Behave: The Biology of Humans at Our Best and Worst

The study of hormones is called endocrinology. For an introduction, see Appendix 2. This chapter asks how hormones, such as testosterone, oxytocin/vasopressin, and stress hormones determine behavior in the hours to days before it occurs.

Testosterone

Across abundant species, males have higher testosterone levels than females, are more aggressive than females, and are most aggressive at periods when testosterone is highest (adolescence and mating season). However, studies show castration reduces but does not fully eliminate aggression. This means testosterone is correlative to aggression, not causative. In other words, it is not testosterone that “makes us aggressive.” This incites an argument that will run through both Sapolsky’s discussion of testosterone and oxytocin/vasopressin. Although we tend to think of hormones as simply causing events in the body to occur (higher testosterone = higher aggression), the actual effects of hormones are context dependent. Just like the morality of a behavior depends on its context, so too does the effect of a hormone.

Testosterone increases confidence and optimism, boosts impulsivity, and creates a pleasure response when spiked. How each of these effects plays out in our actions, however, is context dependent. In primates, testosterone tends to increase aggression. However, when testosterone is increased, subjects are more aggressive to subordinates but not superiors. Testosterone spikes when we are challenged by others, but this does not intrinsically lead to aggression; it can in fact lead to tending and befriending behaviors. This seeming disparity in because testosterone is not about aggression; it is about doing whatever it takes to attain and maintain status, which leads to finding mates. Since this requires aggression in the social contexts of most species, we tend to link testosterone and aggression.

Oxytocin and Vasopressin

Where testosterone has been traditionally associated with aggression, the neuropeptides oxytocin and vasopressin have been associated with love and tenderness. This is also a mistake.

Oxytocin is related to maternal characteristics; it prepares the body for birth and lactation and triggers nurturing instincts across vertebrates. Similarly, vasopressin plays a role in paternal behavior in species where males take a role in child rearing, including humans. In pair bonding primates, oxytocin strengthens bonding and is increased by physical touch and grooming in females and intercourse in males. In humans, oxytocin spikes at the beginning of romantic relationships. Coupled men, when dosed with oxytocin, are less likely to pursue new females than single men: the hormone facilitates bonding, including bonding between child and parent, not “free love.” Oxytocin also inhibits amygdala response and is thereby associated with feelings of safety, higher ratings of strangers as trustworthy, and higher prosociality.

But, like testosterone, oxytocin and vasopressin have some complexities. Alongside fostering prosociality, they foster social competence: increased reliability at detecting social relationships, better Theory of Mind (ToM) predictions, and more time invested in studying the eyes of others. Again, Sapolsky argues, the way to interpret these range of effects is through context. For instance, these “free love” hormones increase aggression when offspring are threatened. Oxytocin also predicts prosociality toward in-group members, but aggression towards out-group members. Oxytocin is not about free love but about bonding to specific individuals and doing whatever is necessary to maintain that bond.

Stress

Stress and its attendant hormonal cascades are also important in behavior. Stress stimuli in the environment activate the fight or flight response, releasing epinephrine (adrenaline) and norepinephrine (noradrenaline), and temporarily shutting down parasympathetic “maintenance projects” like tissue repair and immune system function to divert more energy to escape or attack. While good for hunting or fleeing, the constant anticipatory stress of modern human life can cause chronic stress-related syndromes. Here, Sapolsky gives another example of an evolutionarily adaptive behavior—a short burst of stress diverting energy from immune system function to free it for flight—that in our modern world has become maladaptive.

Stress has other negatives. Stress hormones make us more attentive to images of angry faces and increases fear-learning and fear-related memory consolidation. Chronic stress also weakens PFC control of the amygdala, making fear extinction, or the unlearning of fear, more difficult. This makes sense, as the more we are in stress in an evolutionary environment, the better it is to react immediately without thinking about it if we want to survive.

Stress disrupts working memory, impairs our ability to focus or shift attention between tasks, impairs risk assessment, and increases perseverative behavior (repeating the same behavior again and again regardless of whether it is working). Under stress, emotional information is processed more quickly and less accurately, often leading to reactive aggression. Aggression, however, reduces stress, leading often to “stress inducement displacement aggression” (132)—in other words, taking it out on someone else. This explains certain human social phenomena, such as increases in spousal abuse during economic downturn. Stress also causes more selfish behavior. Perhaps because female stress responses in the brain trigger more oxytocin, female stress reactions are generally of a more “tend and befriend” than “fight or flight” nature.

In summary, hormones are the body’s chemical messengers. Different hormones are associated with different, context-dependent behaviors. As best shown in the stress example, overall hormone levels have longer-lasting predictive effects on behavior than the instantaneity of a message bouncing around in the brain or a stimulus causing that message to fire. Hormone effects structure our moods, our development, and our patterns of behavior throughout the day, throughout the year, and throughout life.