Our sex defines our identity before we are even named - "It's a boy" or "It's a girl" is the traditional welcome we receive in this world. Similarly, throughout our entire life, our gender defines our diverse social roles. Yet, this book shows how nature presents us with only one model, that of bisexuality. Endocrinian manipulations have shown that it is possible to change from one sexuality to another in a reversible manner. This book is key reading in the debate about the genetic character, or not, of homosexuality. Claude Aron, a specialist in in the physiology of reproduction, is also an honorary professor of the Louis-Pasteur University in Strasbourg.