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. 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. 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, 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.
Examples from other societies further reveal the variation in local understandings of incest. In Chinese societies, there is a strong taboo against marriage of persons with the same surname no matter how distantly related. There are often local taboos against marriage between people of certain surnames on the grounds that these surnames belong to clans which were closely related in the past. Similarly, although marriage between first cousins is forbidden in some contemporary jurisdictions it is both legal and acceptable in others.
Although anthropologists have observed and studied violations of incest taboos (in other words, cases of incest), all anthropological theories of the incest taboo are concerned with the formal proscription against incest (as defined locally), not with actual cases of incest (however defined). These theories are motivated by two major questions: first, given the variation in how different societies define incest, and in which relationships are proscribed, is there any general pattern or universal function of incest taboos? Second, given that people do commit incest, why do so many (indeed, arguably, all) societies proscribe certain forms of incest? These questions are not concerned with the specific effects of incest on specific people — a matter usually left to psychologists.
Anthropologists reject this explanation for two reasons. First, inbreeding does not directly lead to congenital birth defects per se; it leads to an increase in the frequency of 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 this type of heritable birth defect die (or are killed) before they reproduce, the ultimate effect of inbreeding will be to decrease the frequency of defective genes in the population.
One might complain that a society would have to have a fairly advanced understanding of genetics to recognize this potential "benefit" of incest, whereas the increased prevalence of birth defects is relatively easy to spot.
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. In that particular case, the prohibition against relations is not based on or motivated by concerns over biological closeness.
Sociologist Ian Robertson gives three main social reasons why incest taboo exist as a cultural universals. The first is that early human beings-living primarily in small kinship groups of hunters and gatherers- needed to protect themselves by forming alliances with other groups. By forcing their children to marry into families outside their own, each group widened its social links and provided itself with allies in time of famine or other hazards. These groups faced the alternatives of marrying out or dying. Marriage in most traditional societies is a practical alliance between groups, not a love match between individuals. That is why marriages were often arranged by the parents, often when their offspring were still children and sometimes even before they are born. The second reason for the incest taboo is that the family itself could not function without it, for the statuses of family members would be utterly and hopelessly confused. As Kingsley Davis points out: " The incestuous child of a father-daughter union would be a brother of his own mother, i.e. the son of his own sister; a stepson of his own grandmother; possibly a brother of his own uncle; and certainly a grandson of his own father." The third reason is that without an incest taboo, sexual rivalry among family members would disrupt the normal roles and attitudes of the various relatives. The father, for example, might experience role conflict as both the disciplinarian and the lover of his daughter; the mother might be jealous of both; and the child, of course, would be caught in the middle. Faced with constant conflict and tension, the family institution might simply disintegrate. The incest taboo has developed over time because it is vital to the survival of the family and thus of society itself. Of course, neither traditional nor modern societies consciously appreciate the reasons for the taboo. They simply accept it as natural and moral.
Another theory suggests that the taboo expresses a psychological revulsion that people naturally experience anyway at the thought of incest.
Under this view, advanced by evolutionary psychologists, the incest taboo is primarily caused not by social condemnation, but rather by genes for incest avoidance, which would tend to prosper, by ensuring that an individual's children (possibly containing those same genes) are not unhealthy due to inbreeding. Furthermore, the benefits of sex (as opposed to asexual reproduction) are mysterious (see evolution of sex), but whatever they are, they would tend to be reduced by incest. Genes that prevented incest would tend to inhabit bodies that had more of these benefits, and therefore tend to become more widely spread.
Evolutionary psychologists (e.g. Steven Pinker in How the Mind Works) suggest that genetic influence is at work in the Westermarck effect, whereby people raised in close proximity (whether related or not) tend to feel little sexual attraction to each other, after maturity.
Most anthropologists reject this explanation, since incest does in fact occur. They suggest that the taboo itself may be the cause of the psychological revulsion.
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. Lévi-Strauss first exposed this Alliance theory in the Elementary Structures of Kinship (1949).
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 do 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.
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