incest, sexual relations between persons to whom marriage is prohibited by custom or law because of their close kinship. Ideas of kinship, however, vary widely from group to group, hence the definition of incest also varies. Customs prescribing whom a person may and may not marry are found among all human groups, and these apparently antedated knowledge of the genetic effects of the intermarriage of close relatives. Even modern prohibitions of incest are based only in part on the observed fact that inherited defects tend to be transmitted in intensified form when both parents possess the same genes. In many societies, the marriage of parents and offspring, or brothers and sisters, is prohibited and abhorred—this is the incest taboo, much discussed in the anthropological literature. Only in some royal families, as in ancient Egypt and among the Inca, were such marriages customary, perhaps with the goal of conserving royal prerogatives and property; such marriages may have been largely symbolic. Theories concerning the incest taboo include sociological and psychological interpretations. In anthropology, it is often considered in relation to rules of exogamy, by which marriage serves as a means of social alliance between groups who might otherwise be disposed to fight one another. Incest is a recurrent theme in mythology and literature across the world, and it has played an important role in psychoanalytical speculation and theory (see Oedipus complex). For the contemporary legal aspects of incest, see consanguinity.

Sexual relations between persons who, because of the nature of their kinship ties, are prohibited by law or custom from intermarrying. The incest taboo is generally universal, although it is imposed differently in different societies. Usually, the closer the genetic relationship between two people, the stronger and more highly charged is the taboo prohibiting or discouraging sexual relations. Some sociobiologists consider that inbred populations have diminished reproductive success and become gene pools for hereditary disorders. Some cultural anthropologists argue instead that the incest prohibition, with the corresponding rules of exogamy, acts to require males to seek sexual and marital partners outside the group, thereby establishing useful alliances. Other theories emphasize the need to control sexual jealousies within the family or to prepare children to function with restraint in adult society. No single explanation seems satisfactory, causing some to question whether incest should be treated as a unitary subject. Most cases of incest that come before criminal courts concern sexual intercourse between fathers and relatively young daughters (see child abuse).

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