Incest taboo

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.

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 so-called 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.

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.^{[citation needed]}

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.^{[dubious – discuss]} Additionally, protecting the health of closer relatives and their inbred offspring is more evolutionarily advantageous than punishing said relative, especially in a context where predation and starvation are significant factors, as opposed to a rich welfare state.

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. 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:

Incest is sexual intercourse between individuals related in certain prohibited degrees of kinship. In every society there are rules prohibiting incestuous unions, both as to sexual intercourse and recognized marriage. The two prohibitions do not necessarily coincide. There is no uniformity as to which degrees are involved in the prohibitions. The rules regulating incest must be investigated in every society by means of the genealogical method. The prohibition may be so narrow as to include only one type of parent–child relationship (though this is very rare), or those within the elementary family; or so wide as to include all with whom genealogical or classificatory kinship can be traced. The more usual practice is that unions with certain relatives only are considered incestuous, the relationships being regulated by the type of descent emphasized. In some societies unions with certain persons related by affinity are also considered incestuous. What penalties fall on (a) the individuals concerned; (b) the community as a whole? Are such penalties enforced by authority, or are they believed to ensue automatically by the action of a supernatural force? Is there any correlation between the severity of the penalty and the nearness of the blood-tie of the partners in guilt? Should children be born as the result of incestuous unions, how are they treated? Are there any methods, ritual or legal, by which persons who fall within the prohibited degrees and wish to marry can break the relationship and become free to marry?