Murder, the most extreme but usually unintended form of interpersonal violence, is explained as the escalation beyond normal bounds of face-to-face dominance competition. No neurohormonal substrate has been identified as a direct cause of face-to-face violence, but testosterone appears to play an indirect role, motivating dominant behavior. The relationship between testosterone and dominance is reciprocal, with elevations in the hormone as much a consequence of competition as its cause. A heightened testosterone may be especially prevalent among young men in "honor subcultures," where threats to one's personal reputation are ubiquitous, increasing the probability that dominance competitions, often triggered by trivial disagreements, result in tragic consequences.