Genetically speaking, the big 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 26 can have such an important consequence, but it does. Is this relatively small genetic variance really sufficient to explain the 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 takes us on a fascinating exploration into the science of sex and gender, and takes a scientific look at what makes men tick. From the most basic questions - why are there only two sexes in humans? Why is there sex at all? - to an examination of maleness - is there a genetic cause for men's promiscuity? Is there such a thing as the male homosexual gene? Can science offer an explanation for the rise of patriarchal society? Sykes's conclusions will surprise some people and are bound to cause controversy. His own research has shown that the all-important male Y chromosome is getting smaller. As the generations pass the female X chromosome is taking over. It is cannibalising parts of the Y chromosome. Women are winning the evolutionary battle of the sexes. His conclusion is that men, slowly but surely, are headed for extinction.