In the widest sense, incest refers to the marital or sexual relationship between close relatives. There are two main connotations of the term incest. In ethnography and anthropology concept of incest denotes cultural rules regulating prohibitions on marriages between relatives. In a modern context, incest most often refers to sexual abuse of a child or minor by a member of the family. In this entry, we will focus on the first conceptualization of incest, while the implications of the modern usage of the term are a focus of the entry on sexual violence.

In an ethnographic context concept of incest is closely related to concepts of endogamy and exogamy. Endogamy refers to cultural rules that impose or allow marriages between members of the same group (family or clan), while exogamy refers to rules that forbid in-group marriages and thus only allow out-group marriages. Although the prohibition of incest is culturally universal, rules vary according to the minimal distance of the familial relationship that is needed for the marriage to be allowed, and whether the forbidden closeness is determined only through blood relationship (consanguineal kinship) or through marriage too (affinal kinship).

In Primitive Marriage (1865), John F. McLennan defined incest as endogamy, and by studying marriage rules in several societies, concluded that the prohibition of endogamy serves as an evolutionary survival mechanism as it promotes reciprocity, cooperation, and harmony between groups that intermarry. Following McLennan, other notable anthropologists and sociologists also emphasized the importance of endogamy for intergroup solidarity, such as Henry Lewis Morgan, Edward Tylor, Sir James Frazer, Bronislaw Malinowski, Claude Levi-Strauss, George Murdock, and Emile Durkheim.

The second explanation for the universal cultural prohibition of incest is based on biological reasoning, i.e., that incest avoidance is instinctive as it is a product of biological evolution. Explanation for this instinctive avoidance of incest claims that inbreeding (children born of relations between close genetic relatives) leads to increased chances of genetic diseases and other negative congenital conditions. Modern biology has shown that this claim has validity, as inbreeding can result in increased frequency of homozygosity of deleterious recessive alleles. The instinctive basis for incest avoidance was first proposed by Edward Westermark in his 1891 book, The History of Human Marriage. Sociobiology and other related theoretical approaches emphasize the importance of the instinctive basis of incest avoidance over cultural and social explanations. The main weakness of this line of reasoning is that it does not explain the cultural practice of prescribing cross-cousin marriage, i.e., marrying parent's opposite-sex sibling’s children, while simultaneously prohibiting parallel cousin marriage (marrying parent’s same sex sibling’s children). This practice is found among various societies around the world. This version of incest prohibition is not based on biological closeness, as both parallel and cross-cousins are equally close biologically.

In the book Totem and Taboo: Resemblances between the Psychic Lives of Savages and Neurotics (1913), Sigmund Freud proposes the third type of explanation for the rules of incest avoidance. He explores the myth (that he invented) about the killing and eating of the violent father in the primal horde. Freud starts with Darwin's theory that states that in early primitive human societies, a single alpha male possessed a harem of females, with all other males prohibited from forming relations with those females. Freud assumes that in a single event in a distant human past, the band of brothers that was expelled from the group by their alpha-male father returned and killed and ate him. Brothers both feared and respected the father. After the act of murdering and eating the father, the sons are in a state of remorse and guilt, and they give up having sexual relations with the women belonging to the deceased father; in that way, they create a new symbolic order, and that is the order of the law. The respect and remorse that the brothers felt toward their father, to Freud, is the symbolic origin of the Oedipus complex and totemism. Even more, this hypothetical singular event represents the true origins of human society, and of all religions, as an effect of collective guilt and ambivalence regarding the killing of the father figure (the true original sin).

References:

Engels. The Origin of the Family, Private Property and the State (2010, in German 1884);

Fromm. The Sane Society (1955);

Freud, Sigmund. Totem and Taboo: Resemblances between the Psychic Lives of Savages and Neurotics (1913),

Lévi-Strauss, Claude. The Elementary Structures of Kinship (1949);

Malinowski, Bronislaw. Sex and Repression in Savage Society (1927);

McLennan, John F. Primitive Marriage (1865);

Morgan, Henry Lewis. Systems of Consanguinity and Affinity of the Human Family (1871);

     -    Ancient Society (1877);