The strongest genetic predictor for violence and aggression is . . . having a Y chromosome. Four times more males than females suffer from antisocial personality disorder, which is characterized by impulsive aggression, and a glance at any police blotter shows which sex commits more violent crimes. While the way we raise boys has something to do with that, there is good reason to believe that testosterone is not exactly an innocent by-stander. Men average several times the amount of testosterone that women do, for one thing, and a pile of studies implicate testosterone in aggression. Call it the original cause of ‘roid rage.

Trouble is, scientists have not nailed down exactly how testosterone increases the likelihood that a man will punch his fist through a wall (or someone’s face). A new study provides the first evidence of how testosterone’s effects on the brain might do that. Elevated levels of this steroid hormone, scientists in the Netherlands find in a study to be published in the August issue of Psychological Science, seem to mess up the brain’s ability to notice and understand angry facial expressions.