<P> A comprehensive report by USA Today tracked all mass killings from 2006 through 2017 in which the perpetrator willfully killed 4 or more people . For mass killings by firearm for instance, it found 271 incidents with a total of 1,358 victims . Mother Jones listed seven mass shootings, defined as indiscriminate rampages in public places resulting in four or more victims killed, in the U.S. for 2015 . An analysis by Michael Bloomberg's gun violence prevention group, Everytown for Gun Safety, identified 110 mass shootings, defined as shootings in which at least four people were murdered with a firearm, between January 2009 and July 2014; at least 57% were related to domestic or family violence . </P> <P> Other media outlets have reported that hundreds of mass shootings take place in the United States in a single calendar year, citing a crowd - funded website known as Shooting Tracker which defines a mass shooting as having four or more people injured or killed . In December 2015, The Washington Post reported that there had been 355 mass shootings in the United States so far that year . In August 2015, The Washington Post reported that the United States was averaging one mass shooting per day . An earlier report had indicated that in 2015 alone, there had been 294 mass shootings that killed or injured 1,464 people . Shooting Tracker and Mass Shooting Tracker, the two sites that the media have been citing, have been criticized for using a broader criteria--counting four victims injured as a mass shooting--thus producing much higher figures . </P> <P> Several possible factors may work together to create a fertile environment for mass murder in the United States . Most commonly suggested include: </P> <Ol> <Li> Higher accessibility and ownership of guns . The US has the highest per - capita gun ownership in the world with 88.8 firearms per 100 people; the second highest is Yemen with 54.8 firearms per 100 people . </Li> <Li> Mental illness and its treatment (or the lack thereof) with psychiatric drugs . This is controversial . Many of the mass shooters in the U.S. suffered from mental illness, but the estimated number of mental illness cases has not increased as significantly as the number of mass shootings . </Li> <Li> The desire to seek revenge for a long history of being bullied . </Li> <Li> Desire for fame and notoriety . </Li> <Li> The copycat phenomenon . </Li> <Li> Failure of government background checks due to incomplete databases and / or staff shortages . </Li> <Li> The widespread chronic gap between people's expectations for themselves and their actual achievement, and individualistic culture . </Li> </Ol>

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