Why does the U.S. lead the world in mass shootings?

The United States has had five times the number of mass shootings in the last 50 years than any other country, a new study by University of Alabama criminologist Adam Lankford finds.

"For decades, people have wondered if the dark side of American exceptionalism is a cultural propensity for violence," Lankford writes, "and in recent years, perhaps no form of violence is seen as more uniquely American than public mass shootings."

According to the study, presented this week at the American Sociological Association's annual meeting in Chicago, there were 291 documented mass shootings in the world between 1966 and 2012, with 90 (31 percent) occurring in the United States.

The Philippines, with 18 mass shootings between 1966 and 2012, was a distant second, according to the study, followed by Russia (15), Yemen (11), and France (10).

Why do so many mass shootings happen in America? There are several factors, according to Lankford:

? America's high rate of gun ownership
? The idolization of fame among U.S. mass shooters
? And what Lankford calls the "the dark side of American exceptionalism"

The failure of the U.S. health care system to treat mental illness is partly to blame for America's disproportionate share of mass shootings, but it is not a unique factor. What is unique, Lankford says, is America's gun-toting culture.

According to data cited by the study, there were approximately 88.8 firearms per 100 people living in the United States in 2007, well above Yemen's 54.8 firearms per 100 people — the world's second-highest rate of gun ownership.

"Because of its world-leading firearm ownership rate, America does stand apart," Lankford writes, "and this appears connected to its high percentage of mass shootings."

What's more, American mass shooters are 3.6 times more likely to arm themselves with multiple weapons than mass shooters in other countries.

Lankford, author of "The Myth of Martyrdom: What Really Drives Suicide Bombers, Rampage Shooters, and Other Self-Destructive Killers," studied how the motives of mass shooters in the United States differed from those elsewhere. And America's cultural obsession with fame clearly plays a role.

"Increasingly in America — perhaps more than in any other country on the globe — fame is revered as an end unto itself," he writes. "Some mass shooters succumb to terrible delusions of grandeur and seek fame and glory through killing. They accurately recognize that the only way they can guarantee that their names and faces adorn magazines, newspapers, and television is by slaughtering unarmed men, women, or children."

That, combined with the failed pursuit "American dream," drives some — especially those with serious mental health issues — to mass violence, Lankford argues.

"It's a common theme," said Lankford, who studied the killers' manifestos, journals, and online diaries. "They do have high aspirations, and when they struggle, they look to blame someone."

That's why mass shootings often occur at U.S. schools and workplaces.

"There's a sense of bullying, of mistreatment," Lankford said. "They're often blaming their boss, their teacher, fellow students, or coworkers for the system being rigged."

Following the mass shooting at a historic black church in Charleston, S.C., in June, President Barack Obama called once again for a national debate on America’s gun laws.

"I’ve had to make statements like this too many times," an exasperated Obama said at the White House. "Communities like this have had to endure tragedies like this too many times. We don’t have all the facts, but we do know that, once again, innocent people were killed in part because someone who wanted to inflict harm had no trouble getting their hands on a gun.

“It is in our power to do something about it. I say that recognizing the politics in this town foreclose a lot of those avenues right now,” the president added. “But it’d be wrong for us not to acknowledge it, and at some point, it’s going to be important for the American people to come to grips with it and for us to be able to shift how we think about the issue of gun violence collectively.”

In his study's conclusion, Lankford agreed.

"Unfortunately, the most obvious step the United States could take to reduce mass shootings may also be the most politically challenging," he writes. "Reduce firearms availability."