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.
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 so-called Westermarck effect discourages adults from engaging in sexual relations with individuals with whom they grew up.
Nevertheless, anthropologists have found that the institution of marriage, and rules concerning appropriate and inappropriate sexual behavior, exist in every society.
The more usual practice is that unions with certain relatives only are considered incestuous, the relationships being regulated by the type of descent emphasized.
[8]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.
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.
[citation needed] This excerpt also suggests that the relationship between sexual and marriage practices is complex, and that societies distinguish between different sorts of prohibitions.
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.
[22] 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.
[23] 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.
Therefore, it is argued that the prohibition against incestuous relations in most societies is not based on or motivated by concerns over biological closeness.
[35] Steve Stewart-Williams argues against the view that incest taboo is a Western phenomenon, arguing that while brother–sister marriage was reported in a diverse range of cultures such Egyptian, Incan, and Hawaiian cultures, it was not a culture-wide phenomenon, being largely restricted to the upper classes.
[2] The anthropologist Claude Lévi-Strauss developed a general argument for the universality of the incest taboo in human societies.
Through exogamy, otherwise unrelated households or lineages will form relationships through marriage, thus strengthening social solidarity.
This theory is based in part on Marcel Mauss's theory of The Gift, which (in Lévi-Strauss' words) argued: that exchange in primitive societies consists not so much in economic transactions as in reciprocal gifts, that these reciprocal gifts have a far more important function than in our own, and that this primitive form of exchange is not merely nor essentially of an economic nature but is what he aptly calls "a total social fact", that is, an event which has a significance that is at once social and religious, magic and economic, utilitarian and sentimental, jural and moral.
[37]It is also based on Lévi-Strauss's analysis of data on different kinship systems and marriage practices documented by anthropologists and historians.
Lévi-Strauss called attention specifically to data collected by Margaret Mead during her research among the Arapesh.
[39] Some anthropologists argue that nuclear family incest avoidance can be explained in terms of the ecological, demographic, and economic benefits of exogamy.
Young men entering the age system would then find a dire shortage of marriageable girls, and extended families would be in danger of dying out.
Thus, by parading this avoidance of their daughters, senior men make these girls available for younger age-sets and their marriages form alliances that mitigate the rivalries for power.
[43] An extreme example of this principle, and an exception to the incest taboo, is found among members of the ruling class in certain ancient states, such as the Inca, Egypt, China, and Hawaii; brother–sister marriage (usually between half-siblings) was a means of maintaining wealth and political power within one family.