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 is 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 stimuli, 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 contracting the night hand in an experimental setting showed more aggressive responses to insult than did subjects contracting the left hand. The next time someone shakes a fist at you, check whether it is the right or left hand. If the person is from a different group and holds a fundamentally different suite of ideological beliefs, which hand is clenched is the least of your worries.
Favoritism toward those who are like us, combined with hatred toward those who are not, is common in animals. As noted above, in-group favoritism or parochialism can lead to heightened levels of cooperation within groups, while simultaneously increasing the level of hostility towards those outside. Our promiscuous brain facilitated this co-evolutionary process, inviting the hormone oxytocin into the mix. Among mammals, including humans, oxytocin is released in females during labor and breastfeeding, and in both males and females during social bonding and parenting. This has led many to think of oxytocin as the cuddle hormone or love drug. Floating within the human brain, oxytocin boosts trust in games of cooperation, and greatly increases our ethnocentric biases. The Dutch psychologist Carsten De Dreu and his colleagues ran a series of experiments that required male subjects to spray oxytocin or a
control up their noses. When oxytocin shoots up the nose, it goes straight to the brain. Relative to the
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