Professor Pinker ends with a treatise on brain science, a fluent home run for a psychologist-turned-historian. Neuro-plasticity, the human brain's ability to change in response to experience, means that people are less likely to resort to violence in their daily lives than their forebears; other behavioural strategies work better. That may not have been quite what Lincoln meant, though the belief in man's improvability is as uplifting in this magisterial work as it was in the president's speech.