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The incest taboo refers to the prohibition, both formal and unstated, against incest in many societies. There are various theories that seek to explain how and why an incest taboo originates. Some advocates maintain that some sort of incest taboo is universal, while others dispute its universality.
The following excerpt from Notes and Queries, the most well-established field manual for ethnographic research, illustrates the scope of ethnographic investigation into the matter.
As this excerpt suggests, anthropologists are interested in the gulf between cultural rules and actual behavior, and many ethnographers have observed that incest occurs in societies with prohibitions against incest. It should be further noted that in these theories anthropologists are generally concerned solely with brother-sister incest, and are not claiming that all forms of incest are taboo (these theories are further complicated by the fact that in many societies people related to one another in different ways, and sometimes distantly, are classified together as siblings). Moreover, the definition restricts itself to sexual intercourse; this does not mean that other forms of sexual contact do not occur, or are proscribed, or prescribed. It should also be noted that in these theories anthropologists are primarily concerned with marriage rules and not sexual behavior. In short, anthropologists were not studying "incest" per se; they were asking informants what they meant by "incest," and what the consequences of "incest" were, in order to map out social relationships within the community.
This excerpt also suggests that the relationship between sexual and marriage practices is complex, and that societies distinguish between different sorts of prohibitions. In other words, although an individual may be prohibited from marrying or having sexual relations with many people, different sexual relations may be prohibited for different reasons, and with different penalties.
For example, homozygotes. A homozygote encoding a congenital birth-defect will produce children with birth-defects, but homozygotes that do not encode for congenital birth-defects will decrease the number of carriers in a population. If children born with birth-defects die (or are killed) before they reproduce, the ultimate effect of inbreeding will be to decrease the frequency of defective genes in the population. Second, anthropologists have pointed out that in the Trobriand case a man and the daughter of his father's sister, and a man and the daughter of his mother's sister, are equally distant genetically. Therefore, the prohibition against relations is not based on or motivated by concerns over biological closeness.
Finally, Claude Lévi-Strauss has argued that the incest taboo is in effect a prohibition against endogamy, and the effect is to encourage exogamy. Through exogamy, otherwise unrelated households or lineages will form relationships through marriage, thus strengthening social solidarity.
This theory was debated intensely by anthropologists in the 1950s. It appealed to many because it used the study of incest taboos and marriage to answer more fundamental research interests of anthropologists at the time: how can an anthropologist map out the social relationships within a given community, and how does these relationships promote or endanger social solidarity? Nevertheless, anthropologists never reached a consensus, and with the Vietnam War and the process of de-colonization in Africa, Asia, and Oceania, anthropological interests shifted away from mapping local social relationships.
Claude Lévi-Strauss, The Elementary Structures of Kinship George Homans and David Schneider, Marriage, Authority, and Final Causes: A Study of Unilateral Cross-Cousin Marriage Rodney Needham, Structure and Sentiment: A Test Case in Social Anthropology