In the aftermath of mass killings, people ask why weren’t the shooters stopped. The same kinds of questions are asked about people like Christoper Harper-Mercer, who slaughtered nine people in Roseburg, Or., and shooters in Charleston, S.C., Isla Vista, Ca., and the Washington Navy Yard. What seems telling about the killers, says the New York Times, is how much they look and seem like so many others who do not inflict harm. “The big problem is that the kind of pattern that describes them describes tens of thousands of Americans, even people who write awful things on Facebook or the Internet,” said Northeastern University criminologist James Alan Fox. “We can't round up all the people who scare us.” The mass public killings that have drawn such intense attention are a phenomenon that largely did not occur until two generations ago.
Grant Duwe, a criminologist at the Minnesota Department of Corrections, has studied more than 1,300 mass murders that took place from 1900 to 2013. He classifies 160 as mass public shootings, ones in which at least four people were shot and killed in a concentrated period, excluding those in family settings or involving other crimes. The episode he says “introduced the nation to the idea of mass murder in a public space” was in 1966, when Charles Whitman climbed a tower at the University of Texas at Austin and killed 16 people. The Congressional Research Service charted an increase in these shootings since then, from an average of one per year during the 1970s to four in the 2000s and a slight uptick in the last few years. The murderers are always white males; most are single, separated or divorced.