Responding to recent discussions of brother-sister marriage in Roman Egypt, this article re-examines the Greek and Egyptian evidence for the practice, both papyrological and literary. Exploring possible antecedents in Egypt and Greece and the distinctive development of Egyptian inheritance practice, we argue that the brother-sister marriages involved real siblings, and that by the beginning of Roman rule such marriages were legitimised by a Ptolemaic law and the prevalent belief that they followed ancient Egyptian custom. But new circumstances introduced by Roman rule, particularly the increasing importance of private property ownership, encouraged the practice to become popular through much of northern Egypt. The explanation for brother-sister marriage in Egypt must be sought in the immediate local historical context, not that of the Eastern Mediterranean generally.

Scholars over the last few decades have been unable to find a convincing explanation for the widespread practice of brother-sister marriage among the common people in Roman Egypt, a social practice seemingly disregarding one of the most fundamental taboos. This paper now argues that these ‘incestuous’ marriages were in fact marriages between a biological child and an adopted one, a practice documented also for other parts of the Eastern Mediterranean. Due to the severe mortality regime before the demographic transition, up to 30 per cent of all fathers did not have a male heir, and therefore adopting the son-in-law was a common succession and inheritance strategy in many pre-modern societies.