How do organisms whose behavior is apparently determined by ``selfish genes'' become social beings, let alone altruists and saints? Ridley, former science editor of the Economist, looks to the growing field of evolutionary psychology for answers. This new discipline draws on insights from anthropology, economics, and politics, as well as on the evolutionary trends the author explored in The Red Queen: Sex and the Evolution of Human Nature (1994). Other organisms besides humans have learned to cooperate. The social insects have long been taken as models for human society; the division of labor they exhibit is one of the key advantages of social living. Vampire bats nest in large groups, and it is common for a successful hunter to share its meal with a neighbor, in hopes that the favor will be returned at a later date. This discovery leads to a digression on the famous ``prisoner's dilemma'' of game theory; the first studies seemed to show that the selfish player invariably wins. It now appears that a cooperative player with a ``tit for tat'' strategy will outlast the purely selfish one. Communal hunting raises interesting issues, too. Surplus meat is often traded for sex with an attractive female neighbor. Early modern humans so effectively hunted large animals that many--the mammoth, for example--became extinct. Another negative effect of large-scale cooperation is war. It is evidently difficult even for highly sophisticated social beings to abandon the notion that only their own tribe is really human and that others must be exterminated. The other side of the coin is trade, which depends on mutual trust. ``Trust is as vital a form of social capital as money is a form of actual capital,'' Ridley argues in a concluding chapter in which he attempts to draw lessons for the modern political arena. A provocative look at some of the central questions about what makes us human; strongly recommended.