Introduction

Why should not brothers and sisters marry each other? Clearly the children of Adam and Eve must have had little choice in the matter of mates, as Augustine makes clear in his most focused discussion of sibling relationships, in De civitate Dei (The City of God). Augustine observes that at the beginning of human history men must have married their sisters:

Once a larger population came into existence, however, brothers were enjoined to look elsewhere for wives, for, Augustine argues, one of the goods of marriage is to form new alliances, to create friendship and affection between different families. Thus to have a father and a father-in-law as one and the same person, as Adam's children did, is to lose the opportunity to bring another man into a close kin relationship. Lévi-Strauss appositely notes how the Arapesh tribe explained to Margaret Mead that a man must marry another man's sister, and give his sister in marriage, so as to acquire brothers-in-law, otherwise ‘with whom will you hunt, with whom will you garden, whom will you go to visit?’ Augustine proposes – perhaps thinking of Egyptian royal marriage customs or Greek myth – that although other religions permit brother–sister marriage, in practice it is shunned:

Although ‘human nature’, as Augustine opines, generally does shrink from brother–sister incest (and still more from same-sex incest), nevertheless medieval writers treat forbidden love between siblings, and not just as occurring problematically in the book of Genesis.