Why are mass killers always male?

There is no shortage of explanations for the overwhelming maleness of the monster population. Some of the answers reveal a lot—and yet nothing at all. Testosterone fuels aggression. Stipulated. Boys take longer to mature than girls. Stipulated. And like the forebrains of young females, those of young males are not fully myelinated until the late 20s or even early 30s. The forebrain is where executive functions—impulse control, reflection, awareness of consequences—live. In the case of males, who are already trip-wired for aggression, that provides a lot of years to behave badly.

There are, too, the social factors: violent video games, a culture of physical aggression fueled by contact sports and the general tendency of all societies to turn their men into hunters and warriors, putting those jobs off-limits to women or at least making them optional.

But there’s more, and a lot of it has to do with status. Males, for better or worse, are ferociously protective of their position in any tribe, community, or society, and any threat to that position goes to the core of their identity and self-esteem.