An incest taboo is any cultural rule or norm that prohibits sexual relations between certain members of the same family, mainly between individuals related by blood. All known human cultures have norms that exclude certain close relatives from those considered suitable or permissible sexual or marriage partners, making such relationships taboo. However, different norms exist among cultures as to which blood relations are permissible as sexual partners and which are not. Sexual relations between related persons which are subject to the taboo are called incestuous relationships.
Some cultures proscribe sexual relations between clan-members, even when no traceable biological relationship exists, while members of other clans are permissible irrespective of the existence of a biological relationship. In many cultures, certain types of cousin relations are preferred as sexual and marital partners, whereas in others these are taboo. Some cultures permit sexual and marital relations between aunts/uncles and nephews/nieces. In some instances, brother–sister marriages have been practised by the elites with some regularity. Parent–child and sibling–sibling unions are almost universally taboo.The Tapestry of Culture An Introduction to Cultural Anthropology, Ninth Edition, Abraham Rosman, Paula G. Rubel, Maxine Weisgrau, 2009, AltaMira Press. . p.101
Origin Debate about the origin of the incest taboo has often been framed as a question of whether it is based in nature or nurture.
One explanation sees the incest taboo as a cultural implementation of a biologically evolved preference for sexual partners with whom one is unlikely to share genes, since inbreeding may have detrimental outcomes. The most widely held hypothesis proposes that the Westermarck effect discourages adults from engaging in sexual relations with individuals with whom they grew up. The existence of the Westermarck effect has achieved some empirical support.Inbreeding, Incest, and the Incest Taboo: The State of Knowledge at the Turn of the Century, Arthur P. Wolf and William H. Durham (Editors), Stanford University Press, 2004, . Introduction
Another school argues that the incest prohibition is a cultural construct which arises as a side effect of a general human preference for group exogamy, which arises because intermarriage between groups construct valuable alliances that improve the ability for both groups to thrive. According to this view, the incest taboo is not necessarily universal, but is likely to arise and become more strict under cultural circumstances that favour exogamy over endogamy, and likely to become more lax under circumstances that favor endogamy. This hypothesis has also achieved some empirical support.
Limits to biological evolution of taboo While it is theoretically possible that natural selection may, under certain genetic circumstances, select for individuals that instinctively avoid mating with (close) relatives, incest will still exist in the gene pool because even genetically weakened, inbred individuals are better watchposts against predators than none at all, and weak individuals are useful for the stronger individuals in the group as looking out for predators without being able to seriously compete with the stronger individuals.E. O. Wilson, Consilience: The Unity of Knowledge, New York, Knopf, 1998The Blind Watchmaker: Why the Evidence of Evolution Reveals a Universe without Design, Richard Dawkins, 1986 Gorrell J.C., McAdam A.G., Coltman D.W., Humphries M.M., Boutin S., Jamieson C.; McAdam, Andrew G.; Coltman, David W.; Humphries, Murray M.; Boutin, Stan (June 2010). "Adopting kin enhances inclusive fitness in asocial red squirrels". Nature Communications 1Wright, Sewall (1922). "Coefficients of inbreeding and relationship". American Naturalist 56
Research
Modern anthropology developed at a time when a great many human societies were illiterate, and much of the research on incest taboos has taken place in societies without legal codes, and, therefore, without written laws concerning marriage and incest. Nevertheless, anthropologists have found that the institution of marriage, and rules concerning appropriate and inappropriate sexual behavior, exist in every society.Marvin Harris 1997 Culture, People and Nature: An Introduction to General Anthropology 7th edition Longman pp. 250, 253 The following excerpt from Notes and Queries on Anthropology (1951), a well-established field manual for ethnographic research, illustrates the scope of ethnographic investigation into the matter:
These theories anthropologists are generally concerned solely with brother–sister incest, and are not claiming that all sexual relations among family members are taboo or even necessarily considered incestuous by that society. 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, and others who are just as closely related genetically are not considered family members.
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. For example, in some Inuit societies in the Arctic, and traditionally in Bali, mothers would routinely stroke the penises of their infant sons; such behavior was considered no more sexual than breastfeeding.Briggs, Jean Louise Never in anger: portrait of an Eskimo family 1970 Harvard University Press, Cambridge (Mass.)Gregory Bateson, Steps to an ecology of mind: collected essays in anthropology, psychiatry, evolution, and epistemology preface by Mark Engel 1972 Chandler, San Francisco 112–115
In these theories, anthropologists are primarily concerned with marriage rules and not actual 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 woman and her brother,Bronislow Malinowski 1929 The Sexual Life of Savages in North-West Melanesia: An Ethnographic Account of Courtship, Marriage and Family Life Among the Natives of the Trobriand Highlands, British New Guinea Boston: Beacon Press 389, 392 and between a woman and her father,Bronislaw Malinowski 1929 The Sexual Life of Savages in North-West Melanesia: An Ethnographic Account of Courtship, Marriage and Family Life Among the Natives of the Trobriand Highlands, British New Guinea Boston: Beacon Press 384 but they describe these prohibitions in very different ways: relations between a woman and her brother 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.Bronislaw Malinowski 1929 The Sexual Life of Savages in North-West Melanesia: An Ethnographic Account of Courtship, Marriage and Family Life Among the Natives of the Trobriand Highlands, British New Guinea Boston: Beacon Press 450–451 A man and his father's sister will often have a flirtatious relationship, and, far from being taboo, Trobriand society encourages a man and his father's sister or the daughter of his father's sister to have sexual relations or marry.Bronislaw Malinowski 1929 The Sexual Life of Savages in North-West Melanesia: An Ethnographic Account of Courtship, Marriage and Family Life Among the Natives of the Trobriand Highlands, British New Guinea Boston: Beacon Press 449–450
Instinctual and genetic explanations
An explanation for the taboo is that it is due to an instinctual, inborn aversion that would lower the adverse genetic effects of inbreeding such as a higher incidence of congenital birth defects. Since the rise of modern genetics, belief in this theory has grown.Alexander, Richard 1977 "Natural Selection and the Analyusis of Human Sociology" in The Changing Scenes in the Natural Sciences, 1776–1976 pp. 283–337 Academy of Natural Science Special Publication 12
Birth defects and inbreeding The increase in frequency of birth defects often attributed to inbreeding results directly from an increase in the frequency of homozygous alleles inherited by the offspring of inbred couples. This leads to an increase in homozygous allele frequency within a population, and results in diverging effects. Should a child inherit the version of homozygous alleles responsible for a birth defect from its parents, the birth defect will be expressed; on the other hand, should the child inherit the version of homozygous alleles not responsible for a birth defect, it would actually decrease the ratio of the allele version responsible for the birth defect in that population. The overall consequences of these diverging effects depends in part on the size of the population.
In small populations, as long as children born with inheritable 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; over time, the gene pool will be healthier. However, in larger populations, it is more likely that large numbers of carriers will survive and mate, leading to more constant rates of birth defects.Thornhill, Nancy, ed. 1993 The Natural History of Inbreeding and Outbreeding. Chicago: UNiversity of Chicago Press Besides recessive genes, there are also other reasons why inbreeding may be harmful, such as a narrow range of certain immune systems genes in a population increasing vulnerability to infectious diseases (see Major histocompatibility complex and sexual selection). The biological costs of incest also depend largely on the degree of genetic proximity between the two relatives engaging in incest. This fact may explain why the cultural taboo generally includes prohibitions against sex between close relatives but less often includes prohibitions against sex between more distal relatives. Children born of close relatives have decreased survival. Many mammal species, including humanity's closest primate relatives, avoid incest.
Westermarck effect
The Westermarck effect, first proposed by Edvard Westermarck in 1891, is the theory that children reared together, regardless of biological relationship, form a sentimental attachment that is by its nature non-erotic.Westermarck, Edvard A. (1921). The history of human marriage, 5th edn. London: Macmillan Melford Spiro argued that his observations that unrelated children reared together on Israeli Kibbutzim nevertheless avoided one another as sexual partners confirmed the Westermarck effect.Spiro, M. (1965). Children of the Kibbutz. New York: Schocken. Joseph Shepher in a study examined the second generation in a kibbutz and found no marriages and no sexual activity between the adolescents in the same peer group. This was not enforced but voluntary. Looking at the second generation adults in all kibbutzim, out of a total of 2769 marriages, none were between those of the same peer group.