Mass public shootings have become such a part of American life in recent decades that the most dramatic of them can be evoked from the nation’s collective memory in a word or two: Luby’s. Jonesboro. Columbine. And now, Virginia Tech. Since Aug. 1, 1966, when Charles Whitman climbed a 27-story tower on the University of Texas campus and started picking people off, at least 100 Americans have gone on shooting sprees.

And all through those years, the same questions have been asked: What is it about modern-day America that provokes such random violence? Is it the decline of traditional morals? The depiction of violence in entertainment? The ready availability of lethal firepower?

Northeastern University criminologist James Alan Fox blames guns, at least in part. He notes that seven of the eight deadliest mass public shootings have occurred in the past 25 years.

"I know that there were high-powered guns before," he said. "But this weaponry is just so much more pervasive than it was."

Australia had a spate of mass public shooting in the 1980s and ’90s, culminating in 1996, when Martin Bryant opened fire at the Port Arthur Historical Site in Tasmania with an AR-15 assault rifle, killing 35 people.

Within two weeks the government had enacted strict gun control laws that included a ban on semiautomatic rifles. There has not been a mass shooting in Australia since.

Yet Grant Duwe, a criminologist with the Minnesota State Department of Corrections, said the availability of guns was not a factor in his exhaustive statistical study of mass murder during the 20th century.

Duwe found that the prevalence of mass murders, defined as the killing of four or more people in a 24-hour period, tends to mirror that of homicide generally. The increase in mass killings during the 1960s was accompanied by a doubling in the overall murder rate after the relatively peaceful 1940s and ’50s.

In fact, Duwe found that mass murder was just as common during the 1920s and early 1930s as it is today. The difference is that then, mass murderers tended to be failed farmers who killed their families because they could no longer provide for them, then killed themselves. Their crimes embodied the despair and hopelessness of the Dust Bowl and the Great Depression, the sense that they and their families would be better off in the hereafter than in the here and now.

On Dec. 29, 1929, a 56-year-old tenant farmer from Vernon, Texas, named J.H. Haggard shot his five children, aged 6 to 18, in their beds as they slept. Then he killed himself. He left a note that said only, "All died. I had ruther be ded. Look in zellar."

Despondent men still kill their families today. But public shooters like Virginia Tech’s Seung-Hui Cho are different. They are angrier and tend to blame society for their failures, sometimes singling out members of particular ethnic or socio-economic groups.

"It’s society’s fault … Society disgusts me," Kimveer Gill wrote in his blog the day before he shot six people to death and injured 19 in Montreal last year.

In the videos and essays he left behind, Cho ranted about privileged students and their debauched behavior.

He also mentioned the Columbine killings, referring to Dylan Klebold and Eric Harris as "martyrs." Imitation undoubtedly plays a role in mass shootings as well, said Daniel A. Cohen, a historian at Case Western Reserve University in Cleveland.

"Certain types of crimes gain cultural resonance in certain periods," Cohen said.

So many post office employees gunned down their co-workers during the 1980s and early ’90s that they spawned a neologism. To "go postal," according to the Webster’s New World College Dictionary, is "to become deranged or go berserk."

The most recent postal shooting was in January 2006 when Jennifer San Marco, a former employee who had been fired a few years earlier because of her worsening mental state, walked into a letter sorting facility in Goleta, Calif., and killed six people with a handgun.

Criminologist Fox speculates that the increasing popularity of workplace killings, and public shootings generally, may be partly due to decreasing economic security and increasing inequality. America increasingly rewards its winners with a disproportionate share of wealth and adoration, while treating its losers to a heaping helping of public shame.

"We ridicule them. We vote them off the island. We laugh at them on `American Idol,’" Fox said.

But there has also been an erosion of community in America over the past half-century, and many scholars believe it has contributed to the rise in mass shootings.

"One would think that there’s some new component to alienation or isolation," said Jeffrey S. Adler, a professor of history and criminology at the University of Florida.

People used to live in closer proximity to their families and be more involved with civic and religious institutions. They were less likely to move from one part of the country to another, finding themselves strangers in an unfamiliar environment.

Even so, the small-town America of yesteryear wasn’t completely immune. On March 6, 1915, businessman Monroe Phillips, who had lived in Brunswick, Ga., for 12 years, killed six people and wounded 32 before being shot dead by a local attorney. Phillips’ weapon: an automatic shotgun.

Remarkably, violence in today’s media seems to have little to do with mass public shootings. Only a handful of them have ever cited violent video games or movies as inspiration for their crimes. Often they are so isolated and socially awkward that they are indifferent to popular culture.

Ultimately, it is impossible to attribute the rise in mass shootings to any single cause. The crimes only account for a tiny fraction of homicides.

And a significant fraction of those who commit them, including Cho, either kill themselves or are killed by police before they can be questioned by investigators.

Via Yahoo