Behave: The Biology of Humans at Our Best and Worst

Behave starts from an enormous question: why do some humans do wonderful things and others terrible ones? Sapolsky's answer is long, and on purpose. For him, no behavior has just one cause. Every act is the result of several layers of influence operating on different time scales.

The book's strategy is to take a behavior, like an act of aggression, and go back in time: what happened one second before, minutes before, hours before, years before, in childhood, in the genes, in the evolution of the species. Each chapter asks what the biology of that period helps explain.

The moral of this structure is that answers like it's the gene, it's the upbringing, or it's the brain are simplistic. The layers blend together, and understanding that changes how you look at other people's behavior and your own.

In the instant right before an act, what decides things are regions of the brain. Sapolsky highlights three players. The amygdala, tied to fear and quick reactions to threats. The prefrontal cortex, which plans, holds back impulses, and does what is hard, like following a rule. And the reward system, which depends on dopamine and is linked to the anticipation of pleasure.

The classic scene is a road rage fight. The amygdala fires at the sound of a horn, and the prefrontal cortex tries to hold back the arm. When the brake works, the person breathes and moves on. When it fails, the argument escalates. Tiredness, hunger, and stress weaken the brake.

The author recalls the case of Phineas Gage, a worker who had an accident with an iron rod through his head in the 19th century and underwent a personality change, as an example of how injuries to the frontal region alter behavior.

One layer earlier, what sets off the brain comes from outside: a face, a word, a smell, a noise. The world sends signals all the time, and many get in without our noticing. This changes reactions within seconds.

The book cites research in which images of faces from different groups activate the amygdala in different ways, even in people who say they have no prejudice, which helps explain automatic biases. Not everything the brain does by reflex is endorsed by reason.

The good news is that context can also be rearranged. Calm environments, clear rules, and contact with people as individuals, rather than labels, change the responses. This applies to a classroom, a bank line, or a police station.