Testosterone has been the misunderstood human hormone since the early 1970s, blamed for everything to rape to low intelligence to road rage. But the authors argue that the picture is more complex: testosterone is related to things as diverse as criminal violence and the way people smile; it affects our language ability and the way we navigate in the space around us; it helps predict what occupation we will enter and whether or not we will marry, have extra-marital affairs, and divorce. It affects both men and women.