New analysis has shown that although the U.S. only makes up 5 percent of the global population, the country has seen 31 percent of global public mass shootings between 1966 and 2012. The research, which is set to be presented at the 110th Annual Meeting of the American Sociological Association (ASA), was based 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.

The report claims to be the first of its kind to quantify all reported mass public shootings which have resulted in the deaths of at least four people. However, gang related crime, drive-by shootings, hostage situations and robberies have not been taken into consideration.

Author Adam Lankford, an associate professor of criminal justice at the University of Alabama, says: "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. That is not a coincidence.

Lankford says that previously much of the correlation between public mass shootings and gun ownership rates had been speculative. With his study, based on quantitative assessment of empirical evidence from 171 countries, Lankford is confident he has evidence of a positive association between the two. "A nation's civilian firearm ownership rate is the strongest predictor of its number of public mass shooters," he states.

Another aspect Lankford looked at was how U.S. public mass shootings varied from those in other countries. He found that shooters in other countries were 3.6 times more likely to use multiple 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.

However, Lankford was surprised to discover that the average number of victims was higher in other countries (8.81 victims) than it was in the United States (6.87 victims).

Speaking to the root causes of public mass shootings in the U.S., Lankford said: "In the U.S., 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 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."