Editing Human sexuality (section)

===Sexual orientation===
{{Main|Sexual orientation}}

There is considerably more evidence supporting innate causes of sexual orientation than learned ones, especially for males. This evidence includes the cross-cultural correlation of homosexuality and childhood [[gender nonconformity]], moderate genetic influences found in [[twin studies]], evidence for prenatal hormonal effects on brain organization, the fraternal birth order effect, and the finding that in rare cases where infant males were raised as girls due to physical deformity, they nevertheless turned out attracted to females. Hypothesized social causes are supported by only weak evidence, distorted by numerous confounding factors.<ref name=Bailey16 />

Cross-cultural evidence also leans more toward non-social causes. Cultures that are very tolerant of homosexuality do not have significantly higher rates of it. Homosexual behavior is relatively common among boys in British single-sex boarding schools, but adult Britons who attended such schools are no more likely to engage in homosexual behavior than those who did not. In an extreme case, the [[Sambia people]] ritually require their boys to engage in homosexual behavior during adolescence before they have any access to females, yet most of these boys become heterosexual.<ref name=LeVay/><ref name=Balthazart/>

It is not fully understood why genes causing homosexuality persist in the gene pool. One hypothesis involves [[kin selection]], suggesting that homosexuals invest heavily enough in their relatives to offset the cost of not reproducing as much directly. This has not been supported by studies in Western cultures, but several studies in [[Samoa]] have found some support for this hypothesis. Another hypothesis involves [[Sexual conflict|sexually antagonistic]] genes, which cause homosexuality when expressed in males but increase reproduction when expressed in females. Studies in both Western and non-Western cultures have found support for this hypothesis.<ref name=Bailey16/><ref name=BussCh5>{{cite book |last=Buss |first=David |date=2019 |title=Evolutionary Psychology: The New Science of the Mind |chapter=Men's Long-Term Mating Strategies|url=https://books.google.com/books?id=Sn6JDwAAQBAJ |publisher=Routledge |isbn=9780429590061 }}</ref>