Incest between twins
Incest between twins is a subclass of sibling incest and includes both heterosexual and homosexual relationships. While in modern Western European culture, such behaviour is considered taboo and is quite rare, incest between twins is a common feature in Indo-European, Indonesian and Oceanian mythology, and there are a few societies in which the prohibition on it is limited or it is partially accepted.
In Balinese culture
In traditional Balinese culture, it was common for a set of twins of the opposite sex to marry each other, since it was assumed that they had sex in utero. The standard anthropological explanation of this custom is based in explications of the conflicts between descent and affinity in Balinese society. Twin incest was a common feature of Balinese folklore—several southeast Asian new myths prominently feature a twin or sibling couple. In one of the more common stories, the brother marries his sister and she bears his child, but on discovering that they are siblings, they are forced to part. As in many other mythologies, the Balinese deities frequently marry their siblings without any of the issues faced by human incestuous couples.
In European culture
Twin incest is a prominent feature in ancient Germanic mythology, and its modern manifestations, such as the relationship between Siegmund and Sieglinde in Richard Wagner's Die Walküre, and a feature in some Greek mythology, such as the story of Byblis and Kaunos. There are strong parallels between the Germanic portrayals of twin incest and those of the Balinese Ramayana, and some scholars have speculated an early Indo-European link. The theme also appears in English literature, such as the incest between the twins Polydore and Urania in Delarivier Manley's The New Atlantis. Sexual conflict and incest between twins is also a prominent feature of the contemporary German author Kerstin Hensel's "Grotesque Literature".
In a 1983 review of the scholarly literature on twin homosexuality and twin incest, Ray Bixler concluded that "most same sex homosexual twins, if reared with their co-twins, do not attempt or even want to seduce them in adulthood". His study drew on Edvard Westermarck's hypothesis that sexual desire is generally absent in relationships between members of a nuclear family.
One case of incest between twins, in which twins who were adopted by separate families as babies later married without knowing they were brother and sister, was mentioned in a House of Lords debate on the Human Fertility and Embryology Bill in January 2008. According to the charity Adults Affected by Adoption, there had been other cases of this sort that had involved siblings. The story was widely publicised in the British press, although its truthfulness was called into question.
Czech identical twins Elijah and Milo Peters are male porn actors who work condomless, performing both anal and oral sex on each other in video performances since 2009. They consider each other to be both brothers and romantic partners, and report that they do not have sex with any men besides each other when they are not on film. 
In popular culture
- In the critically acclaimed book series and HBO TV series, Game of Thrones features fraternal twins Jaime and Cersei Lannister, who frequently engage in intercourse, beginning when they were children. As a result, three children have been born.
- The 1975 novel Gemini by Michel Tournier depicted the deeply sexual relationship between twin brothers Jean and Paul and their struggle to come to terms with the implications of their sexualized world consisting of only each other versus their individuality in the outside world.
- Episode Bombshell of Law and Order: Special Victims Unit deals with incest between adult twins.
- Boon, James A. (1990). Affinities and Extremes: Crisscrossing the Bittersweet Ethnology of East Indies History, Hindu-Balinese Culture and Indo-European Culture. Chicago University Press. p. 113. ISBN 978-0-226-06463-5.
- Errington, Atkinson, Shelley, Jane Monnig (1990). Power and Difference: Gender in Island Southeast Asia. Stanford University Press. p. 227. ISBN 0-8047-1781-8.
- Boon, James A. (1990). Affinities and Extremes: Crisscrossing the Bittersweet Ethnology of East Indies History, Hindu-Balinese Culture and Indo-European Culture. Chicago University Press. p. 107. ISBN 978-0-226-06463-5.
- Pollak, Ellen (2003). Incest and the English Novel, 1684-1814. The Johns Hopkins University Press. p. 103. ISBN 978-0-8018-7204-4.
- Marven, Lynn (2005). Body and Narrative in Contemporary Literatures in German: Herta Müller, Libuse Moníková, Kerstin Hensel. Clarendon Press. pp. 220–2. ISBN 978-0-19-927776-6.
- Bixler, Ray H. (August 1983). "Homosexual Twin Incest Avoidance". The Journal of Sex Research 19 (3): 296–302. doi:10.1080/00224498309551190. JSTOR 3812342.
- Westermarck, Edvard (1922). The History of Human Marriage, Vol. II. New York: Allerton, p. 193.
- "Parted-at-birth twins 'married'". BBC News. 2008-01-11. Retrieved 2008-05-18.
- Henley, Jon (2008-01-15). "Did a pair of twins really get married by mistake?". guardian.co.uk. Retrieved 2008-05-18.
- Rogers, Thomas (May 21, 2010). "Gay Porn's Most Shocking Taboo". Salon. Retrieved 2012-05-23.
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