Social aggression is an escalating hazard for individuals and society. It is most frequently observed as impulsive–reactive aggression in antisocial personality disorder (APD), but in psychopathic aggressive personalities instrumental social aggression is more prominent. However, the psychobiological mechanisms underlying human social aggression are still poorly understood. Here we propose a psychobiological mechanism that may explain human social aggression wherein the steroid hormones cortisol and testosterone play a critical role. High levels of testosterone and low levels of cortisol have been associated with social aggression in several species but it seems that in those individuals wherein these hormonal markers combine social aggression is most violent. In this review we discuss fundamental and clinical research which underscores the potential of the testosterone–cortisol ratio as a possible marker for criminal aggressive tendencies.