Editing Biology and sexual orientation (section)

====Twin studies====
Twin studies are one method of testing genetic and environmental influences, although they cannot reveal what kind of environmental influence this may include (social or non-social).{{Sfn|Bailey|2003|p=110–111}} Identical or monozygotic twins share their genes, while fraternal or dizygotic twins are only as genetically similar as any other sibling pair. When twins both share a trait, they are ''concordant'' for this trait; and when they differ they are [[Discordant Twins|''discordant'']]. If identical twins have a higher rate of concordance for a trait than fraternal twins, it indicates that genes may contribute to the trait.<ref name="Bailey_2016" />{{rp|74–76}}

The 2016 Bailey et al. meta-analysis of all twin studies on sexual orientation found that the [[median]] concordance for homosexual or non-heterosexual orientation in twins in unbiased probability samples is 24% for monozygotic identical twins, and 15% for dizygotic twins.<ref name="Bailey_2016" />{{rp|74–76}} According to Rice et al. the identical twin concordance for homosexuality is similar to identical twin concordance for two other traits influenced by prenatal androgens: [[cryptorchidism]] and [[hypospadias]] (feminized male gonads) which have an identical twin concordance around 25%, despite twins sharing genes and prenatal environments.<ref name=":4">{{Cite journal |last1=Rice |first1=William R. |last2=Friberg |first2=Urban |last3=Gavrilets |first3=Sergey |date=2012 |title=Homosexuality as a Consequence of Epigenetically Canalized Sexual Development |url=http://scottbarrykaufman.com/wp-content/uploads/2012/12/Rice-et-al.-2012.pdf |journal=The Quarterly Review of Biology |volume=87 |issue=4 |pages=343–368 |doi=10.1086/668167 |pmid=23397798 |issn=0033-5770}}</ref>{{rp|356}}[[File:Gary_and_Larry_Lane_photo.jpg|alt=|thumb|250x250px|Identical twin studies are a useful mechanism for assessing the role of genes and environment.]]Twin studies have also found that among twins with differing sexual orientations, homosexual twins were significantly more [[gender nonconforming]] than their heterosexual co-twin, and that this was noticeable from a young age.{{Sfn|Bailey|2003|p=110–111}} Bailey states:
{{Quote|text=What kind of environmental factor can cause genetically identical twins reared in the same family from birth—often dressed alike and given the same toys—to differ in their sexual orientations? It is a fascinating question that we haven’t begun to answer well. One hint comes from the childhood behavior findings. When identical twins differed in their sexual orientation, the gay one tended to recall being much more feminine than the straight one. This means that the environmental factors that cause the twin differences are there early on, by childhood. Based on other things we know, such as studies of children with cloacal exstrophy, I suspect that these factors operate in the womb.{{Sfn|Bailey|2003|p=110–111}}}}
Identical twins reared apart from birth are another method of studying the origin of psychological traits. Unfortunately, such twin pairs are rare. Three sets of male twin pairs exist in the literature. In the first pair found by Thomas Bouchard, both male twins reared apart from birth were homosexual. In a second pair found by Whitham, both males were also homosexual. In the third pair found by Bouchard, the male twins were neither definitively concordant nor discordant, as both had relations with males and females, thus this pair may be concordant for bisexuality. Among the female twin pairs; four female twin pairs were all discordant, although the small number of cases prevent any strong conclusions.{{Sfn|LeVay|2017|p=89–90}}

According to William Rice and colleagues, the concordance of homosexuality among twins raises the possibility that homosexuality is not caused by genes nor atypical levels of hormones, but an epigenetic mechanism controlling how sensitive fetuses are to prenatal hormones.<ref name=":4" /><ref>{{Cite journal |last=Tasos |first=Emmanouil |date=2022 |title=To What Extent are Prenatal Androgens Involved in the Development of Male Homosexuality in Humans? |url=https://www.researchgate.net/publication/352125966 |journal=Journal of Homosexuality |language=en |volume=69 |issue=11 |pages=1928–1963 |doi=10.1080/00918369.2021.1933792 |pmid=34080960 |issn=0091-8369 |via=ResearchGate}}</ref>{{Rp|page=22}}