Incest, inbreeding, and their consequences
Bittles, A.H. (2001) Incest, inbreeding, and their consequences. In: Smelser, N.J. and Baltes, P.B., (eds.) International Encyclopedia of the Social and Behavioral Sciences. Pergamon Press, Oxford, England, pp. 7254-7259.
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Inbreeding describes unions between couples known to share at least one common ancestor. While now rare in most Western societies, 20 percent to over 50 percent of current marriages in regions such as North Africa, West, Central, and South Asia are between couples related as second cousins or closer. The term incest is generally applied to any sexual union closer than permissible under prevailing religious or legal norms, and most commonly to mating between first-degree relatives, i.e., father 13daughter, mother 13son, or brother 13sister, who have 50 percent of their genes in common. Incest avoidance is observed in virtually all human societies, and according to the Westermarck hypothesis it can be explained in terms of negative imprinting against close associates of early childhood. The adverse biological outcomes associated with inbreeding are caused by the expression of detrimental recessive genes. The closer the biological relationship between parents, the greater the probability that their offspring will inherit identical copies of one or more mutant genes. Thus the offspring of incestuous matings are four times more likely to have inherited identical gene copies from each parent than children born to first cousins, with associated adverse effects that may include developmental delay, physical anomalies, and intellectual handicap.
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