other advanced countries. It doesn't happen in other places with this kind of frequency."^[17][18] After a mass shooting in December 2015, he said that mass shootings in the United States has "no parallel in the world."^[19]

Survivors of mass shootings can suffer from posttraumatic stress disorder.^[20][21] There have been many shootings around the world where a large number of victims have died.^[22] Survivors have written about other mass shooting incidents.^[23]

One paper studied Swedish police officers' reactions to a mass shooting.^[24] 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.^[25] The survivors of the 2011 Norway attacks recounted their experience to GQ.^[26] 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.^[27]

In the United States, the majority of mass shooters are male.^[28][29] Notable American perpetrators include, among others, Nidal Malik Hasan, Charles Whitman, James Eagan Holmes, Seung-Hui Cho, and Aaron Alexis. Notable mass shooters from outside the United States include, among others, 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.^[30]

Mass shootings can be motivated by terrorism and caused by mental illness, among many other reasons.^[28] One criminologist claims that the individualistic culture in the United States puts the country at greater risk for mass shootings than other countries.^[31] Others believe that people with mental illness are scapegoated as the cause of mass shootings.^[32] Mass shooting expert and former FBI profiler Mary O'Toole characterizes some mass shooting perpetrators as "injustice collectors."^[33] Author Dave Cullen described killer Eric Harris as an injustice collector in his 2009 book Columbine.^[34] He expanded on the concept in a 2015 New Republic essay on injustice collectors,^[35] 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]} In a 2011 column, criminologist James Alan Fox contended that mass murderers typically experience "years of disappointment and failure that produce a mix of profound hopelessness and deep-seated resentment."^[36]

Some people have considered whether media attention revolving around the perpetrators of mass shootings is a factor in sparking further incidents.^[37] 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.^[38]

Some in law enforcement have decided against naming mass shooting suspects in media-related events, in order to avoid giving them notoriety.^[39]

Responses to mass shootings take a variety of forms, depending on the country and political climate.

After the Port Arthur massacre in Australia, the government changed gun laws in Australia.^[40][41] After experiencing several deadly mass shootings, Great Britain enacted tough gun laws.^[42]

In the United States, support for gun law reform varies considerably by political party, with Democrats generally more supportive and Republicans generally more opposed. Some in the U.S. believe that tightening gun laws would prevent future mass shootings.^[43] Some politicians in the U.S. introduced legislation to reform the background check system for purchasing a gun.^[44] A vast majority of Americans support tighter background checks. "According to a poll [...] by Quinnipiac University in Connecticut, 93 percent of registered voters said they would support universal background checks for all gun buyers."^[45]

Others contend 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.^[46] Some contend that mass shootings are a side effect of gun control itself, as they sometimes occur in locations where ordinary citizens cannot legally be armed (and therefore, typically are not).^{[citation needed]}

As of October 2015, U.S. President Barack Obama had given speeches on eleven different mass shootings during his seven-year presidency.^[47]

• School shooting

• Mass Shooting Tracker
• 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.