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Prohibited sexual relations between members of a close kinship group, such as between parents and children or between brothers and sisters. The term is often expanded to include not only actual inter-course but other sexual acts as well.

While the incest taboo is nearly universal and exists in nearly all societies, notions of kinship vary greatly from culture to culture. Thus, some cultures would consider sexual relations between first cousins incest, while others would not. The same premise holds true for inter-course between a stepfather and stepdaughter. The very rare exceptions to incest, such as those found in ancient Egyptian and Incan societies, usually involve mandatory incestuous unions within royal families, which may have been motivated by economic or theocratic considerations.

In classical psychoanalytic theory, the psychosexual development of children between the ages of three and...