Editing Incest taboo (section)

===Instinctual and genetic explanations===
{{seealso|Inbreeding depression}}
An explanation for the taboo is that it is due to an instinctual, inborn aversion that would lower the adverse [[Heredity|genetic effects]] of [[inbreeding]] such as a higher incidence of congenital [[birth defect]]s. Since the rise of modern genetics, belief in this theory has grown.<ref>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</ref><ref>{{cite journal | last1 = Bittles | display-authors = etal  | year = 1991 | title = Reproductive Behavior and Health in Consangueneous Marriages | journal = Science | volume = 2 | issue = 52| pages = 789–794 | doi = 10.1126/science.2028254 | pmid = 2028254 | bibcode = 1991Sci...252..789B | s2cid = 1352617 }}</ref><ref name="doi10.1038/ng1094-117">{{Cite journal | last1 = Bittles | first1 = A. H. | last2 = Neel | first2 = J. V. | doi = 10.1038/ng1094-117 | title = The costs of human inbreeding and their implications for variations at the DNA level | journal = Nature Genetics | volume = 8 | issue = 2 | pages = 117–121 | year = 1994 | pmid =  7842008| s2cid = 36077657 }}</ref><ref name=moral>{{Cite journal | last1 = Lieberman | first1 = D. | last2 = Tooby | first2 = J. | last3 = Cosmides | first3 = L. | doi = 10.1098/rspb.2002.2290 | title = Does morality have a biological basis? An empirical test of the factors governing moral sentiments relating to incest | journal = Proceedings of the Royal Society B: Biological Sciences | volume = 270 | issue = 1517 | pages = 819–826 | year = 2003 | pmid =  12737660| pmc = 1691313}}</ref>{{failed verification|date=November 2011}}

====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.<ref>{{cite journal | last1 = Livingstone | first1 = Frank B | year = 1969 | title = Genetics, Ecology, and the Origins of Incest and Exogamy | journal = Current Anthropology | volume = 10 | pages = 45–62 | doi=10.1086/201009| s2cid = 84009643 }}</ref> 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.<ref>Thornhill, Nancy, ed. 1993 ''The Natural History of Inbreeding and Outbreeding''. Chicago: UNiversity of Chicago Press</ref> Besides recessive genes, there are also other reasons why inbreeding may be harmful, such as a narrow range of certain [[immune system]]s 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.<ref>{{cite journal |last1=Antfolk |first1=Jan |last2=Lieberman |first2=Debra |last3=Santtila |first3=Pekka |title=Fitness Costs Predict Inbreeding Aversion Irrespective of Self-Involvement: Support for Hypotheses Derived from Evolutionary Theory |journal=PLOS ONE |date=2012 |volume=7 |issue=11|doi=10.1371/journal.pone.0050613 |pages=e50613 |pmid=23209792 |pmc=3509093|bibcode=2012PLoSO...750613A |doi-access=free }}</ref> Children born of close relatives have decreased survival.<ref name="doi10.1038/ng1094-117"/><ref name="moral"/> Many mammal species, including humanity's closest [[primate]] relatives, avoid incest.<ref name="WolfDurham2005"/>

====Westermarck effect====
{{main|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.<ref>Westermarck, Edvard A. (1921). ''The history of human marriage'', 5th edn. London: Macmillan</ref> [[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.<ref>Spiro, M. (1965). Children of the Kibbutz. New York: Schocken.</ref> 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.<ref>{{Cite journal | last1 = Shepher | first1 = J. | title = Mate selection among second generation kibbutz adolescents and adults: Incest avoidance and negative imprinting | doi = 10.1007/BF01638058 | journal = Archives of Sexual Behavior | volume = 1 | issue = 4 | pages = 293–307 | year = 1971 | pmid =  24179077| s2cid = 25602623 }}</ref>

However, according to a book review by John Hartung of a book by Shepher, out of 2516 marriages documented in Israel, 200 were between couples reared in the same kibbutz. These marriages occurred after young adults reared on kibbutzim had served in the military and encountered tens of thousands of other potential mates, and 200 marriages is higher than what would be expected by chance. Of these 200 marriages, five were between men and women who had been reared together for the first six years of their lives.<ref>{{cite journal | last1 = Hartung | first1 = John | year = 1985 | title = Review of ''Incest: A Biological View'' by J. Shepher | doi = 10.1002/ajpa.1330670213 | journal = American Journal of Physical Anthropology | volume = 67 | pages = 167–171 }}</ref>

A study in Taiwan of marriages where the future bride is adopted in the groom's family as an infant or small child found that these marriages have higher infidelity and divorce and lower fertility than ordinary marriages; it has been argued that this observation is consistent with the Westermarck effect.<ref>Wolf, A. 1995. ''Sexual attraction and childhood association: a Chinese brief for Edward Westermarck''. Stanford, CA: Stanford University Press.</ref>

====Third-parties' objections====
Another approach is looking at moral objections to third-party incest. This increases the longer a child has grown up together with another child of the opposite sex. This occurs even if the other child is genetically unrelated.<ref name=moral/> Humans have been argued to have a special kin detection system that besides the incest taboo also regulates a tendency towards [[altruism]] towards kin.<ref>{{Cite journal | last1 = Lieberman | first1 = D. | last2 = Tooby | first2 = J. | last3 = Cosmides | first3 = L. | doi = 10.1038/nature05510 | title = The architecture of human kin detection | journal = Nature | volume = 445 | issue = 7129 | pages = 727–731 | year = 2007 | pmid =  17301784| pmc = 3581061| bibcode = 2007Natur.445..727L }}</ref>

====Counter arguments====
One objection against an instinctive and genetic basis for the incest taboo is that incest does occur.<ref>Claude Lévi-Strauss, 1969 ''The Elementary Structures of Kinship'' revised edition, translated from the French by James Harle Bell and John Richard von Sturmer. Boston: Beacon Press. 17</ref><ref>Cicchetti and Carlson eds. 1989 ''Child Maltreatment: Theory and Research on the Causes and Consequences of Child Abuse and Neglect''. New York, Cambridge University Press</ref><ref>Glaser and Frosh 1988 ''Child and Sexual Abuse'' Chicago: Dorsey Press.</ref> Anthropologists have also argued that the social construct "incest" (and the incest taboo) is not the same thing as the biological phenomenon of "inbreeding". For example, there is equal genetic relation between a man and the daughter of his father's sister and between a man and the daughter of his mother's sister, such that biologists would consider mating incestuous in both instances, but [[Trobrianders]] consider mating incestuous in one case and not in the other. Anthropologists have documented a great number of societies where marriages between some first cousins are prohibited as incestuous, while marriages between other first cousins are encouraged. Therefore, it is argued that the prohibition against incestuous relations in most societies is not based on or motivated by concerns over biological closeness.<ref>Claude Lévi-Strauss, 1969 ''The Elementary Structures of Kinship'' revised edition, translated from the French by James Harle Bell and John Richard von Sturmer. Boston: Beacon Press. 13–14</ref> Other studies on cousin marriages have found support for a biological basis for the taboo.<ref>{{Cite journal | last1 = Kushnick | first1 = G. | last2 = Fessler | first2 = D. M. T. | doi = 10.1086/659337 | title = Karo Batak Cousin Marriage, Cosocialization, and the Westermarck Hypothesis | journal = Current Anthropology | volume = 52 | issue = 3 | pages = 443–448 | year = 2011 | url = https://openresearch-repository.anu.edu.au/bitstream/1885/50038/2/01_Kushnick_Karo_Batak_Cousin_Marriage%2c_2011.pdf | hdl = 1885/50038 | s2cid = 20905611 | hdl-access = free }}</ref><ref>{{Cite journal | last1 = Fessler | first1 = D. M. T. | title = Neglected Natural Experiments Germane to the Westermarck Hypothesis | doi = 10.1007/s12110-007-9021-1 | journal = Human Nature | volume = 18 | issue = 4 | pages = 355–364 | year = 2007| pmid = 26181312 | s2cid = 2039872 }}</ref><ref>{{Cite journal | last1 = McCabe | first1 = J. | doi = 10.1525/aa.1983.85.1.02a00030 | title = FBD Marriage: Further Support for the Westermarck Hypothesis of the Incest Taboo | journal = American Anthropologist | volume = 85 | pages = 50–69 | year = 1983| doi-access =  }}</ref> Also, current supporters of genetic influences on behavior do not argue that genes determine behavior absolutely, but that genes may create predispositions that are affected in various ways by the environment (including culture).<ref name=AmPs2010>{{Cite journal | last1 = Confer | first1 = J. C. | last2 = Easton | first2 = J. A. | last3 = Fleischman | first3 = D. S. | last4 = Goetz | first4 = C. D. | last5 = Lewis | first5 = D. M. G. | last6 = Perilloux | first6 = C. | last7 = Buss | first7 = D. M. | doi = 10.1037/a0018413 | title = Evolutionary psychology: Controversies, questions, prospects, and limitations | journal = American Psychologist | volume = 65 | issue = 2 | pages = 110–126 | year = 2010 | pmid = 20141266 | url = http://homepage.psy.utexas.edu/homepage/Group/BussLAB/pdffiles/evolutionary_psychology_AP_2010.pdf | access-date = 2015-08-28 | archive-date = 2015-08-20 | archive-url = https://web.archive.org/web/20150820063400/http://homepage.psy.utexas.edu/homepage/Group/BussLAB/pdffiles/evolutionary_psychology_AP_2010.pdf | url-status = dead }}</ref>

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. Stewart-Williams argues that these marriages were largely political (their function being to keep power and wealth concentrated in the family) and there is no evidence the siblings were attracted to each other and there is in fact some evidence against it (for example, [[Cleopatra]] married two of her brothers but did not have children with them, only having children with unrelated lovers). Stewart-Williams suggests that this was therefore simply a case of social pressure overriding anti-incest instincts. Stewart-Williams also observes that anti-incest behaviour has been observed in other animals and even many plant species (many plants could self-pollinate but have mechanisms that prevent them from doing so).<ref>Stewart-Williams, Steve. The ape that understood the universe: How the mind and culture evolve. Cambridge University Press, 2018, pp 135-136</ref>