This is one of the great recurring themes of human history, the balance between cooperation and conflict. It is the obsession of governments and families, of lovers and rivals. It is the key to economics. It is, as we shall see, one of the oldest themes in the history of life, for it is repeated right down to the level of the gene itself. And the principal cause of it is sex. Sex, like marriage, is a cooperative venture between two rival sets of genes. Your body is the scene of this uneasy coexistence.
