Behave: The Biology of Humans at Our Best and Worst

The question

Robert Sapolsky is a neuroendocrinologist at Stanford University who spent many summers studying wild baboons in Kenya, research he described in his memoir A Primate’s Memoir. Behave, published in 2017, is his attempt to bring together everything biology can say about human behaviour, especially behaviour at its most extreme: violence, aggression, cruelty, and also cooperation, altruism and empathy.

His question is simple to state. Someone has just done something, pulled a trigger, touched another’s arm, given away something precious. Why did they do it? Sapolsky’s insistence is that the question has no single answer, and that every discipline that offers one, whether neuroscience, endocrinology, genetics, psychology or anthropology, sees only part of it.

The path through it

The book’s structure is its argument. Sapolsky starts at the moment of the behaviour and moves backward in time, a chapter at a time.

One second before: what was happening in the brain? He describes the amygdala, central to fear and aggression; the frontal cortex, which helps people do the harder thing when it is the right thing; and the dopamine system, which is less about pleasure than about anticipating reward.

Seconds to minutes before: what in the environment triggered those neurons? He shows how subtle cues, a face, a smell or a word, can shift behaviour without conscious awareness.

Hours to days before: what hormones were circulating? He treats testosterone, which he argues does not simply cause aggression but amplifies whatever behaviour a social context rewards; oxytocin, which strengthens bonds within a group and can increase hostility toward outsiders; and stress hormones, which over time damage the frontal cortex.

Days to months before: how had the brain changed? Neurons rewire with experience, and chronic stress or enrichment changes the brain’s structure.

Adolescence: the frontal cortex is the last part of the brain to mature, not fully developed until the mid-twenties. That, Sapolsky argues, explains much about adolescent risk-taking, and also adolescents’ capacity for idealism.

Childhood and the womb: early experiences, including adversity, poverty and the mother’s stress during pregnancy, shape the developing brain in lasting ways.

Genes: genes matter, but their effects depend heavily on environment. Sapolsky dismantles simple claims about genes “for” aggression or intelligence.

Culture, over centuries: societies differ in how they raise children, how they value honour, whether they are individualist or collectivist, and those differences trace back to ecology and history.

Evolution, over millions of years: natural selection shaped the capacities for cooperation and competition that all of this builds on.

The last part of the book applies the whole picture to large themes: why people divide the world into us and them; hierarchy and obedience; morality; empathy; and war and peace.

Where it arrives

Sapolsky arrives at two conclusions. The first is that behaviour can only be understood by integrating all these levels at once. A neuroscientist who points to the amygdala, or a sociologist who points to poverty, is right but incomplete; each cause works through the others.

The second, which he develops at length in the book’s chapter on criminal justice and then in his next book, Determined (2023), is that the more we understand the causes of behaviour, the less room there seems to be for free will. If a person’s actions follow from their brain, which was shaped by their hormones, experiences, upbringing, genes and culture, none of which they chose, then praise and blame look very different. He argues that this should make the justice system less punitive, by analogy with how we stopped blaming people with epilepsy for their seizures once their cause was understood.

What remains open

The free will conclusion is the most debated. Many philosophers are compatibilists, holding that freedom and responsibility are compatible with determinism, because what matters is whether a person acts on their own reasons, not whether their reasons have causes. Sapolsky engages with this view and rejects it, but the debate continues.

Some of the research he discusses, especially from social psychology, has been affected by the replication crisis, and some findings, such as specific priming effects, are less secure than they seemed in 2017. The book’s broader account of the brain and hormones rests on stronger evidence.

What it asks of someone deciding

The book asks whether you are explaining behaviour by a single cause. A manager who blames a team’s failure on one person’s character, or a policymaker who attributes crime to one factor, is usually missing most of the picture.

It asks what conditions you create. Stress, threat and humiliation make the worst behaviour more likely, through mechanisms Sapolsky describes in detail. Organisations that understand this can design environments that bring out the better side of people.

And it asks how you draw the line between us and them. Sapolsky shows how quickly and arbitrarily people form groups and turn hostile to outsiders, and how those boundaries can also be redrawn. The site’s idea page Groupthink and the Silence of Rooms shows one consequence of strong group feeling.