Evolutionary psychology suggests that a threat to status could be a trigger for extremely violent behavior, Harrison said. After examining 90 male mass murderers from 1996 to 2008, Harrison and a colleague found that a threat to status (being bullied or a job loss, for example) triggered the violence in 88 percent of the cases.

“Anything that attacks a man’s status, then, is really a reproductive threat,” Harrison said. …

“For the men, it’s almost like going to war. Men often have assault weapons and big-time military stuff; they dress in quasi-military attire, and they assault a physical place like a building; it’s almost random, without much personal connect,” said psychologist and Temple University professor Frank Farley. “For women — my gosh, it’s so different. It’s up close and personal. It’s the personal, family life. Most infanticides are by mothers.”