Genetically speaking, the only difference between men and women is that where women have two X chromosomes, men have one X and one Y. It is surprising that one chromosome difference out of our total of forty-six can have such an important consequence, but it does. Is this relatively small genetic variance really sufficient to explain the huge differences between the sexes, not just the physical but the psychological, social, even cultural? Drawing on his own work at the forefront of modern genetics and the exciting theories of evolutionary biology, Bryan Sykes explores the mysteries of the science of sex and gender, and takes a scientific look at what makes men tick. He addresses the most basic issues of why there are only two sexes in humans and, even, why there is sex at all. He also raises more far-reaching questions, such as: Is there a genetic cause for men's greed, aggression and promiscuity? Is there such a thing as the male homosexual gene? And what do genes tell us about the future for men? Sykes's conclusions will surprise some people and are bound to cause controversy. The all-important male Y chromosome is getting smaller and, as the generations pass, the female genome