How do men become men and women become women? How does a child establish gender identity? By what processes is the human being directed toward reproductive maturity as either male or female? In Man and Woman, Boy and Girl, John Money and Anke Ehrhardt offer a comprehensive account of sexual differentiation using genetics, embryology, endocrinology and neuro-endocrinology, psychology, and anthropology. Their multidisciplinary approach to gender identity avoids the old arguments over nature versus nurture. Money and Ehrhardt focus instead on the interaction of hereditary endowment and environmental influence. Money and Ehrhardt's work will lead many readers to the conclusion that the differences between man and man, or woman and woman, can be as great as between man and woman. A new model of sexual differentiation emerges from this conclusion. It indicates that the social roles of men and women, rather than being fixed by membership in a sexual caste, should be related to individual biography, achievement, and incentives. Still the most thorough treatment of the subject, this latest printing contains a new preface by John Money.