Homosexuality and genetics

SIGNIFICANCE: The debate over whether individuals choose to whom they are attracted or their orientation is determined primarily by genetic or social factors is ongoing. Interest persists in part because individuals’ sexual orientation appears to extend beyond sexuality to influence gender and in part because individuals erroneously believe that social acceptance and treatment of gay people may differ depending upon whether gay and lesbian individuals are free agents or are responding to biological imperatives.

Biological vs. Environmental Factors

Sexual orientation is a fundamental aspect of human sexuality. In most societies, the only accepted expression of sexuality was heterosexuality (women mating with men). Sexual orientation may be closely linked to sexual experience, but many factors (social, religious, or logistic) can decrease the correlation. As a result, the frequency of homosexuality (a sexual orientation or attraction to persons of the same sex) varies. The Kinsey Institute reported that, according to the 2018 (US) National Survey of Sexual Health and Behavior, about 8 percent of adult men and 7 percent of adult women surveyed identified as bisexual, gay, or lesbian, but the percentage of people who have had same-gender sexual relations is higher. A 2022 Gallup poll found that 7.1 percent of adults in the United States identified themselves as lesbian, gay, bisexual, or transgender (LGBT). In general, there appears to be a continuum, from exclusive heterosexuality (87 to 93 percent) to exclusive homosexuality (1 to 4 percent) with many people falling somewhere between. Like most complex behaviors, homosexuality is probably influenced by both biological and environmental factors. The exact mechanism may differ for individuals who appear to exhibit similar behavioral patterns.

Genetic Influences

The genetic basis of homosexuality has been assessed using twin studies and pedigree analysis. Lesbians are approximately three times as likely as heterosexual women to have lesbian sisters and generally have more lesbian relatives as well, which suggests that genes as well as environmental factors influence homosexuality in women. Similarly, among men, in sexual orientation among (MZ) twins is greater than that for dizygotic (DZ) twins or non-twin brothers. Since MZ twins share 100 percent of their genes but are not always either both straight or both gay, sexual orientation cannot be 100 percent due to genes.

Heritability of homosexuality has been estimated at 30 to 75 percent for men and at 25 to 76 percent for women. The different rates of and frequency, with lesbians typically representing a smaller proportion of the population than gay men, suggests that men’s and women’s sexuality may have different origins. The X-linked locus associated with homosexuality in some men (Xq28, according to D. H. Hamer and S. Hu, 1993) does not appear to be associated with lesbianism (according to Hu et al., 1995). Further, research suggests that men’s orientation is bimodal in distribution relative to the Kinsey scale of sexual orientation, whereas women’s orientation is distributed more continuously and is more likely than men’s to change through adulthood.

A study in 2019 found that genetics along with environment play a part in shaping a person's sexuality. After analyzing DNA from more than 470,000 people in the United States and the United Kingdom, researchers found a handful of genes that were connected to same-sex sexual behavior. The study was led by Andrea Ganna, group leader of the European Molecular Biological Laboratory at the Institute of Molecular Medicine in Finland. Ganna and her team discovered five genetic variants that were statistically associated with same-sex behaviors.

Neurohormonal Influences

Gay and lesbian adults do not differ from their heterosexual counterparts in terms of circulating levels of sex hormones. Instead, the neuroendocrine theory predicts that prenatal exposure to high levels of androgens masculinizes brain structures and influences sexual orientation. Consistent with this, women with congenital adrenal hyperplasia (CAH) who experience atypically high levels of androgens prenatally appear to be somewhat more likely to engage in same-sex sexual fantasies and behavior compared to the control group of women (Meyer-Bahlburg, et al. 2008), whereas XY women with complete androgen insensitivity syndrome (cAIS) do not exhibit increased expression of lesbianism. Exposure to the synthetic estrogen DES, which is also thought to have a demasculating effect on the brain, also appears to influence women’s sexuality modestly and to induce higher levels of homosexuality.

Stress hormones generally reduce the production of sex hormones. The level and timing of stress experienced by women during pregnancy may therefore also affect the amount of sex hormones experienced prenatally and hence the sexual differentiation and organizational phase of early brain development. Studies suggest that some women who experience stress during pregnancy may be more likely to have homosexual children, but the data are still preliminary.

Given that most gays and lesbians do not have one of the aforementioned hormonal conditions and most individuals who do have them are heterosexual, the neuroendocrine theory alone does not appear to account for the origin of homosexuality.

Neuroanatomical Influences

Although stereotypes exist, there is no overall lesbian or gay physique. There is some evidence that gay men’s brains may differ from heterosexual men’s in some structures where sexual dimorphism also occurs (for example, interstitial nuclei of the anterior hypothalmus 3, suprachiasmatic nucleus in the anterior hypothalamus and the anterior commissure), presumably due to the organizational effects of sex hormones. Structure size varies considerably both within and between sexes; however, all three structures appear to differ significantly in size for gay versus heterosexual men. It is not yet clear whether these differences cause homosexual activity or are caused by it.

Evolutionary Perspective

Evolutionary biologists have suggested that homosexuality may persist because there is little cost associated with the behavior. In situations in which homosexuality is not exclusive (that is, most individuals engage in heterosexual as well as homosexual liaisons), homosexuals would experience little or no decline in reproductive success. This could occur when marriage is compulsory, where there are strict gender roles and religious requirements, or when homosexual behavior is situational or opportunistic. Similarly, in situations in which individuals are exclusively homosexual and experience no direct individual fitness (that is, no offspring are produced), homosexuals can reduce the reproductive cost by increasing their inclusive fitness via contributions to relatives’ offspring. Consistent with the latter hypothesis, there is some evidence that gay men exhibit increased levels of empathy, an accepted indicator of altruism.

Homosexuality is one of the three most common expressions of human sexual orientation and has been observed throughout human history and across religions and cultures. Like other complex behavioral traits, sexual orientation appears to be influenced by both biological and environmental factors. There is some evidence that situational or opportunistic homosexuality may differ from obligatory homosexuality and that the mechanisms influencing sexual orientation may be different in gay men and lesbians.

Key Terms

• concordancethe presence of a trait in both members of a pair of twins
• heritabilitythe proportion of phenotypic variation that is due to genes rather than the environment
• sex-linked traitscharacteristics that are encoded by genes on the X or Y chromosome

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