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, and the Book of Samuel treats the marriage of a royal prince to his half-sister as unusual, rather than wicked. In ancient times, tribal nations preferred endogamous marriage – marriage to one's relatives the ideal marriage was usually that to a cousin, and it was often forbidden for an eldest daughter to even marry outside the family. (See Affinity (Catholic canon law).) The laws of many countries regarding prohibited relationships do not necessarily follow the biblical prohibitions nor those of any particular church.
The various Christian denominations set their own categories of prohibited incestuous relationships, which have changed from time to time. (See also Forbidden relationships in Judaism.) The Karaites reject the authority of Talmudic opinions and interpret the biblical prohibitions differently. Jewish views on incest are based on the biblical categories of forbidden relationships and have been subject to rabbinic interpretations in the Talmud. Leviticus 18:7–18 and 20:11–21, but also in Deuteronomy. These prohibitions are found predominantly in Incest in the Bible refers to sexual relations between certain close kinship relationships which are prohibited by the Hebrew Bible.
