Wright starts out by claiming that the environment that shaped the evolution of humans was... human society itself. "The evolution of human beings has consisted largely of adaptation to one another". He then provides an entertaning overview of human behavior, from sex to family life to friendship to social status.

He shows how natural selection favors men who are good at deceiving women and, at the same time, favors women who are good at spotting deception. He shows why men are obsessed with sex (they have to compete for scarce female eggs) and why women are jealous (they have to compete for scarce male investment), why men of all ages are attracted to younger women (they are actually attracted to the ability to produce babies) and why women are attracted to wealthier men (they are actually attracted to the ability to provide for the babies), how the Madonna-whore dichotomy is firmly rooted in the mind of men. He shows that monogamy means equality for men, but not necessarily for women. So far so good. Then Wright moves into modern territory, and examines modern (post-1960s) sexuality with the eye of the Darwinist. Wright reaches the cynical conclusion that morality is simply the set of rules that increase the odds to pass one's genes to the next generation.

Sex is a good example. Darwin had already realized that animals were subject to one kind of pressure that came from members of their own species: sexual selection. Males have to compete in order to mate with a female. Females get to choose which male they mate with. Males seem to be indiscriminate in their sexual appetite, whereas females seem to be very discriminating. This simple asymmetry of behavior explains many traits that would not be easy to explain with standard Darwinian theory (for example, why some animals have very colored traits, and proudly display them, thus helping their predators spot them). But sexual selection often prevails: males who were not equipped to compete against other males (e.g., bulls with no horns) and to attract females (e.g., peacocks with small tails) were excluded from sex, and ther traits are thus extinct. Darwin did not explain where sexual selection came from, though. George Williams found the answer. It comes from a simple physical fact: women can reproduce only about once a year, whereas men can reproduce every day of the year (if they find a woman willing to, of course). For a woman the main "investment" to reproduction is giving birth and nurturing the baby, a lengthy and complex consequence of a few minutes of sex. For a man the main investment is just those few minutes of sex. Thus the different sexual behavior.

Wright employs a concept from game theory, non-zero sumness, to explain altruism.

Human nature does not come out looking too good. Human nature is merely a machine that has been fine-tuned over millions of years to maximize a mathematical equation (that of survival of our genes). Morality is mere convenience. To become moral animals, Wright claims, humans must first realize how thoroughly amoral they are.

Wright emphasizes that evolutionary psychology deals with a new version of the unconscious (the genetic repertory), that makes Freud's version look amateurish and very unscientific.

The best thing about the book is that it is written as a long analysis of Darwin's theory, life, books, ad eve psyche. Darwin himself becomes the main test case for Darwin's theory and the theries that descended from it. As Wright puts it, Darwin was an animal too. Thus the theory of natural selection must apply to Darwin as well as to a chimp. This is a narrative ploy worthy of the best postmodernist novelists.