Incest taboo

The incest taboo refers both to the fact that in many societies specific forms of incest are prohibited, and to various theories that claim, and seek to explain why, some sort of incest taboo is universal.

All theories of the incest taboo are concerned with the formal proscription against incest (as defined locally), not with actual cases of incest (however defined). In general, 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). It should also be noted that in these theories anthropologists are primarily concerned with marriage rules and not sexual behavior.

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, Trobriand Islanders prohibit both sexual relations between a man and his mother, and between a woman and her father, but they describe these prohibitions in very different ways: relations between a man and his mother fall within the category of forbidden relations among members of the same clan; relations between a woman and her father do not. This is because the Trobrianders are matrilineal; children belong to the clan of their mother and not of their father. Thus, sexual relations between a man and his mother's sister (and mother's sister's daughter) are also considered incestuous, but relations between a man and his father's sister are not. Indeed, a man and his father's sister will often have a flirtatious relationship, and a man and the daughter of his father's sister may prefer to have sexual relations or marry.

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.

Similarly, although marriage between first cousins is forbidden in some contemporary jurisdictions it is both legal and acceptable in others.