The Better Angels of Our Nature is a gloriously optimistic book. In it, well-known cognitive psychologist Steven Pinker put forward an argument that contradicted what most people believed at the time. Pinker said that despite humanity's biological tendency toward violence, we are, in fact, less violent today than ever before. To prove it, he laid out pages of detailed statistical evidence. For him, much of the credit for the decline goes to the Enlightenment,...