Behave: The Biology of Humans at Our Best and Worst

by Robert M. Sapolsky

Putting facts into nice cleanly demarcated buckets of explanation has its advantages—for example, it can help you remember facts better. But it can wreak havoc on your ability to think about those facts.

This is because the boundaries between different categories are often arbitrary, but once some arbitrary boundary exists, we forget that it is arbitrary and get way too impressed with its importance.

When we’re scared, we secrete the same hormone as would some subordinate fish getting hassled by a bully. The biology of pleasure involves the same brain chemicals in us as in a capybara. Neurons from humans and brine shrimp work the same way. House two female rats together, and over the course of weeks they will synchronize their reproductive cycles so that they wind up ovulating within a few hours of each other. Try the same with two human females (as reported in some but not all studies), and something similar occurs. It’s called the Wellesley effect, first shown with roommates at all-women’s Wellesley College.8 And when it comes to violence, we can be just like some other apes—we pummel, we cudgel, we throw rocks, we kill with our bare hands. So some of the time an intellectual challenge is to assimilate how similar we can be to other species. In other cases the challenge is to appreciate how, though human physiology resembles that of other species, we use the physiology in novel ways. We activate the classical physiology of vigilance while watching a scary movie. We activate a stress response when thinking about mortality. We secrete hormones related to nurturing and social

behavior has occurred—one that is reprehensible, or wonderful, or floating ambiguously in between. What occurred in the prior second that triggered the behavior? This is the province of the nervous system. What occurred in the prior seconds to minutes that triggered the nervous system to produce that behavior? This is the world of sensory stimuli, much of it sensed unconsciously. What occurred in the prior hours to days to change the sensitivity of the nervous system to such stimuli? Acute actions of hormones.

The cortex is the gleaming, logical, analytical crown jewel of layer 3. Most sensory information flows there to be decoded. It’s where muscles are commanded to move, where language is comprehended and produced, where memories are stored, where spatial and mathematical skills reside, where executive decisions are made.

Nauta studied what brain regions sent axons to the frontal cortex and what regions got axons from it. And the frontal cortex was bidirectionally enmeshed with the limbic system, leading him to propose that the frontal cortex is a quasi member of the limbic system. Naturally, everyone thought him daft. The frontal cortex was the most recently evolved part of the very highbrow cortex—the only reason why the frontal cortex would ever go slumming into the limbic system would be to preach honest labor and Christian temperance to the urchins there.

In PTSD sufferers the amygdala is overreactive to mildly fearful stimuli and is slow in calming down after being activated.13 Moreover, the amygdala expands in size with long-term PTSD.

Instability activated the frontal cortex plus the amygdala. Being unsure of your place is unsettling.

Additionally, the shortcut projections form stronger, more excitable synapses in the BLA than do the ones from the sensory cortex; emotional arousal enhances fear conditioning through this pathway. This shortcut’s power is shown in the case of a man with stroke damage to his visual cortex, producing “cortical blindness.” While unable to process most visual information, he still recognized emotional facial expressions via the shortcut.

Someone does something lousy and selfish to you in a game, and the extent of insular and amygdaloid activation predicts how much outrage you feel and how much revenge you take. This is all about sociality—the insula and amygdala don’t activate if it’s a computer that has stabbed you in the back.

The dissociation between fear and aggression is evident in violent psychopaths, who are the antithesis of fearful—both physiologically and subjectively they are less reactive to pain; their amygdalae are relatively unresponsive to typical fear-evoking stimuli and are smaller than normal.34

What does the frontal cortex do? Its list of expertise includes working memory, executive function (organizing knowledge strategically, and then initiating an action based on an executive decision), gratification postponement, long-term planning, regulation of emotions, and reining in impulsivity.35

the frontal cortex makes you do the harder thing when it’s the right thing to do.

The frontal cortex also mediates “executive function”—considering bits of information, looking for patterns, and then choosing a strategic action.40

Importantly, increase cognitive load on the frontal cortex, and afterward subjects become less prosocial*—less charitable or helpful, more likely to lie.46 Or increase cognitive load with a task requiring difficult emotional regulation, and subjects cheat more on their diets afterward.*

For example, you’re learning a piece of music on the piano, there’s a difficult trill, and each time as you approach it, you think, “Here it comes. Remember, tuck my elbow in, lead with my thumb.” A classic working-memory task. And then one day you realize that you’re five measures past the trill, it went fine, and you didn’t have to think about it. And that’s when doing the trill is transferred from the frontal cortex to more reflexive brain regions (e.g., the cerebellum). This transition to automaticity also happens when you get good at a sport, when metaphorically your body knows what to do without your thinking about

Across primate species, the bigger the size of the average social group, the larger the relative size of the frontal cortex.

And there’s another nonpathological circumstance where the PFC silences, producing emotional tsunamis: during orgasm.

Some supporting evidence: (a) drugs like cocaine, heroin, and alcohol release dopamine in the accumbens; (b) if tegmental release of dopamine is blocked, previously rewarding stimuli become aversive; (c) chronic stress or pain depletes dopamine and decreases the sensitivity of dopamine neurons to stimulation, producing the defining symptom of depression—“anhedonia,” the inability to feel pleasure.

Some rewards, such as sex, release dopamine in every species examined.

A monkey has learned that when he presses a lever ten times, he gets a raisin as a reward. That’s just happened, and as a result, ten units of dopamine are released in the accumbens. Now—surprise!—the monkey presses the lever ten times and gets two raisins. Whoa: twenty units of dopamine are released. And as the monkey continues to get paychecks of two raisins, the size of the dopamine response returns to ten units. Now reward the monkey with only a single raisin, and dopamine levels decline.

What was an unexpected pleasure yesterday is what we feel entitled to today, and what won’t be enough tomorrow.

In other words, dopamine is not about the happiness of reward. It’s about the happiness of pursuit of reward that has a decent chance of occurring.*99

As a subtle example of information being communicated, when female pandas ovulate, their vocalizations get higher, something preferred by males. Remarkably, the same shift and preference happens in humans.

Across species the dominant sensory modality—vision, sounds, whichever—has the most direct access to the limbic system.

There’s also subliminal cuing about beauty.18 From an early age, in both sexes and across cultures, attractive people are judged to be smarter, kinder, and more honest. We’re more likely to vote for attractive people or hire them, less likely to convict them of crimes, and, if they are convicted, more likely to dole out shorter sentences.

Part of the increased success probably reflects the fact that winning stimulates testosterone secretion, which increases glucose delivery and metabolism in the animal’s muscles and makes his pheromones smell scarier.

Success in everything from athletics to chess to the stock market boosts testosterone levels.

Testosterone makes people cocky, egocentric, and narcissistic.

Testosterone boosts impulsivity and risk taking, making people do the easier thing when it’s the dumb-ass thing to do.7 Testosterone does this by decreasing activity in the prefrontal cortex and its functional coupling to the amygdala and increasing amygdaloid coupling with the thalamus—the source of that shortcut path of sensory information into the amygdala.

So testosterone does subtle things to behavior. Nonetheless, this doesn’t tell us much because everything can be interpreted every which way. Testosterone increases anxiety—you feel threatened and become more reactively aggressive. Testosterone decreases anxiety—you feel cocky and overconfident, become more preemptively aggressive. Testosterone increases risk taking—“Hey, let’s gamble and invade.” Testosterone increases risk taking—“Hey, let’s gamble and make a peace offer.” Testosterone makes you feel good—“Let’s start another fight, since the last one went swell.” Testosterone makes you feel good—“Let’s all hold hands.” It’s a crucial unifying concept that testosterone’s effects are hugely context dependent.

Spray oxytocin up a woman’s nose (a way to get the neuropeptide past the blood-brain barrier and into the brain), and she’ll find babies to look more appealing.

A female rodent giving birth increases vasopressin and vasopressin receptor levels throughout the body, including the brain, of the nearby father.

Among pair-bonding tamarin monkeys, lots of grooming and physical contact predicted high oxytocin levels in female members of a pair. What predicted high levels of oxytocin in males? Lots of sex.

when a dog and its owner (but not a stranger) interact, they secrete oxytocin.30

In the words of Sue Carter, exposure to oxytocin is “a physiological metaphor for safety.”

Oxytocin, the luv hormone, makes us more prosocial to Us and worse to everyone else.

We love stress that is mild and transient and occurs in a benevolent context.

glucocorticoids decrease activation of the (cognitive) medial PFC during processing of emotional faces. Collectively, stress or glucocorticoid administration decreases accuracy when rapidly assessing emotions of faces.69

Stated most broadly, sustained stress impairs risk assessment.

Stress can disrupt cognition, impulse control, emotional regulation, decision making, empathy, and prosociality.

Oxytocin is not a universal luv hormone. It’s a parochial one.

context of delayed frontocortical maturation. If by adolescence limbic, autonomic, and endocrine systems are going full blast while the frontal cortex is still working out the assembly instructions, we’ve just explained why adolescents are so frustrating, great, asinine, impulsive, inspiring, destructive, self-destructive, selfless, selfish, impossible, and world changing. Think about this—adolescence and early adulthood are the times when someone is most likely to kill, be killed, leave home forever, invent an art form, help overthrow a dictator, ethnically cleanse a village, devote themselves to the needy, become addicted, marry outside their group, transform physics, have hideous fashion taste, break their neck recreationally, commit their life to God, mug an old lady, or be convinced that all of history has converged to make this moment the most consequential, the most fraught with peril and promise, the most demanding that they get involved and make a difference. In other words, it’s the time of life of maximal risk taking, novelty seeking, and affiliation with peers. All because of that immature frontal cortex.

covered in chapter 4, marinating the brain in excess glucocorticoids, particularly during development, adversely effects cognition, impulse control, empathy, and so on.33 There is impaired hippocampal-dependent learning in adulthood. For example, abused children who develop PTSD have decreased volume of the hippocampus in adulthood.

By age five, the lower a child’s socioeconomic status, on the average, the (a) higher the basal glucocorticoid levels and/or the more reactive the glucocorticoid stress response, (b) the thinner the frontal cortex and the lower its metabolism, and (c) the poorer the frontal function concerning working memory, emotion regulation, impulse control, and executive decision making; moreover, to achieve equivalent frontal regulation, lower-SES kids must activate more frontal cortex than do higher-SES kids.

Inequality emerged when “stuff”—things to possess and accumulate—was invented following animal domestication and the development of agriculture.

Moreover, a culture highly unequal in material resources is almost always also unequal in the ability to pull the strings of power, to have efficacy, to be visible. (For example, as income inequality grows, the percentage of people who bother voting generally declines.) Almost by definition, you can’t have a society with both dramatic income inequality and plentiful social capital. Or translated from social science–ese, marked inequality makes people crummier to one another.

If you want to improve health and quality of life for the average person in a society, you spend money on public goods—better public transit, safer streets, cleaner water, better public schools, universal health care. But the more income inequality, the greater the financial distance between the wealthy and the average and thus the less direct benefit the wealthy feel from improving public goods. Instead they benefit more from dodging taxes and spending on their private good—a chauffeur, a gated community, bottled water, private schools, private health insurance.

As noted, desert cultures are prone toward monotheistic religions; rain forest dwellers, polytheistic ones. Nomadic pastoralists’ deities tend to value war and valor in battle as an entrée to a good afterlife. Agriculturalists invent gods who alter the weather. As noted, once cultures get large enough that anonymous acts are possible, they start inventing moralizing gods. Gods and religious orthodoxy dominate more in cultures with frequent threats (war, natural disasters), inequality, and high infant mortality rates.