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Biology and Sexual Orientation

Biology and sexual orientation is the subject of research into the role of biology in the development of human sexual orientation. No simple, single cause for sexual orientation has been conclusively demonstrated, but research suggests that it is by a combination of genetic, hormonal, and environmental influences,[1] with biological factors involving a complex interplay of genetic factors and the early uterine environment.[2] Biological factors which may be related to the development of a heterosexual, homosexual, bisexual or asexual orientation include genes, prenatal hormones, and brain structure.

Empirical studies

Twin studies

A number of twin studies have attempted to isolate biological factors in sexual orientation. As Bearman and Bruckner (2002) describe it, early studies concentrated on small, select samples, which showed very high genetic influences;[3] however, they were also criticized for non-representative selection of their subjects.[4] Later studies, performed on increasingly representative samples, showed much lesser concordance among monozygotic (MZ, colloquially "identical") twins, although still significantly larger than among dizygotic (DZ) twins.

For example, a meta-study by Hershberger (2001)[5] compares the results of eight different twin studies: among those, all but two showed MZ twins having much higher concordance of sexual orientation than DZ twins, suggesting a non-negligible genetic component. Two additional examples: Bailey and Pillard (1991) in a study of gay twins found that 52% of monozygotic (MZ) brothers and 22% of the dizygotic (DZ) twins were concordant for homosexuality.[6] Also, Bailey, Dunne and Martin (2000) used the Australian twin registry to obtain a sample of 4,901 twins.[7] Self reported zygosity, sexual attraction, fantasy and behaviours were assessed by questionnaire and zygosity was serologically checked when in doubt. MZ twin concordance for homosexuality was found to be 30%.

A recent study of all adult twins in Sweden (more than 7,600 twins)[8] found that same-sex behavior was explained by both heritable factors and individual-specific environmental sources (such as prenatal environment, experience with illness and trauma, as well as peer groups, and sexual experiences), while influences of shared-environment variables such as familial environment and societal attitudes had a weaker, but significant effect. Women showed a statistically non-significant trend to weaker influence of hereditary effects, while men showed no effect of shared environmental effects. The use of all adult twins in Sweden was designed to address the criticism of volunteer studies, in which a potential bias towards participation by gay twin may influence the results (see below).

Overall, the environment shared by twins (including familial and societal attitudes) explained 0–17% of the choice of sexual partner, genetic factors 18–39% and the unique environment 61–66%. The individual's unique environment includes, for example, circumstances during pregnancy and childbirth, physical and psychological trauma (e.g., accidents, violence, and disease), peer groups (other than those shared with a twin), and sexual experiences. In men, genetic effects explained .34–.39 of the variance, the shared environment .00, and the individual-specific environment .61–.66 of the variance. Corresponding estimates among women were .18–.19 for genetic factors, .16–.17 for shared environmental, and .64–.66 for unique environmental factors.


Twin studies have received a number of criticisms including self-selection bias where homosexuals with gay siblings are more likely to volunteer for studies. Nonetheless, it is possible to conclude that, given the difference in sexuality in so many sets of identical twins (who are genetically identical), sexual orientation cannot be purely caused by genetics.[9]

Another issue is the recent finding that even monozygotic twins can be different and there is a mechanism which might account for monozygotic twins being discordant for homosexuality. Gringas and Chen (2001) describe a number of mechanisms which can lead to differences between monozygotic twins, the most relevant here being chorionicity and amniocity.[10] Dichorionic twins potentially have different hormonal environments and receive maternal blood from separate placenta. Monoamniotic twins share a hormonal environment, but can suffer from the 'twin to twin transfusion syndrome' in which one twin is "relatively stuffed with blood and the other exsanguinated".[11] If one twin receives less testosterone and the other more, this could result in different levels of brain masculinisation.

Chromosome linkage studies

Chromosome linkage studies of sexual orientation have indicated the presence of multiple contributing genetic factors throughout the genome. In 1993, Dean Hamer and colleagues published findings from a linkage analysis of a sample of 76 gay brothers and their families.[12] Hamer et al. found that the gay men had more gay male uncles and cousins on the maternal side of the family than on the paternal side. Gay brothers who showed this maternal pedigree were then tested for X chromosome linkage, using twenty-two markers on the X chromosome to test for similar alleles. In another finding, thirty-three of the forty sibling pairs tested were found to have similar alleles in the distal region of Xq28, which was significantly higher than the expected rates of 50% for fraternal brothers. This was popularly dubbed as the 'gay gene' in the media, causing significant controversy. Sanders et al. in 1998 reported on their similar study, in which they found that 13% of uncles of gay brothers on the maternal side were homosexual, compared to 6% on the paternal side.[13]

A later analysis by Hu et al. replicated and refined the earlier findings. This study revealed that 67% of gay brothers in a new saturated sample shared a marker on the X chromosome at Xq28.[14] Although two other studies (Bailey et al., 1999; McKnight and Malcolm, 2000) failed to find a preponderance of gay relatives in the maternal line of homosexual men,[13] a rigorous replication of the maternal loading was reported on samples in Italy in England. One study by Rice et al. in 1999 failed to replicate the Xq28 linkage results.[15] Meta-analysis of all available linkage data indicates a significant link to Xq28, but also indicates that additional genes must be present to account for the full heritability of sexual orientation. A recent study of 894 heterosexual and 694 homosexual men found no evidence of sex linkage.[16]

Sexual orientation and evolution

Sexual practices that significantly reduce the frequency of heterosexual intercourse also significantly decrease the chances of successful reproduction, and for this reason, they would appear to be maladaptive in an evolutionary context following a simple Darwinian model of natural selection—on the assumption that homosexuality would reduce this frequency. Several theories have been advanced to explain this contradiction, and new experimental evidence has demonstrated their feasibility.[46]

Some scholars[46] have suggested that homosexuality is adaptive in a non-obvious way. By way of analogy, the allele (a particular version of a gene) which causes sickle-cell anemia when two copies are present may also confer resistance to malaria with a lesser form of anemia when one copy is present (this is called heterozygous advantage). [47]

The so-called "gay uncle" hypothesis posits that people who themselves do not have children may nonetheless increase the prevalence of their family's genes in future generations by providing resources (food, supervision, defense, shelter, etc.) to the offspring of their closest relatives. This hypothesis is an extension of the theory of kin selection. Kin selection was originally developed to explain apparent altruistic acts which seemed to be maladaptive. The initial concept was suggested by J.B.S. Haldane in 1932 and later elaborated by many others including John Maynard Smith, W. D. Hamilton and Mary Jane West-Eberhard.[48] This concept was also used to explain the patterns of certain social insects where most of the members are non-reproductive.

Brendan Zietsch of the Queensland Institute of Medical Research proposes the alternative theory that men exhibiting female traits become more attractive to females and are thus more likely to mate, provided the genes involved do not drive them to complete rejection of heterosexuality.[49]

In a 2008 study, its authors stated that "There is considerable evidence that human sexual orientation is genetically influenced, so it is not known how homosexuality, which tends to lower reproductive success, is maintained in the population at a relatively high frequency." They hypothesized that "while genes predisposing to homosexuality reduce homosexuals' reproductive success, they may confer some advantage in heterosexuals who carry them." and their results suggested that "genes predisposing to homosexuality may confer a mating advantage in heterosexuals, which could help explain the evolution and maintenance of homosexuality in the population."[50] However, in the same study, the authors noted that "nongenetic alternative explanations cannot be ruled out" as a reason for the heterosexual in the homosexual-heterosexual twin pair having more partners, specifically citing "social pressure on the other twin to act in a more heterosexual way" (and thus seek out a greater number of sexual partners) as an example of one alternative explanation. Also, the authors of the study acknowledge that a large number of sexual partners may not lead to greater reproductive success, specifically noting there is an "absence of evidence relating the number of sexual partners and actual reproductive success,either in the present or in our evolutionary past".

Important new evidence on a plausible mechanism for the evolution of "gay genes" has emerged from the work of Camperio-Ciani.[23] They found in two large, independent studies that the female relatives of homosexual men tended to have significantly more offspring than those of the heterosexual men. Female relatives of the homosexual men on their mother's side tended to have more offspring than those on the father's side. This indicates that females carrying a putative "gay genes" complex are more fecund than women lacking this complex of genes, and thereby can compensate for any decreased fertility of the males carrying the genes. This is a well known phenomenon in evolution known as "sexual antagonism," and has been widely documented for many traits that are advantageous in one sex but not in the other. This provides solid experimental evidence of how "gay genes" could not only survive but thrive over the course of evolution.

Biological differences in gay men and lesbians


Some studies have found correlations between physiology of people and their sexuality. These studies provide evidence which they claim suggests that:

  • Gay men report, on an average, slightly longer and thicker penises than non-gay men.[51]
  • Gay men and straight women have, on average, equally proportioned brain hemispheres. Lesbian women and straight men have, on average, slightly larger right brain hemispheres.[52]
  • The VIP SCN nucleus of the hypothalamus is larger in men than in women, and larger in gay men than in heterosexual men.[53]
  • The average size of the INAH-3 in the brains of gay men is approximately the same size as INAH 3 in women, which is significantly smaller, and the cells more densely packed, than in heterosexual men's brains.[28]
  • The anterior commissure is larger in women than men and was reported to be larger in gay men than in non-gay men,[27] but a subsequent study found no such difference.[54]
  • Gay men's brains respond differently to fluoxetine, a selective serotonin reuptake inhibitor.[55]
  • The functioning of the inner ear and the central auditory system in lesbians and bisexual women are more like the functional properties found in men than in non-gay women (the researchers argued this finding was consistent with the prenatal hormonal theory of sexual orientation).[56]
  • The suprachiasmatic nucleus was found by Swaab and Hopffman to be larger in gay men than in non-gay men,[57] the suprachiasmatic nucleus is also known to be larger in men than in women.[58]
  • The startle response (eyeblink following a loud sound) is similarly masculinized in lesbians and bisexual women.[59]
  • Gay and non-gay people's brains respond differently to two putative sex pheromones (AND, found in male armpit secretions, and EST, found in female urine).[24][60][61]
  • One region of the brain (amygdala) is more active in gay men than non-gay men when exposed to sexually arousing material.[62]
  • Finger length ratios between the index and ring fingers may be different between non-gay and lesbian women.[56][63][64][65][66][67]
  • Gay men and lesbians are significantly more likely to be left-handed or ambidextrous than non-gay men and women;[68][69][70] Simon LeVay argues that because "[h]and preference is observable before birth [71]... [t]he observation of increased non-right-handness in gay people is therefore consistent with the idea that sexual orientation is influenced by prenatal processes," perhaps heredity.[28]
  • A study of 50 gay men found 23% had counterclockwise hair whirl, as opposed to 8% in the general population. This may correlate with left-handedness.[72]
  • Gay men have increased ridge density in the fingerprints on their left thumbs and pinkies.[72]
  • Length of limbs and hands of gay men is smaller compared to height than the general population, but only among white men.[72]


Recent studies suggest the presence of subtle differences in the way gay people and non-gay people process certain kinds of information. Researchers have found that:

  • Gay men[73] and lesbians are more verbally fluent than heterosexuals of the same sex[74][75][76] (but two studies did not find this result).[77][78]
  • Gay men may receive higher scores than non-gay men on tests of object location memory (no difference was found between lesbians and non-gay women).[79]

Political aspects

Whether genetic or other physiological determinants as the basis of sexual orientation is a highly politicized issue. The Advocate, a U.S. gay and lesbian newsmagazine, reported in 1996 that 61% of its readers believed that "it would mostly help gay and lesbian rights if homosexuality were found to be biologically determined".[80] A cross-national study in the United States, the Philippines, and Sweden found that those who believed that "homosexuals are born that way" held significantly more positive attitudes toward homosexuality than those who believed that "homosexuals choose to be that way" and/or "learn to be that way". [81][82]

Equal protection analysis in U.S. law determines what groups are considered suspect classes and therefore eligible for heightened scrutiny based on several factors, one of which is immutability. Evidence that sexual orientation is biologically determined (and therefore perhaps immutable in the legal sense) would strengthen the legal case for heightened scrutiny of laws discriminating on that basis.[83][84][85]

The perceived causes of sexual orientation have a significant bearing on the status of sexual minorities in the eyes of social conservatives. The Family Research Council, a conservative Christian think tank in Washington, D.C., argues in the book Getting It Straight that finding people are born gay "would advance the idea that sexual orientation is an innate characteristic, like race; that homosexuals, like African-Americans, should be legally protected against 'discrimination;' and that disapproval of homosexuality should be as socially stigmatized as racism. However, it is not true." On the other hand, some social conservatives such as Reverend Robert Schenck have argued that people can accept the "inevitable... scientific evidence" while still morally opposing homosexuality.[86] As well, National Organization for Marriage board member and fiction writer Orson Scott Card has supported biological research on homosexuality, writing that "our scientific efforts in regard to homosexuality should be to identify genetic and uterine causes... so that the incidence of this dysfunction can be minimized.... [However, this should not be seen] as an attack on homosexuals, a desire to 'commit genocide' against the homosexual community.... There is no 'cure' for homosexuality because it is not a disease. There are, however, different ways of living with homosexual desires."[87]

Some advocates for the rights of sexual minorities resist linking that cause with the concept that sexuality is biologically determined or fixed at birth. They argue that sexual orientation can shift over the course of one's life.[88] At the same time, others resist any attempts to pathologise or medicalise 'deviant' sexuality, and choose to fight for acceptance in a moral or social realm.[86] The Atlantic Monthly has stated that "Some, recalling earlier psychiatric "treatments" for homosexuality, discern in the biological quest the seeds of genocide. They conjure up the specter of the surgical or chemical "rewiring" of gay people, or of abortions of fetal homosexuals who have been hunted down in the womb."[89] Simon LeVay has said, in response to letters from gays and lesbians making such criticisms, that the research "has contributed to the status of gay people in society."[86]


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42. ^ An Evolutionary Look at Human Homosexuality Greg Cochran, original publication date unknown
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44. ^
Despite some degree of logical plausibility, there is ultimately very little to be said in favor of these contentions. In its focus on the reduced reproductive rates of homosexual men and women, the account ignores other mechanisms by which genetic traits endure across generations. More importantly, the account is offered without any evidence whatsoever about which microbe might work how to generate homosexual interests. A peer-reviewed science journal turned this account away, but it nevertheless found its way into the pages of the public press.... the ease with which theories of homosexuality seep into public discourse raises important ethical questions about the way in which researchers ought to communicate their various theories to the public. Given that an unfounded theory of homosexuality can do more damage than good, researchers should raise the bar in regard to the views they propound about its origin.
45. ^ a b Crain, C. "Did a Germ Make You Gay?" in Out Magazine, August 1999 :
On the one hand, William Byne, a brain researcher at the Mount Sinai School of Medicine in New York, suspects that Cochran and Ewald are guilty of pathologizing homosexuality. "It's hard for most people to entertain the idea that homosexuality might be a natural variant of human sexual behavior," says Byne. On the other hand, Michael Bailey, a professor of psychology at Northwestern University, gives Cochran and Ewald the benefit of the doubt. Bailey does worry that homophobes could use the germ theory as political ammunition—as "proof" that homosexuality is a disease. But that would be "a totally illegitimate conclusion," in Bailey's opinion. Not everything caused by a germ is a disease, he insists. "Suppose we found that a form of genius was also caused by a virus. Would that mean that genius is a disease?"
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[edit] External links
• Molecular Genetic Study of Sexual Orientation A genetic study of gay brothers at Northwestern University.