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.
See J. Shepher, Incest: A Biosocial View (1983); J. B. Twitchell, Forbidden Partners: The Incest Taboo in Modern Culture (1986).