Narrator: During the Indonesian massacres of the mid-1960s, paramilitary groups descended on villages to carry out executions. They brought their weapons, their orders, and their hatred. But they also brought something else: traditional gamelan orchestras. When an unrepentant veteran of the massacres was later asked why they brought music to a slaughter, he gave a chillingly simple answer: "Well, to make it more beautiful." How can a human being find beauty in unspeakable violence? What happens in the brain and body to produce acts of both horrific cruelty and breathtaking altruism? These are the questions at the heart of Robert M. Sapolsky’s masterwork, Behave: The Biology of Humans at Our Best and Worst. The book argues that to understand any behavior, we can’t look for a single cause. Instead, we must peel back the layers of time, from the second before the act to the millennia of evolution that shaped our species.

Narrator: To understand why a behavior happens, Sapolsky begins with the brain in the final second before the action. This is the domain of neurobiology, where the brain acts as the final common pathway for everything that came before it—genes, hormones, childhood, and culture. Two key structures are often in a tense dialogue: the amygdala and the frontal cortex. The amygdala is the brain’s alarm bell, central to fear, anxiety, and aggression. It learns fear and injects a sense of distrust and vigilance into our decisions. The tragic case of Charles Whitman, the 1966 University of Texas tower shooter, provides a stark illustration. Whitman, a seemingly normal young man, left notes expressing confusion about his own violent impulses. An autopsy later revealed a glioblastoma tumor pressing on his amygdala. While not a simple cause-and-effect, it highlights the amygdala's role in aggression.

In contrast, the frontal cortex is the brain’s CEO. It’s the last part of the brain to fully mature, not coming online until our mid-twenties. Its primary job is to make you do the harder thing when it’s the right thing to do. It reins in the amygdala's impulses, weighs consequences, and makes long-term plans. This tension is famously explored in the "runaway trolley" problem. Most people would pull a lever to divert a trolley, sacrificing one person to save five. This is a rational, utilitarian calculation handled by the dorsolateral prefrontal cortex (dlPFC). But very few people would push a large person off a bridge to stop the same trolley. The personal, hands-on nature of this act activates emotional regions like the amygdala and the ventromedial prefrontal cortex (vmPFC), creating a gut feeling that it’s just wrong. In that one second, our behavior is the outcome of this complex negotiation between ancient impulses and modern reason.

Narrator: Zooming out to the seconds, minutes, and hours before a behavior, we find the powerful, often invisible, influence of sensory cues and hormones. Our brains are constantly processing information from our environment, much of it unconsciously. In one study, subjects in a lab were more likely to behave dishonestly if there was a briefcase in the room, a subliminal cue for corporate ruthlessness. These cues shape our actions without our awareness.

Hormones also play a crucial modulatory role, but not in the way most people think. Testosterone, for example, doesn’t create aggression out of thin air. Instead, it amplifies pre-existing aggressive tendencies. It makes us more willing to do whatever it takes to maintain status. In a context where status is earned through aggression, testosterone fuels that aggression. But as a study using the Ultimatum Game showed, if being fair and generous is what earns status, testosterone can actually make people more prosocial. Similarly, oxytocin, often called the "love hormone," is more complicated. It enhances bonding and trust, but primarily with our in-group, or "Us." When it comes to out-groups, or "Them," oxytocin can increase xenophobia and preemptive aggression. Hormones, Sapolsky shows, don't command our behavior; they turn up the volume on the tendencies that are already there.

Narrator: The brain that acts in the moment was sculpted over a lifetime of experience, starting in the womb. Adolescence is a particularly volatile period, defined by a critical mismatch in brain development. The limbic system, responsible for emotion and reward, is fully developed, while the frontal cortex, the brain’s source of impulse control, is still a work in progress. This explains why teenagers are prone to risk-taking, intense emotions, and a powerful need for peer acceptance. They feel the thrill of a reward more intensely and are less equipped to regulate the impulse to chase it.

Going back even further, early childhood experiences have a profound and lasting impact. The horrific case of Romanian orphans under the Ceaușescu regime revealed the biological cost of neglect. Children raised in understaffed institutions with little stimulation or affection grew up with lower IQs, severe attachment problems, and a host of psychiatric issues. Their brains were literally smaller, with an overactive, enlarged amygdala. This demonstrates that a nurturing environment is not a luxury but a biological necessity for building a healthy brain. Even before birth, the prenatal environment shapes the developing brain. Maternal stress, for instance, can expose a fetus to high levels of glucocorticoids, altering its brain development and predisposing it to anxiety and depression later in life.

Narrator: Many people think of genes as a deterministic blueprint for behavior, but Sapolsky argues this is fundamentally wrong. Genes don’t determine anything on their own; their effects are almost always dependent on the environment. This is the crucial concept of gene/environment interaction. The most famous example is the MAO-A gene, sometimes sensationalized as the "warrior gene." A landmark study by Avshalom Caspi followed a large cohort of children into adulthood. It found that having the low-activity version of the MAO-A gene, on its own, did not predict antisocial behavior. However, for individuals who also suffered severe childhood abuse, that genetic variant tripled the likelihood of being convicted of a violent crime. The gene only mattered in the context of a specific environment.

This means asking "what a gene does" is the wrong question. The right question is "what does a gene do in a particular environment?" Furthermore, the concept of "heritability" is one of the most misunderstood in science. A trait with high heritability doesn't mean it's unchangeable. It simply means that in the range of environments studied, much of the variation between individuals is due to genetic differences. As Sapolsky’s plant analogy shows, if you study a gene only in one type of environment, you artificially inflate its importance and miss the powerful role the environment plays.

Narrator: One of the most automatic and powerful tendencies in human behavior is the creation of Us/Them dichotomies. Our brains are hardwired to categorize people into in-groups and out-groups with stunning speed. Brain imaging studies show that when a white subject sees a black face for just a fraction of a second, the amygdala activates before the conscious mind even registers the face. This isn't evidence of innate racism, but of an automatic, deeply ingrained categorization process.

This tendency is so powerful that it can be triggered by the most trivial distinctions. In his "minimal group" experiments, Henri Tajfel showed that people would favor members of their own group even when the group was formed on a basis as meaningless as over- or underestimating the number of dots on a screen. This in-group favoritism is amplified by hormones like oxytocin, which makes us more prosocial to "Us" and more hostile to "Them." This biological reality helps explain everything from schoolyard cliques to ethnic conflicts, showing that our worst behaviors often stem from a biological mechanism originally designed to foster cooperation within small, related groups.

Narrator: Sapolsky saves his most radical argument for last. After meticulously building a case that our behavior is the product of a cascade of factors—from neurobiology to hormones, childhood, genes, culture, and evolution—he confronts the concept of free will. If every action is the result of a chain of events that stretches back long before we were born, in what sense are we truly "free" to choose? Sapolsky concludes that we are not. Free will, he argues, is an illusion.

This has staggering implications for our justice system, which is built on the ideas of blame, culpability, and retribution. If a person's actions are the result of a brain tumor, a history of abuse, a particular genetic makeup, and a culture of violence, how can we punish them as if they had complete freedom to choose otherwise? Sapolsky is not arguing that dangerous people should be let loose. Instead, he advocates for a model akin to quarantining someone with a contagious disease. We must protect society from harm, but the goal should be management and prevention, not retribution. This means abandoning our satisfying sense of blame and embracing a more complex, and perhaps more compassionate, understanding of human action.

Narrator: The single most important takeaway from Behave is that human behavior is staggeringly complex, and any attempt to explain it with a single, simple cause is doomed to fail. There is no one gene, one hormone, one brain region, or one childhood event that makes us who we are. We are the sum of a multitude of interacting influences, cascading across time.

This understanding leaves us with a profound challenge. If our actions are not born of a freely willed choice in the moment, but are the product of our biology and all that has acted upon it, how do we make sense of praise, blame, and justice? Can we learn to hate the act without hating the actor? Sapolsky’s work doesn't provide easy answers, but it forces us to confront the biology of our own humanity, in all its beautiful and terrible complexity.