A "mass shooting" is an incident involving multiple victims of firearm violence. There is no widely accepted definition of the term "mass shooting" but the FBI defines a "mass murder" as "four or more murdered during an event with no "cooling-off period" between the murders."

Different media outlets and research groups use different definitions for the term "mass shooting" For example , Crime violence research group "Gun Violence Archive" defines a "mass shooting" as " FOUR or more shot (injured or killed) in a single event [incident], at the same general time and location, not including the shooter,” differentiating between mass shooting and mass murder and not counting shooters as victims.

The United States’ Congressional Research Service acknowledges that there is not a broadly accepted definition and defines a “public mass shooting”^[1] as an event where someone selects four or more people and kills them in an indiscriminate manner, echoing the FBI’s definition of the term "mass murder." But adding the indiscriminate factor.^[2]

There are a variety of definitions of mass shooting:^[3][4]

The lack of a single definition can lead to alarmism in the news media, with some reports conflating categories of different crimes.^[16]

An act of mass shooting is typically defined as terrorist when it “appears to have been intended” to intimidate or to coerce people;^[17] although a mass shooting is not, in itself, an act of terrorism. A U.S. Congressional Research Service report explicitly excluded from its definition of public mass shootings those in which the violence is a means to an end, for example where the gunmen “pursue criminal profit or kill in the name of terrorist ideologies.”^[1]

Mass shootings have occurred on the African continent, including the 1927 shooting in South Africa perpetrated by Stephanus Swart, the 2016 Grand Bassam attack in C’ôte d’Ivoire/Ivory Coast, and the 1994 Kampala wedding massacre in Kampala, Uganda. Whilst incidents of mass violence resulting from terrorism and ethnic conflict have occurred on the continent, “mass shootings” as generally understood are rare in Africa.

Other shootings include the 2013 Meet al-Attar shooting in Egypt.

On 2nd April 2015, armed terrorists stormed a public university in the North Eastern part of the country and killed 148 people.

Several mass shootings have occurred in Asia, including the 1878 Hyderabad shooting and 1983 Pashupatinath Temple shooting in India, the 1938 Tsuyama massacre in Japan, the 1948 Babrra massacre in Pakistan, the 1993 Chongqing shooting and the 1994 Tian Mingjian incident in China, as well as the 2001 Nepalese royal massacre.

One of the earliest documented cases of a mass shooting in world history was the 1878 Hyderabad shooting, in which 6 were killed and a further 4 were injured by a sepoy in the British Indian Army in Hyderabad, Sindh, British Raj.

South Korea has suffered multiple mass shootings in the South Korean Army, mainly due to soldiers' stress, conflicts from its violence, and sequestration from society.

Japan has as few as two gun-related homicides per year. These numbers include all homicides in the country, not just mass shootings.^[18]

There have been many mass shootings in Israel, including the 1972 Lod Airport Massacre, which killed 26 and injured 80, the 2002 Bat Mitzvah massacre in Hadera, the 2014 Jerusalem synagogue attack in Jerusalem and the June 2016 June 2016 Tel Aviv shooting at the popular Sarona centre complex in Tel Aviv.

There have been two mass shootings by Jews in Israel. Ami Popper was convicted of murdering seven Palestinian men in a mass shooting carried out in 1990. In 1994 Baruch Goldstein murdered 29 Muslims worshipping and injuring a further 125 in Hebron. Also known as the Cave of the Patriarchs massacre.

There have been some mass shootings in Europe.^[19] Recent examples including the 1987 Hungerford massacre, the 1996 Dunblane massacre and the 2010 Cumbria shootings in the United Kingdom, the 1990 Puerto Hurraco massacre in Spain, the 2001 Zug massacre in Switzerland, the 2002 Erfurt school massacre, the 2009 Winnenden school shooting, the 2011 Frankfurt Airport shooting and the 2016 Munich shooting in Germany, the 2007 Jokela school shooting and the 2008 Kauhajoki school shooting in Finland, the 2010 Bratislava shooting in Slovakia, the 2012 Toulouse and Montauban shootings the January 2015 Charlie Hebdo shooting (and related attacks) and the November 2015 Paris attacks in France and the 2018 Macerata attack in Italy. The deadliest mass shooting of all time (to have been perpetrated by a single person) occurred in Europe with the 2011 Norway attacks in Norway, in which 69 people were shot and killed (in addition to 8 others by a bomb).^[19]

Notable mass shootings that occurred in the Russian Empire, Soviet Union (Russian Soviet Federative Socialist Republic) and Russia include the Pogroms in the Russian Empire, 1992 Tatarstan shooting in Tatarstan, the 2002 Yaroslavsky shooting in Yaroslavsky, the 2002 Moscow theater hostage crisis, the 2012 Moscow shooting and the 2014 Moscow school shooting in Moscow, the 2004 Beslan school siege in Beslan and the 2013 Belgorod shooting in Belgorod.

Notable mass shootings in Canada include the 1989 École Polytechnique massacre, the 1992 Concordia University massacre and the 2006 Dawson College shooting in Montreal, the 2012 Danzig Street shooting and the 2018 Danforth shooting in Toronto, the 2014 Edmonton killings in Edmonton and the 2017 Quebec City mosque shooting in Quebec City.

Notable mass shootings in Mexico include the 2010 Chihuahua shootings in Chihuahua.

The United States has had the most mass shootings out of any country on earth. ^{[21][22][23][24][25]}

In one 2017 study published in Time magazine by criminologist Adam Lankford, it was estimated that 31% of public mass shootings occur in the US, although it has only 5% of the world’s population. ^[26] The study concludes that “The United States and other nations with high firearm ownership rates may be particularly susceptible to future public mass shootings, even if they are relatively peaceful or mentally healthy according to other national indicators.”^[27]

Criminologist Gary Kleck criticized Lankford’s findings stating the study merely shows a proportional relationship, but fails to prove that gun ownership causes mass shootings. Kleck claims that Lankford has been unwilling to share a list of his cases, provide a list of the number of attacks per country, or even list his sources so that others can check his numbers. ^[28] Backlash from economist and gun rights advocate John Lott also raised objections to Lankford’s methodology and refusal to share his data. He speculated that Lankford had overlooked a significant number of mass shootings outside the US, which if accounted for would adjust the nation's share closer to 2.88%, slightly below the world average. ^[29][30]

Mass shootings have also been observed to be followed by an increase in the purchase of weapons, but does not seem to create an increased feeling of needing guns in either gun owners nor non-owners. ^[31]

Notable mass shootings in Argentina include the 2004 Carmen de Patagones school shooting in Carmen de Patagones.

Notable mass shootings in Brazil include the 2011 Realengo massacre in Rio de Janeiro and the Suzano school shooting in Suzano.

Notable mass shootings in Australia include the 1987 Hoddle Street massacre in Hoddle Street, Clifton Hill, Melbourne and the 1996 Port Arthur Massacre in Port Arthur, Tasmania. There were 13 mass shootings with five or more deaths between 1979 and 1996, and depending on definition between 3 and 9, following stricter gun control laws.^[32]

Notable mass shootings in New Zealand include the 1990 Aramoana massacre^[33] in which 14 people were killed (including the perpetrator) in Aramoana and the 2019 Christchurch mosque shootings in Christchurch, which resulted in 51 deaths^[34] and is the largest mass shooting in New Zealand history.^[35]

After mass shootings, some survivors have written about their experiences and their experiences have been covered by journalists. A survivor of the Knoxville Unitarian Universalist church shooting wrote about his reaction to other mass shooting incidents.^[36] 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.^[37] The survivors of the 2011 Norway attacks recounted their experience to GQ.^[38] In addition, one paper studied Swedish police officers’ reactions to a mass shooting.^[39]

Survivors of mass shootings can suffer from Post-traumatic stress disorder.^[40][41]

Mass shooters from outside the United States include Anders Behring Breivik (Norway, 2011), Robert Steinhauser and Tim Kretschmer (Germany, 2002 and 2009), William Unek (Africa, 1954 and 1957), Marc Lépine, Valery Fabrikant and Alexandre Bissonnette (Canada, 1989, 1992 and 2017), Pekka-Eric Auvinen and Matti Juhani Saari (Finland, 2007 and 2008), Genildo Ferreira de França (Brazil, 1997), Friedrich Leibacher (Switzerland, 2001), Ľubomír Harman (Slovakia, 2010), Tristan van der Vlis (Netherlands, 2011), Richard Komakech (Uganda, 1994), Omar Abdul Razeq Abdullah Rifai (Egypt, 2013), Farda Gadirov (Azerbaijan, 2009), Martin Bryant and Benjamin Hoffmann (Australia, 1996 and 2019), Michael Robert Ryan and Derrick Bird (England, 1987 and 2010), Thomas Hamilton (Scotland, 1996), Ljubiša Bogdanović (Serbia, 2013), Woo Bum-kon (South Korea, 1982), Heinz Schmidt (Germany, 1913), Stanisław Ławrynowicz and Janusz Obrąbalski (Poland, 1925), Zdenek Kovar (Czech Republic, 2015), Dipendra Bir Bikram Shah Dev (Nepal, 2001), Jessie Javier Carlos (Philippines, 2017), Mohammed Merah (France, 2012), David Ali Sonboly (Germany, 2016), Vladislav Roslyakov (Crimea, 2018), Guilherme Monteiro and Luiz Henrique de Castro (Brazil, 2019) and Brenton Tarrant (New Zealand, 2019).

Perpetrators of mass shootings in the U.S. include Eric Harris and Dylan Klebold, Edward Charles Allaway, James Edward Pough, Carl Robert Brown, Omar Mateen, Robert A. Hawkins, James Oliver Huberty, Nathan Dunlap, George Hennard, Dylann Roof, Adam Lanza, Nidal Malik Hasan, Charles Whitman, Jeff Weise, Gang Lu, Patrick Sherrill, Eric Houston, Barry Loukaitis, Laurie Dann, Christopher Harper-Mercer, Gian Luigi Ferri, Mark Essex, Scott Evans Dekraai, Steven Kazmierczak, Jennifer San Marco, James Eagan Holmes, Anthony F. Barbaro, Michael McLendon, Rodrick Shonte Dantzler, Jared Lee Loughner, Seung-Hui Cho, Elliot Rodger, Charles Carl Roberts IV, Rizwan Farook and Tashfeen Malik, Robert Lewis Dear, Mitchell Johnson and Andrew Golden, Aaron Alexis, Wade Michael Page, Patrick Edward Purdy, Gavin Eugene Long, Micah Xavier Johnson, Kyle Aaron Huff, One L. Goh, Randy Stair, Stephen Paddock, Devin Patrick Kelley, William Atchison, Nikolas Cruz, Dimitrios Pagourtzis, David Katz, Robert Bowers, Scott Beierle, Ian Long, Zephen Xaver, Dakota Theriot, Gary Martin, John T. Earnest, Trystan Terrell, Devon Erickson and Alec Maya McKinney, DeWayne Craddock, Santino William Legan, Patrick Crusius, Connor Stephen Betts and Seth Ator. Males account for 98% of mass shooters.^[42][43][44] According to a database compiled by Mother Jones magazine, the race of the shooters is approximately proportionate to the overall U.S. population, although Asians are overrepresented and Latinos underrepresented.^[44] Criminologist James Allen Fox said that most mass murderers do not have a criminal record, or involuntary incarceration at a mental health centre,^[45] but an article in The New York Times in December 2015 about 15 recent mass shootings found that six perpetrators had had run-ins with law enforcement, and six had mental health issues.^[46]

Mass shootings can be motivated by extreme religious ideologies, political ideologies (i.e. neo-Nazism, terrorism), misogyny,^[47] mental illness,^[48][49] and extensive bullying,^[50] among other reasons.^[42] Forensic psychologist Stephen Ross cites extreme anger and the notion of working for a cause—rather than mental illness—as primary explanations.^[51] A study by Vanderbilt University researchers found that “fewer than 5% of the 120,000 gun-related killings in the United States between 2001 and 2010 were perpetrated by people diagnosed with mental illness.”^[52] John Roman of the Urban Institute argues that, while better access to mental health care, restricting high powered weapons, and creating a defensive infrastructure to combat terrorism are constructive, they don't address the greater issue, which is “we have a lot of really angry young men in our country and in the world.”^[53]

Author Dave Cullen, in his the 2009 book Columbine on the infamous 1999 Columbine High School massacre and its perpetrators Eric Harris and Dylan Klebold, described Harris as an “injustice collector.”^[54] He expanded on the concept in a 2015 New Republic essay on injustice collectors,^[55] identifying several notorious killers as fitting the category, including Christopher Dorner, Elliot Rodger, Vester Flanagan, and Andrew Kehoe. Likewise, mass shooting expert and former FBI profiler Mary O’Toole also uses the phrase “injustice collector” in characterizing motives of some mass shooting perpetrators.^[56] In relation, criminologist James Alan Fox contends that mass murderers are “enabled by social isolation” and typically experience “years of disappointment and failure that produce a mix of profound hopelessness and deep-seated resentment.”^[57][58] Jillian Peterson, an assistant professor of criminology at Hamline University who is participating in the construction of a database on mass shooters, noted that two phenomena surface repeatedly in the statistics: hopelessness and a need for notoriety in life or in death.^[59] Notoriety was first suggested as a possible motive and researched by Justin Nutt. Nutt stated in a 2013 article, “those who feel nameless and as though no one will care or remember them when they are gone may feel doing something such as a school shooting will make sure they are remembered and listed in the history books.”^[60]

In a 2019 op-ed for the Los Angeles Times, ^[61]Jillian Peterson and James Densley of The Violence Project think tank presented a new, hopeful, framework to understand mass shootings. Based on a study funded by the National Institute of Justice, Peterson and Densley found mass shooters had four things in common: (1) early childhood trauma and exposure to violence at a young age; (2) an identifiable grievance or crisis point; (3) validation for their belief system, have studied past shootings to find inspiration; and (4) the means to carry out an attack. This new framework highlights the complexity of the pathway to a mass shooting, including how each one can be “socially contagious,”^[62] but also provides a blueprint to prevent the next mass shooting. Each one of the four themes represents an opportunity for intervention. By reducing access to firearms (means), slowing contagion (validation), training in crisis intervention de-escalation (crisis), and increasing access to affordable mental healthcare (trauma), a mass shooting can be averted.

In considering the frequency of mass shootings in the United States, criminologist Peter Squires says that the individualistic culture in the United States puts the country at greater risk for mass shootings than other countries, noting that “many other countries where gun ownership is high, such as Norway, Finland, Switzerland and Israel . . . tend to have more tight-knit societies where a strong social bond supports people through crises, and mass killings are fewer.” He is an advocate of gun control, but contends there is more to mass shootings than the prevalence of guns.^[63] The Italian Marxist academic Franco Berardi argues that the hyper-individualism, social alienation and competitiveness fomented by neoliberal ideology and capitalism creates mass shooters by causing people to “malfunction.”^[64]

A noteworthy connection has been reported in the U.S. between mass shootings and domestic or family violence, with a current or former intimate partner or family member killed in 76 of 133 cases (57%), and a perpetrator having previously been charged with domestic violence in 21.^[65][66]

Moynihan said that “almost all school shooters come from families where the parents are either divorced or alienated,”^[67] and Cook argued that “perhaps they wouldn’t need more gun control if they had better divorce control.”^[68]

Some people have considered whether media attention revolving around the perpetrators of mass shootings is a factor in sparking further incidents.^[69] In response to this, some in law enforcement have decided against naming mass shooting suspects in media-related events to avoid giving them notoriety.^[70]

The effects of messages used in the coverage of mass shootings has been studied. Researchers studied the role the coverage plays in shaping attitudes toward persons with serious mental illness and public support for gun control policies.^[71]

In 2015 a paper written by a physicist and statistician, Sherry Towers, along with four colleagues was published, which proved that there is indeed mass shooting contagion using mathematical modeling.^[72] However, in 2017 Towers said in an interview that she prefers self-regulation to censorship to address this issue, just like years ago major news outlets successfully prevent copycat suicide.^[73]

In 2016 the American Psychological Association published a press release, claiming that mass shooting contagion does exist and news media and social media enthusiasts should withhold the name(s) and face(s) of the victimizer(s) when reporting a mass shooting to deny the fame the shooter(s) want to curb contagion.^[74]

Some news media have weighed in on the gun control debate. After the 2015 San Bernardino attack, the New York Daily News’ front-page headline, “God isn’t fixing this,” was accompanied by “images of tweets from leading Republicans who shared their ‘thoughts’ and ‘prayers’ for the shooting victims.”^[75][76] Since the 2014 Isla Vista killings, satirical news website The Onion has repeatedly republished the story “‘No Way To Prevent This’, Says Only Nation Where This Regularly Happens” with minor edits after major mass shootings, to satirise the popular consensus that there is a lack of political power in the United States to prevent mass shootings.^[77]

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

After the 1996 Port Arthur massacre in Port Arthur, Tasmania, Australia, the government changed gun laws in Australia.

In the aftermath of the Christchurch mosque shootings in Christchurch, New Zealand, the country announced a ban on almost all semiautomatic military-style weapons.^[78]

As a result of the mass shootings Hungerford massacre in Hungerford, England and Dunblane school massacre in Stirling, Scotland, the United Kingdom enacted tough gun laws and a buyback program to remove specific classes of firearms (The Firearms Amendment Act 1988 limiting rifles and shotguns, and the 1997 Firearms Amendment Acts which restricted or made illegal many handguns) from private ownership.^[79] There has been one mass shooting since the laws were restricted, the Cumbria shootings in 2010 which killed 13 people.^[19][80]

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.^[81] Some politicians in the U.S. introduced legislation to reform the background check system for purchasing a gun.^[82] 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.”^[83]

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.^[84]

According to British criminologist Peter Squires who has studied gun violence in different countries, mass shootings may be more due to the “individualistic culture” in the U.S. than its firearm laws.^[85]

Gun control policies may cause a lot of controversy due to divided opinions on who should be able to carry a weapon. An opinion survey was conducted by the firm GfK Knowledge Networks to differentiate between the different attitudes towards gun control. There was a gun policy survey and a mental illness survey. Studies showed that over 85% of those questioned supported national background checks into the mental health records of citizens attempting to purchase a gun. More than 50% of people felt that those with mental health issues were more deviant and threatening than those who had good mental health. The study also found that there is large interest in contributing to mental health awareness as well as simply prohibiting those with mental illness from purchasing guns. Nearly two thirds of respondents supported greater government spending on mental health, with more than 60% of people believing this would reduce gun violence in the USA. (Colleen L. Barry, 2013)

As of June 2016, U.S. President Barack Obama had spoken in the aftermath of fourteen mass shootings during his eight-year presidency, repeatedly calling for more gun safety laws in the United States.^[86] 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.”^[87] After the December 2015 San Bernardino attack, Obama renewed his call for reforming gun-safety laws and also said that the frequency of mass shootings in the United States has “no parallel in the world.”^[88] After the February 2018 attack at Florida’s Parkland school shooting at Stoneman Douglas High School, the school's student survivors, teachers, and parents became strong leaders in the effort to ban assault weapon sales and easy accessibility to weapons.^[89]