Despite having only about 5 percent of the world's population, the United States was the attack site for a disproportionate 31 percent of public mass shooters globally from 1966-2012, according to new research that will be presented at the 110th Annual Meeting of the American Sociological Association (ASA).

"The United States, Yemen, Switzerland, Finland, and Serbia are ranked as the Top 5 countries in firearms owned per capita, according to the 2007 Small Arms Survey, and my study found that all five are ranked in the Top 15 countries in public mass shooters per capita," said study author Adam Lankford, an associate professor of criminal justice at the University of Alabama. "That is not a coincidence."

Lankford's study, which examines the period from 1966-2012, relies on data from the New York City Police Department's 2012 active shooter report, the FBI's 2014 active shooter report, and multiple international sources. It is the first quantitative analysis of all reported public mass shootings around the world that resulted in the deaths of four or more people. By definition, these shootings do not include incidents that occurred solely in domestic settings or were primarily gang-related, drive-by shootings, hostage taking incidents, or robberies.

"My study provides empirical evidence, based on my quantitative assessment of 171 countries, that a nation's civilian firearm ownership rate is the strongest predictor of its number of public mass shooters," Lankford said. "Until now, everyone was simply speculating about the relationship between firearms and public mass shootings. My study provides empirical evidence of a positive association between the two."

As part of his study, Lankford explored how public mass shootings in the U.S. differed from those abroad. He found that public mass shooters in other countries were 3.6 times less likely to have used multiple weapons (typically multiple guns, but occasionally a gun plus another weapon or weapons) than those in the U.S., where more than half of shooters used at least two weapons.

"Given the fact that the United States has over 200 million more firearms in circulation than any other country, it's not surprising that our public mass shooters would be more likely to arm themselves with multiple weapons than foreign offenders," Lankford said. "I was surprised, however, that the average number of victims killed by each shooter was actually higher in other countries (8.81 victims) than it was in the United States (6.87 victims) because so many horrific attacks have occurred here."

The side-effect of America having experienced so many mass shootings may be that our police are better trained to respond to these incidents than law enforcement in other countries, which reduces the number of casualties, Lankford suggested.

In addition to killing fewer people and using more weapons, U.S. public mass shooters were also more likely to attack in schools, factories/warehouses, and office buildings than offenders in other countries. But compared to U.S. shooters, attackers abroad were significantly more likely to strike in military settings, such as bases, barracks, and checkpoints.

While Lankford's study revealed a strong link between the civilian firearm ownership rate and the large number of public mass shooters in the United States, he said there could be other factors that make the U.S. especially prone to public mass shooting incidents.

"In the United States, where many individuals are socialized to assume that they will reach great levels of success and achieve 'the American Dream,' there may be particularly high levels of strain among those who encounter blocked goals or have negative social interactions with their peers, coworkers, or bosses," Lankford explained. "When we add depression, schizophrenia, paranoia, or narcissism into the mix, this could explain why the U.S. has such a disproportionate number of public mass shooters. Other countries certainly have their share of people who struggle with these problems, but they may be less likely to indulge in the delusions of grandeur that are common among these offenders in the U.S., and, of course, less likely to get their hands on the guns necessary for such attacks."

In terms of the study's policy implications, Lankford said, "The most obvious implication is that the United States could likely reduce its number of school shootings, workplace shootings, and public mass shootings in other places if it reduced the number of guns in circulation."

There is evidence that such an approach could be successful, according to Lankford. "From 1987-1996, four public mass shootings occurred in Australia," Lankford said. "Just 12 days after a mass shooter killed 35 people in the last of these attacks, Australia agreed to pass comprehensive gun control laws. It also launched a major buyback program that reduced Australia's total number of firearms by 20 percent. My study shows that in the wake of these policies, Australia has yet to experience another public mass shooting."

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More information: The paper, "Mass Shooters, Firearms, and Social Strains: A Global Analysis of an Exceptionally American Problem," will be presented on Sunday, Aug. 23, at 2:30 p.m. CDT in Chicago at the American Sociological Association's 110th Annual Meeting.