After the recent shooting at the Washington Navy Yard, mass murders have once again occupied the attention of the American news media. From Aurora to Newtown to Washington, DC, one obvious trend among rampage shooters is that they are almost exclusively male. In a recent article for NPR, Linton Weeks asked members of the Homicide Research Working Group why women are so underrepresented among these shooters.

According to Lin Huff-Corzine of the University of Central Florida, women simply don’t kill as much as men. Examining FBI data, she and her colleagues found that:

Between 2001 and 2010, less than 8 percent of mass murder offenders in the U.S. were women, she says, adding that some of the women included in the statistics assisted in a crime but did not pull a trigger.

Candice Batton, the director of the School of Criminology and Criminal Justice at the University of Nebraska, Omaha explained that about 90-91% of homicide perpetrators were male. Men are more lethally violent than women in part due to their tendency to externalize blame for their personal problems, leading to “anger and hostility toward others.”

Batton says that women, on the other hand, “are more likely to develop negative attributions of blame that are internal in nature, that is: ‘The cause of my problems is some failing of my own: I didn’t try hard enough, I’m not good enough.’ And this, in turn, tends to translate into feelings of guilt and depression that are targeted toward oneself.”