Why are there two sexes? How different are they and why? Why can't a woman be more like a man? Or should the question be: why can't a man be more like a woman? Controversy rages around sex and gender, but just what are the differences and how are they determined? Lewis Wolpert, distinguished scientist, broadcaster and author, has tackled depression, religion and old age from a developmental biologist's perspective. Now he enters the gender debate, starting with his argument that men are fundamentally modified females - if the genes present at fertilisation did not do their job properly, we would all be women - and journeying through MRI techniques, the nature of sexual attraction, 'neurosexism' and whether men are really better at maths. With fresh and persuasive research and with his customary intelligence and curiosity, Lewis Wolpert sets out to make his mark on this controversial topic - and makes some surprising discoveries along the way.