Now that we've acknowledged that women can engage in violent behavior, let's not beat around the bush: A majority of violence, both mass and otherwise, is committed by men. (Especially if you bring war and politics into the issue, which might not fit in this article but is worth pondering.)
When you consider that between 1980 and 2008, 90 percent of those who committed homicide were men and males were nine times more likely to offend, it could strike you as less shocking that men committed nearly all mass murders. They seem to be the ones committing crimes, period [source: Cooper and Smith]. (Which might make it all the more shocking that historically there have been only two crimes committed equally as often by women: murdering their own children and shoplifting [source: Lithwick].)
Are men biologically wired to be more impulsive or aggressive? It seems silly to say that men are inherently violent, but there are some natural factors that we should consider. Men are stronger -- that could mean that in a mass killing setting, they'd be more likely to overpower any challenger and less likely to be stymied. So while that doesn't mean they're more likely to plot mass killing, it might lead back to our examples of violent women who stopped before more people were hurt.
A popular myth is that higher levels of testosterone lead to aggression, which might explain why males are more likely to commit mass murders. However, studies point to the fact that violence might lead to testosterone, not the other way around. Testosterone might be used to prepare the body for aggression (and not initiate it) in both men and women [source: Mims]. And to prove once again that nature might just need a helping hand from nurture, one researcher points out one way to absolutely raise a man's testosterone level: hand him a gun [source: Mims]. (The 2006 study that drew this conclusion only included male participants.)
That brings us to another very interesting juncture: For both biological and societal reasons men might commit more violence. A study at Iowa State University determined that video game exposure does, in fact, cause a physiological response in the body. Video game players were less physiologically aroused by real-life violence, and the results were all fairly standard across participants [source: Carnagey, Anderson and Bushman].
But, of course, a caveat: This showed all individuals were desensitized to violence after video game exposure, not just males -- or not just some males. While there might be an argument that men or boys play more video games and thus are more widely becoming inured to real-life violence, that certainly can't explain why some of them turn into mass murderers.