Let's face it, say Barash and Lipton: Males and females, boys and girls, men and women are different. To be sure, these differences are often heightened by distinctions in learning, cultural tradition, and social expectation, but underpinning them all is a fundamental difference that derives from biology. Throughout the natural world, males are those creatures that make sperm; females make eggs. The oft-noticed "gender gap" derives, in turn, from this "gamete gap." In Gender Gap, Barash and Lipton (husband and wife, professor and physician, biologist and psychiatrist) explain the evolutionary aspects of male-female differences.