In the aftermath of the Sandy Hook Elementary School killings, dozens of questions remain unanswered. But one common trait of most mass shootings is rarely discussed, much less clarified: Why are the perpetrators almost always men?
"Mass murderers are almost always male (I'd say at least 98 percent), most often have a motive (e.g., revenge), and most have a definitive tie to the victims - or the victims symbolize something to the killer," said Marissa Harrison, assistant professor of psychology at Penn State Harrisburg, citing research she's conducted or reviewed.
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.
It's not to say that women aren't violent or vengeful, psychologists say; rather, that the violence is of a different nature.
"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."
Male and female murderers seem to prefer different weapons: men use guns more, and women suffocate and drown victims, said Mary Muscari, an associate professor in the Decker School of Nursing at Binghamton University who specializes in child health, mental health and forensics. (A Gallup poll found that 46 percent of men own a gun in America, as opposed to 23 percent of women.)
"People assume they're going to be guys," Muscari said. "I think it's something that needs to be looked at: If we figure out why women don't do it, it might help us understand why men do."
Many point to revenge as a motive, but both women and men are vengeful. Parsing apart male and female responses to revenge, however, yields some gender differences:
"Women are pretty good with revenge too," Muscari said. "Women who have been sexually assaulted often have revenge fantasies." The difference? Women feel guilty about them, Muscari said.
Farley refers to those psychological characteristics as each individual's "bag of traits," and they probably have some genetic influence. Part of most females' bags of traits, he said, are personal connections: relationships, emotion and care. Men are more likely to seek justice. So a woman's violence may be more likely to stem from an overwhelming personal life, an exit out of a seemingly impossible home situation, for example, whereas a man's may be a more abstract search for justice.
"Going to war is not part of (a typical woman's) bag of tricks," he said.
And gender issues in mass murder cases may go beyond the identity of the perpetrator, Muscari pointed out.
"Some (murderers) seem to specifically target women," she said. "What does that mean? What is the overall picture with gender?"
With so many unanswered questions, any sort of solution requires an effort on a massive scale, Farley said.
"We need science to do a project on the scale of going to the moon or the Human Genome Project directed at human violence," he said.