Using a technique of fictional sex scenes followed by scientific explanations, the scientist-turned-writer/broadcaster shows how some women choose the father of their children - by putting two men's sperm armies into battle to increase the chances of being fertilised by the "best man". A constant stream of surprising facts makes this an entertaining page-turner. Baker claims 10 per cent of us were not fathered by our "fathers"; women become more interested in infidelity during their fertile periods; and women are more likely to conceive from casual sex than routine sex with a partner. A must for anyone who wants to understand what is really going on during sex.