A century of genetic research shows that DNA differences inherited from our parents are the consistent life-long sources of our psychological individuality—the blueprint that makes us who we are. Reflecting his own studies of twins and adoptees, Robert Plomin explains what DNA does, and makes a powerful case that it is nature, not nurture, that truly shapes who we are. Intriguingly, recent breakthroughs could give us the power to predict a child's psychological strengths and weaknesses at birth.