WASHINGTON: The US was the site of 31 per cent of the world's public mass shootings from 1966-2012, despite having only 5 per cent of the world's population, according to a new study.According to a research by Adam Lankford , associate professor of criminal justice at The University of Alabama , a combination of American exceptionalism, American gun culture and stressors are potential factors in explaining the commonality of public mass shooters in the US.Previous studies did not include statistics of offenders worldwide, a gap in research that has partly contributed to the assumption that mass shootings are an American problem, the study 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," Lankford said.Lankford's quantitative assessment of 171 countries relied on multiple sources, including active shooter reports from the New York Police Department and the The Federal Bureau of Investigation (FBI), along with multiple international sources.He used data of public mass shootings that resulted in the deaths of four or more people and didn't include data of homicides committed during domestic disputes, hostage situations or robberies."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 US, where more than half of shooters used at least two weapons," according to the study."The US, 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," Lankford said."Given the fact that the US 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," he 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 US (6.87 victims) because so many horrific attacks have occurred here," he added.Lankford's paper, "Mass Shooters, Firearms, and Social Strains: A Global Analysis of an Exceptionally American Problem, will be published in a criminological journal in the coming months.