Why they kill

MASS MURDER certainly wasn't invented with the 1966 Texas Tower shootings. For as long as there has been history, there has been murder -- including horrific mass murder. Certainly in the first half of the 20th century there were examples, such as the case of Howard Unruh, a mentally ill war veteran who killed 13 people in 13 minutes with a Luger pistol on the streets of Camden, N.J., in 1949.

But 1966 was a dramatic turning point. On Aug. 1, Charles Whitman, a student at the University of Texas at Austin, climbed up a 27-story tower and killed 14 people, wounding 31 others, before being shot dead by the police.

After the Whitman killings, the incidents started to climb. Mass murders (and, especially, mass shootings) became increasingly common -- George Hennard in Killeen, Texas; Patrick Edward Purdy in Stockton; James Huberty in San Ysidro; Eric Harris and Dylan Klebold at Columbine High School in Littleton, Colo., to name just a few -- and the body counts began to grow as well. Seven of the eight largest mass shootings in modern U.S. history have occurred in the last 25 years.

The murder of at least 32 people at Virginia Tech on Monday may have been the worst, but it was only one of about 20 mass shootings that occur each year in the United States (a subset of the two dozen or so mass murders). A mass murder is defined as an event in which four or more people are killed in the same episode. Serial killings, by contrast, occur over an extended time.

What accounts for the increase? Is it possible that man (and yes, 95% of all mass murderers are men, who tend to be far more comfortable and better trained in using firearms) has simply grown more evil and bloodthirsty since 1966 than during the previous millenniums of human existence?

Of course not. But several changes have taken place that have made such incidents more common.

One, of course, is the change in the potency of weaponry. Before 1966, the best weapons available to most would-be killers were pistols, rifles, maybe a shotgun. That is no longer the case; today, semiautomatics are all too easily accessible.

But there also have been societal changes that have increased the incidence of mass killing. In studying mass murderers over 25 years, my colleague, Jack Levin, and I have identified five factors that exist in virtually all cases.

First, perpetrators have a long history of frustration and failure and a diminished ability to cope with life's disappointments.