"There's a feeling of entitlement that white men have that black men don't," says Fox, a professor at Northeastern University and co-author of "Extreme Killing." "They often complain that their job was taken by blacks or Mexicans or Jews. They feel that a well-paid job is their birthright. It's a blow to their psyche when they lose that. . . . If you're a member of a group that hasn't historically experienced unemployment, there's a far greater stigma to [losing a job] than those who have."
A perhaps telling statistic: About 60 percent of mass killers are older than 30, an indication, Fox says, that "it takes years to develop the level of frustration and anger" that expresses itself as indiscriminate murder.
It's not just the killer whose race matters, he says, but the victims', too. Incidents in which the dead are poor, nonwhite or of immigrant background tend to get less attention from the media than other mass shootings.
The killing of five children (and wounding of 29) at an elementary school in Stockton, Calif., in 1989, for example, was forgotten long before the attacks at Columbine High School in suburban Denver a decade later, he says, possibly because the young victims in the first crime were refugees from Southeast Asia, and in the latter, the victims were middle- and upper-middle-class teens. The Stockton shooter was a white, unemployed man.
Similarly, the massacres of 13 people in Binghamton, N.Y., in 2009 and seven people at a small college in Oakland, Calif., this year attracted far less attention than the Aurora killings, despite similar body counts. In both of those cases, the alleged perpetrators and almost all of the victims were poor or working-class immigrants.