The research reveals that a nation's civilian firearm ownership rate is the strongest predictor of its number of public mass shooters. It also shows that despite having only about 5 percent of the world's population, the United States accounted for 31 percent of public mass shooters globally from 1966 to 2012.

"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, a criminal justice professor at the University of Alabama, in a press release. "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. According to Lankford, 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.

However, it does not include data on incidents that occurred solely in domestic settings or that were primarily gang-related, drive-by shootings, hostage taking incidents or robberies.

Confirmations and surprises

"The connection between firearms and mass shootings is something people have speculated about for a long time," Lankford told DW in a telephone interview. "This is the first time that empirical evidence confirmed this connection, but the strong association between the two was even surprising for me."

Lankford says the data speaks for itself

Even a country like Switzerland, with its peaceful reputation, reflects the study's findings. "There were only two mass shootings there in the 90s, but given the small size of the population, the resulting rate approaches that of the US," explained Lankford.

The study also found that US shooters were more likely to use multiple weapons. Despite this, the average number of victims per shooter was lower than in other countries.

"That was another surprising finding," said Lankford. "One reason for this could be that the difference in the death toll resulting from multiple weapons as opposed to one weapon in the US is not as dramatic as in other countries. Also, because the US has had so many attacks of this kind, our law enforcement authorities have learned how to respond to these incidents quickly and effectively."

Unclear motives

On the topic of root causes of public mass shootings in the US, Lankford believes that in a country 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, co-workers or bosses."

"When we add depression, schizophrenia, paranoia or narcissism into the mix, this could explain why the US has such a disproportionate number of public mass shooters," Lankford said in the press release.

But whether the study's findings will aid the movement for more gun control in the US is uncertain.

"I didn't come into this study with any gun control agenda - I just let the data speak for itself," Lankford told DW. "Whether people are willing to act on it is another question."

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