Behave: The Biology of Humans at Our Best and Worst

by Robert M. Sapolsky

the greatest crime-fighting tool is a thirtieth birthday.

So kids improve at delayed gratification. Mischel’s next step made his studies iconic—he tracked the kids afterward, seeing if marshmallow wait time predicted anything about their adulthoods. Did it ever. Five-year-old champs at marshmallow patience averaged higher SAT scores in high school (compared with those who couldn’t wait), with more social success and resilience and less aggressivefn9 and oppositional behavior. Forty years postmarshmallow, they excelled at frontal function, had more PFC activation during a frontal task, and had lower BMIs.21 A gazillion-dollar brain scanner doesn’t hold more predictive power than one marshmallow. Every anxious middle-class parent obsesses over these findings, has made marshmallows fetish items.

Evidence for the most basic need provided by a mother comes from a controversial quarter. Starting in the 1990s, crime rates plummeted across the United States. Why? For liberals the answer was the thriving economy. For conservatives it was the larger budgets for policing, expanded prisons, and three-strikes sentencing laws. Meanwhile, a partial explanation was provided by legal scholar John Donohue of Stanford and economist Steven Levitt of the University of Chicago—it was the legalization of abortions. The authors’ state-by-state analysis of the liberalization of abortion laws and the demographics of the crime drop showed that when abortions become readily available in an area, rates of crime by young adults decline about twenty years later. Surprise—this was highly controversial, but it makes perfect, depressing sense to me. What majorly predicts a life of crime? Being born to a mother who, if she could, would have chosen that you not be. What’s the most basic thing provided by a mother? Knowing that she is happy that you exist.fn13,26

Why do we often become attached to a source of negative reinforcement, seek solace when distressed from the cause of that distress? Why do we ever love the wrong person, get abused, and return for more? Psychological insights abound. Because of poor self-esteem, believing you’ll never do better. Or a codependent conviction that it’s your calling to change the person. Maybe you identify with your oppressor, or have decided it’s your fault and the abuser is justified, so they seem less irrational and terrifying. These are valid and can have huge explanatory and therapeutic power.

In other words, in young rats even aversive things are reinforcing in Mom’s presence, even if Mom is the source of the aversive stimuli. As Sullivan and colleagues wrote, “attachment [by such an infant] to the caretaker has evolved to ensure that the infant forms a bond to that caregiver regardless of the quality of care received.” Any kind of mother in a storm. If this applies to humans, it helps explain why individuals abused as kids are as adults prone toward relationships in which they are abused by their partner.

The link between exposure to childhood media violence and increased adult aggression is stronger than the link between lead exposure and IQ, calcium intake and bone mass, or asbestos and laryngeal cancer.

Two caveats: (a) there is no evidence that catastrophically violent individuals (e.g., mass shooters) are that way because of childhood exposure to violent media; (b) exposure does not remotely guarantee increased aggression—instead, effects are strongest on kids already prone toward violence. For them, exposure desensitizes and normalizes their own aggression.fn18

As emphasized by the psychiatrist Stuart Brown, founder of the National Institute for Play, the opposite of play is not work—it’s depression.

Interestingly, kids in (collectivist) Japan play more violent video games than do American kids, yet are less aggressive. Moreover, exposing Japanese kids to media violence boosts aggression less than in American kids.56 Why the difference? Three possible contributing factors: (a) American kids play alone more often, a lone-wolf breeding ground; (b) Japanese kids rarely have a computer or TV in their bedroom, so they play near their parents; (c) Japanese video-game violence is more likely to have prosocial, collectivist themes.

DeCasper and colleague Melanie Spence then showed (using the pacifier-sucking-pattern detection system) that newborns typically don’t distinguish between the sounds of their mother reading a passage from The Cat in the Hat and from the rhythmically similar The King, the Mice, and the Cheese.62 But newborns whose mothers had read The Cat in the Hat out loud for hours during the last trimester preferred Dr. Seuss. Wow.

In further studies, many carried out by Goy’s student Kim Wallen of Emory University, pregnant females received lower doses of testosterone, and only in the last trimester.67 This produced daughters with normal genitalia but masculinized behavior. The authors noted the relevance of this to transgender individuals—the external appearance of one sex but the brain, if you will, of the other.fn28

Put simply, cultures with more income inequality have less social capital.

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.

Thus unequal cultures make people less kind. Inequality also makes people less healthy.

Extensive research has examined the SES/health gradient. Four quick rule-outs: (a) The gradient isn’t due to poor health driving down people’s SES. Instead low SES, beginning in childhood, predicts subsequent poor health in adulthood. (b) It’s not that the poor have lousy health and everyone else is equally healthy. Instead, for every step down the SES ladder, starting from the top, average health worsens. (c) The gradient isn’t due to less health-care access for the poor; it occurs in countries with universal health care, is unrelated to utilization of health-care systems, and occurs for diseases unrelated to health-care access (e.g., juvenile diabetes, where having five checkups a day wouldn’t change its incidence). (d) Only about a third of the gradient is explained by lower SES equaling more health risk factors (e.g., lead in your water, nearby toxic waste dump, more smoking and drinking) and fewer protective factors (e.g., everything from better mattresses for overworked backs to health club memberships).

What then is the principal cause of the gradient? Key work by Nancy Adler at UCSF showed that it’s not so much being poor that predicts poor health. It’s feeling poor—someone’s subjective SES (e.g., the answer to “How do you feel you’re doing financially when you compare yourself with other people?”) is at least as good a predictor of health as is objective SES.

Separate from the size of a population, how about its density? One study surveying thirty-three developed countries characterized each nation’s “tightness”—the extent to which the government is autocratic, dissent suppressed, behavior monitored, transgressions punished, life regulated by religious orthodoxy, citizens viewing various behaviors as inappropriate (e.g., singing in an elevator, cursing at a job interview).46 Higher population density predicted tighter cultures—both high density in the present and, remarkably, historically, in the year 1500.

To invoke some estimates, anatomically modern humans emerged about 200,000 years ago, and behaviorally modern ones about 40,000 to 50,000 years ago; animal domestication is 10,000 to 20,000 years old, agriculture around 12,000. After plant domestication, it was roughly 5,000 more years until “history” began with civilizations in Egypt, the Mideast, China, and the New World. When in this arc of history was war invented?

Critics on the other side of these debates have deeper worries. For one thing, the false picture of, say, Amazonian tribes as ceaselessly violent has been used to justify stealing their land. According to Stephen Corry of Survival International, a human-rights organization that advocates for indigenous tribal peoples, “Pinker is promoting a fictitious, colonialist image of a backward ‘Brutal Savage’, which pushes the debate back over a century and is still used to destroy tribes.”

Evidence for the reality of evolution includes: Numerous examples where changing selective pressures have changed gene frequencies in populations within generations (e.g., bacteria evolving antibiotic resistance). Moreover, there are also examples (mostly insects, given their short generation times) of a species in the process of splitting into two. Voluminous fossil evidence of intermediate forms in numerous taxonomic lineages. Molecular evidence. We share ~98 percent of our genes with the other apes, ~96 percent with monkeys, ~75 percent with dogs, ~20 percent with fruit flies. This indicates that our last common ancestor with other apes lived more recently than our last common ancestor with monkeys, and so on. Geographic evidence. To use Richard Dawkins’s suggestion for dealing with a fundamentalist insisting that all species emerged in their current forms from Noah’s ark—how come all thirty-seven species of lemurs that made landfall on Mt. Ararat in the Armenian highlands hiked over to Madagascar, none dying and leaving fossils in transit? Unintelligent design—oddities explained only by evolution. Why do whales and dolphins have vestigial leg bones? Because they descend from a four-legged terrestrial mammal. Why should we have arrector pili muscles in our skin that produce thoroughly useless gooseflesh? Because of our recent speciation from other apes whose arrector pili muscles were attached to hair, and whose hair stands up during emotional arousal. Enough. Don’t get...

Is the driving force of behavioral evolution always that someone be vanquished? Not at all. One exception is elegant, if specialized. Remember rock/paper/scissors? Paper envelops rock; rock breaks scissors; scissors cut paper. Would rocks want to bash every scissors into extinction? No way. Because then all those papers would enwrap the rocks into extinction. Each participant has an incentive for restraint, producing an equilibrium.