Incest between twins or twincest is a subclass of sibling incest and includes both heterosexual and homosexual relationships.^[1]

Sexual arousal from a twin sister or sister in general is referred to as sororilagnia or sorophilia, while a male analogous term is fratrilagnia,^[2] and the more general term is sibcest.^[3]

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 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.^[4]

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.^[5]

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.^[6]

The theme also appears in English literature, such as the incest between the twins Polydore and Urania in Delarivier Manley's The New Atlantis.^[7]

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".^[8] His study drew on Edvard Westermarck's hypothesis that sexual desire is generally absent in relationships between members of a nuclear family.^[9]

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.^[10] The story was widely publicised in the British press,^[11] although its truthfulness was called into question.^[12]