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, incest between twins is a common feature in Indo-European, Asian (such as Japan, the Philippines, and Bali) and Oceanian mythology, and there are a few societies in which the prohibition on it is limited or it is partially accepted.
In Asian culture
In traditional Balinese culture, it was[when?] 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 mythology. As in many other mythologies, the Balinese deities frequently marry their siblings without any of the incest-related issues faced by similarly-situated human couples.
This was commonplace in Southeast Asian creation myths which prominently featured twin or sibling couples. In these stories, the brother usually wooed and wed his sister, who bore his child(ren), but on discovering that they are siblings, they are often (but not always) forced to part.
According to Tagalog mythology, Malakás ("strong") and Magandá ("beautiful"), the first humans on earth, were fraternal twins born of the same bamboo stalk.
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.
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 Micheal and Radek Cuma aka Milo and Elijah 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 Wendy MacLeod's play The House of Yes and its subsequent film adaptation, twins Jackie O. and Marty Pascal reignite an incestuous relationship - in which they have sex with each other after ritualistically reenacting the assassination of John F. Kennedy - when Marty comes home for Thanksgiving to introduce his fiancee, Lesly, to his exceedingly eccentric family.
- In The Empire Strikes Back, Luke Skywalker and Princess Leia have an extended kiss while Luke recuperates on the Planet Hoth, though neither of them is aware that they are siblings at the time.
- In George R. R. Martin's critically acclaimed A Song of Ice and Fire book series and its HBO TV adaptation, fraternal twins Jaime and Queen Cersei Lannister frequently engage in secret intercourse, beginning when they were children. As a result, three children have been born, posing as the heirs of Cersei's husband, King Robert. Lord Eddard Stark discovers this, but his friend King Robert dies before he can reveal the children's true parentage, and the subsequent political struggle plunges the Seven Kingdoms into a massive civil war.
- 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.
- The novel The Secret History also depicts a sexual relationship between twins Camilla and Charles Macaulay.
- The episode "Bombshell" of Law and Order: Special Victims Unit deals with incest between adult twins, Cassandra Davina (Rose McGowan) and Doug Loveless (Ryan Hurst).
- Incest (as "forbidden love"), especially between siblings, particularly twins, has regularly been featured in Japanese fiction, most visibly in mediums appealing primarily to youth and "otaku" such as manga, anime, videogames, light novels.
- In the anime version of .hack//Legend of the Twilight, Shugo and Rena are a pair of teenage twins who are separated when their parents divorce and are reunited in the online game of The World where their relationship intensifies to romantic feelings.
- In the anime and manga Aki Sora, Sora and his twin sister Nami regularly have sex, even though it begins as Nami raping her twin brother. Sora willingly has sex with his older sister, Aki, although the two sisters do not have sex or possibly even know about each other's relationship with their brother.
- In the anime and manga Yosuga no Sora, fraternal twins Haruka and Sora develop an incestuous relationship at the end of the series, and it is later revealed that they moved overseas to find happiness, got married and are expecting their first child.
- In Boku wa Imouto ni Koi o Suru, Yori and Iku are twins who fall deeply in love with each other and struggle to deal with the consequences of it. At the end of the manga, it is revealed that they are the result of heteropaternal superfecundation.
- In the popular anime Ouran High School Host Club, identical twins Hikaru and Kaoru play up the twincest taboo by giving large quantities of brotherly love to each other. This intrigues and delights many of their female classmates; however, the relationship between the brothers does not appear to actually cross into amatory at any time.
- In the Booker Prize-winning novel The God of Small Things, written by Indian author Arundhati Roy, ends with its two main characters sleeping together.
- In the 2003 film The Dreamers, adult fraternal twins Theo and Isabelle engage in sexual games with one another, for a while incorporating their new friend, Matthew, who is intrigued and disturbed by their world.
- In the short film "Keith & Heath", two identical twin brothers (who are puppets) are the best of friends until a girl threatens to tear them apart. The film ends with the two brothers getting married and kissing, which turns them into humans (played by Jon Cozart) and chaos ensues.
- Sexual contact between adult identical twins is a niche in gay pornography.
- Sexual conflict and incest between twins is a prominent feature of the contemporary German author Kerstin Hensel's "Grotesque Literature".
- In the 2004 teen-comedy film Eurotrip, fraternal twins Jenny and Jamie (Michelle Trachtenberg and Travis Wester), tour Europe when they decide to have some fun and go to a nightclub. Drunk on absinthe, Jenny and Jamie French-kiss and make out with each other, witnessed by a horrified Scotty and Cooper.
- The word 'twincest' is used in the 2014 movie Gone Girl about the possibly over close relationship between the chief protagonist (Ben Affleck) and his sister (Carrie Coon).
- In the Independent 2006 horror film The Hamiltons, twins Wendell (Joseph McKelheer) and Darlene (Mackenzie Firgens) share a bizarre incestuous relationship that separates them from the rest of the siblings.
- In Marvel's The Ultimates, twin siblings Quicksilver and Scarlet Witch share an incestuous relationship in The Ultimates #8, the cover which has come to be known as the "twincest" cover.
- 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.
- 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.
- "Gay Porn's Most Shocking Taboo," Salon, May 20, 2010, http://www.salon.com/2010/05/21/twincest/
- 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.
- The Ultimates #8 (Nov. 2002)
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