Behave

Behave: The Biology of Humans at Our Best and Worst

It is Christmas morning, 1914. The Western Front. Somewhere between the British trenches at Ploegsteert Wood and the German lines opposite, a young German soldier climbs out of his foxhole holding not a rifle but a small lantern. He walks into no man's land. A British soldier watches him from the parapet, finger still on the trigger. Then a second German emerges. And a third. And then, improbably, a British private stands up too. Within an hour, men who had been killing each other for four months are trading schnapps and cigarettes in a frozen field. They bury their dead together. Some play football in the mud. By nightfall they are back in their trenches, and within days most of them are shooting again.

Robert Sapolsky, neuroendocrinologist at Stanford, has spent a career asking why. Not why the truce happened, and not why the killing resumed -- but how a single human nervous system can produce both behaviors, sometimes within the same hour. The Christmas truce of 1914 is the kind of event that breaks every simple story about human nature. It fits neither the story that says we are fundamentally savage, nor the story that says we are fundamentally kind. It says instead that context does almost everything. That the distance between heroism and atrocity can be a matter of seconds, of who is standing next to you, of which brain region fires first.

Behave, published in 2017, is Sapolsky's attempt to explain the full chain of causation behind a single human act -- specifically, the act of one person doing something to another person, whether beautiful or terrible. The book runs to nearly eight hundred pages because Sapolsky refuses to accept short answers. Every time you think you have found the cause, he rewinds the tape further back. The neuroscience explains the brain at the moment of action. But the hormones explain the brain. And the developmental history explains the hormones. And the evolutionary pressures explain the developmental program. Pull on any thread and the whole tapestry unrolls.