The author of the controversial "The Nurture Assumption" tackles the biggest mystery of psychology: What makes people differ so much in personality and behaviour? The search leads Judith Rich Harris into fascinating byways of science - from classic experiments in social psychology to cutting-edge research in neuroscience, from studies of twins to studies of ants. And Harris' solution is startlingly original: the first completely new theory of personality since Freud's. This is the story of a scientific quest, but it is also the story of a courageous woman who refused to be satisfied with "what everyone knows is true".