Exogamy is the custom of marrying outside a specified group of people to which one belongs. In addition to blood relatives, marriage to members of a specific totem or other group may be forbidden.
Different theories have been proposed to account for the origin of exogamy. Edward Westermark said it arose in the aversion to marriage between blood relatives or near kin, that is, in horror of incest. This is very probably the true solution.
McLennan holds that exogamy was due originally to scarcity of women, which obliged men to seek wives from other groups, including marriage by capture, and this in time grew into a custom.
Morgan and Howitt maintain that exogamy was introduced to prevent marriage between blood relations, especially between brother and sister, which had been common in a previous state of promiscuity. Frazer says this is the true solution, that it really introduced group marriage, which is an advance to monogamy, and that the most complete record of this is the classificatory system of relationship. Lang, however, denies there is any group marriage, and says that so-called group marriage is only tribe-regulated licence. Hill-Tout writes that exogamous rules arose for political reasons by marriage treaties between groups. Darwin denies primitive promiscuous intercourse, and says exogamy arose from the strongest male driving the other males out of the group. This is also the opinion of Lang, Atkinson, and Letourneau.
See chapter 18 of the Book of Leviticus for Biblical prohibitions on incest.