Robert Sapolsky, a Stanford neuroscientist and primatologist, attempts a comprehensive biological account of human behavior in this nearly eight-hundred-page synthesis. His method is to start with a single act—a person doing something good, bad, or strange—and then peel back the layers of causation in reverse chronological order. What was happening in the brain a second before? What were the hormones doing minutes earlier? Which adolescent and childhood experiences shaped the relevant neural circuits? What genes did the person inherit, and what cultures and ecologies did their ancestors evolve in? Sapolsky moves through neuroscience, endocrinology, developmental psychology, behavioral genetics, evolutionary biology, and anthropology, treating each as a different time scale on the same question. The book is at its best on subjects Sapolsky has studied for decades: stress and the body, primate social behavior, the biology of aggression and empathy, and the malleability of in-group/out-group thinking. He uses the synthesis to argue against simple distinctions between nature and nurture and against confident claims about free will. The writing is digressive and frequently funny, with footnoted asides drawn from Sapolsky's years studying baboons in Kenya. Behave has become a standard reference for anyone trying to think clearly about why humans do what they do.