their antlers, humans using their fists, or chess masters using their minds. Along with testosterone’s increase is an increase in dopamine, a decrease in cortisol and serotonin, and a decrease in frontal lobe activity and control. Within the environment of a promiscuous brain, this physiological ballet affects our sense of fairness, empathy, moral conscience, attitude toward retribution and justice, as well our willingness to engage in lethal aggression. Brain imaging studies reveal that the prefrontal cortex plays an essential role in regulating our aggressive instincts, when it’s working. When individuals respond aggressively to an unfair offer in a bargaining game, testosterone levels rise and activity decreases in a part of the prefrontal cortex associated with self-control. Thus, testosterone’s effectiveness in human aggression 1s facilitated by a loss of control. Damage within this region of the brain causes abnormal aggressive responses to not only direct insult, but even such trivial matters as being offered a lowish offer in the ultimatum game discussed earlier. Anatomical and functional abnormalities within this region of the frontal lobes are also associated with aggressive pathology, such as psychopathology. There are also individual differences in aggressive tendencies among healthy people, due in part to differences in the patterns of activity between the right and left prefrontal cortices. Heightened activity on the left is associated with greater sensitivity to reward, lowered sensitivity to punishment, and considerably stronger aggressive responses to threatening stimul1, such as an angry face. This is not simply a correlation, as evidenced by studies that experimentally either suppress or increase activity in one hemisphere compared with the other. For example, if you contract your right hand you will increase activity in the left hemisphere of the brain; conversely, contracting the left hand increases activity in the right hemisphere. Subjects co