In ancient times, tribal nations preferred endogamous marriage - marriage to one's relatives;[1] the ideal marriage was usually that to a cousin, and it was often forbidden for an eldest daughter to …

In ancient times, tribal nations preferred endogamous marriage - marriage to one's relatives;^[1] the ideal marriage was usually that to a cousin, and it was often forbidden for an eldest daughter to even marry outside the family.^[1] Marriage to a half-sister, for example, is considered incest by most nations today, but was common behaviour for Egyptian pharaohs; similarly, the Book of Genesis portrays Sarah as marrying Abraham, her half-brother, without criticising the close genetic relationship between them,^[2] and the Book of Samuel treats the marriage of a royal prince to his sister as unusual,^[3] rather than wicked.^[1] The issue of consanguinity is thus a complicated one, and the exact definition of incest varies throughout the Bible and Rabbinical literature.

The Holiness Code of Leviticus elaborates in detail the relationships it regards as incestuous unions,^[4] and two chapters later specifies punishments for specific incestuous unions, but this second list of unions is much shorter than the first;^[5] textual scholars regard these two lists as having originally been independent documents, bound together at a later point.^[6][7][8] The Deuteronomic Code, gives a yet more simple list of forbidden relationships - just parent's daughter (including sister), father's wife (including mother), and mother-in-law.^[9][10] These lists only mention relationships with female relatives; excluding lesbianism, this implies that the list is addressed to men. These lists of forbidden unions compare as follows (the relations highlighted in red are those that are forbidden):