The authors integrate biological and anthropological findings with real-life stories of individuals to address the conundrums that surround male-female behavior and relationships. Drawing on the latest research in evolutionary biology, they trace the multifaceted gender gap to the basic, defining difference between males and females: that one makes sperm, the other, eggs. They show how that distinction explains why women and men differ in essential ways, exploring such questions as: Why are men more attracted than women to pornography, group sex, and one-night stands? Why are women the "gatekeepers" of sex? Why do women have orgasms?