“Unselfish action is a hallmark of humanity. We may sacrifice our lives for the good of our children, for the good of our nation, and sometimes even for the good of a stranger. What motivates such altruistic acts? To a biologist, this question has two very different answers. There is the proximate answer that explains our psychological reasons for acting altruistically, and there is the ultimate answer that explains how an unselfish act increases our Darwinian fitness relative to some selfish alternative. Through the two more-or-less independent sections of Unto Others, Sober and Wilson discuss both proximate and ultimate explanations. They use both sections to also emphasize their belief in the value of pluralistic hypotheses, with natural selection driven by multiple levels of causation and behavior driven by multiple desires… Sober and Wilson…have the laudable goal of stimulating research into levels of selection and motivation as applied to humans and their culture.”