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How Did Homosexuality Evolve? Is There a Gay Gene?

Dr. Thomas Swan has a PhD in psychology from the University of Otago. He has researched several psychological traits and disorders.

Could being gay be determined by our genetic makeup?

Could being gay be determined by our genetic makeup?

Genetic Explanations for Being Gay

In the past few decades, much attention has focused upon the work of geneticists who are searching for a gene that might cause a gay sexual orientation. Proving or disproving the existence of a "gay gene" would settle tendentious political debates about whether homosexuality is or isn't a choice (i.e., nature or nurture).

Despite the clear rewards of such research, homosexuality may be too complex to be explained by a single gene. For example, there is evidence to suggest that several genes may be involved (perhaps hundreds), and that there is a role for epigenetic (i.e., whether genes are turned "on" or "off"), prenatal, and environmental factors.

Furthermore, psychological research and collective experience tell us that the terms “gay” and “straight” by no means represent the continuum of sexuality. There is a diverse spectrum of sexual orientations, which further suggests that several genes and causal factors affect the sexuality of all individuals to some "relative" extent.

Beyond the search for a gay gene, biologists are also presented with the dilemma of whether one should exist in the first place. Gay people are less likely than straight people to pass on their genetic material through reproduction, meaning that any predisposition for being gay should have long-since disappeared from the human genome through natural selection.

This article therefore has a twofold function:

  1. To summarize the evidence for and against gay genes.
  2. To explain how homosexuality might have evolved.

Is There a Gay Gene?

According to a 1991 study conducted by Bailey and Pillard, when one identical twin is gay, there is a 52% chance that the other twin will be as well. The percentage also dropped for reduced genetic linkages, with 22% of fraternal twins and 9.2% of non-twin brothers being gay.

Although some theorists held up this study as proof of a gay gene, others pointed out that identical twins should have shown 100% agreement. Indeed, Bailey and Pillard’s work actually discounted the notion of a genetic "on/off switch” for being gay, while at the same time suggesting some degree of genetic involvement.

Twin studies suggest there is a partial genetic influence on the manifestation of a gay orientation.

Twin studies suggest there is a partial genetic influence on the manifestation of a gay orientation.

The Bailey and Pillard study had several problems. For example, most twins are not separated at birth, meaning that the results might still be explained by environmental or social factors.

These factors might include a similar family environment (e.g., that is more tolerant of homosexuality) or the tendency of twins to mimic each other, which might be greater in siblings with greater similarity. Indeed, the study failed to explain why fraternal twins and non-twin brothers, who have the same level of genetic similarity, produced such different results.

Despite the problems, Dean Hamer subsequently found evidence of genetic consistencies in gay men (shared Xq28 genetic markers). However, his 1993 findings were disputed by George Rice who found no such sharing. The studies might have been inconsistent because Hamer did not utilize a control group or test his subjects’ heterosexual brothers for the same linkage, or because Rice's research was biased (he had professed a belief that a gay gene could not exist).

Following these initial forays into the subject, several studies have identified genes that are more prevalent in gay individuals. In 2008, it was discovered that genes that determine blood type (chromosone 9) and rhesus factor (a protein found on red blood cells; chromosone 1) can produce differences between gay and straight individuals on these biological traits.

In 2014, a team of researchers found support for the Hamer study (Xq28) and identified a further gene (8p12; chromosone 8). In 2017, the same team identified several genes that are more prevalent in gay individuals (on chromosones 8, 13, and 14), some of which relate to thyroid function.

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Finally, in 2019, a large study that included almost half a million British participants found several "gay genes" (on chromosones 4, 7, 11, 12, and 15). The authors nevertheless urged caution, stating that these genes only explain 8%–25% of the gay orientation and that no single gene explains more than 1%. In other words, there may be hundreds or even thousands of genes that contribute to the orientation.

Could sexual orientation be determined in the womb?

Could sexual orientation be determined in the womb?

Epigenetic, Prenatal, and Environmental Factors

Most biologists concede that genetic factors contribute less than 50% to the gay orientation. The remainder may stem from the interaction of these genes with environmental factors.

For example, epigenetics is the study of whether the genes that we all have are active or "switched on" in some individuals and not others. Activation may be hereditary or disease-related, or occur through hormonal, chemical (e.g., drugs), developmental (e.g., aging), or environmental influences (e.g., diet and exercise).

Although the study of epigenetic causes of homosexuality is relatively new, a 2015 study found that it could accurately predict the sexual orientation of 70% of people within their sample.

The specific epigenetic cause may be prenatal hormones. For example, a review of research concerning the prenatal hormonal environment found that many of the traits that define homosexuality are "organized" in the womb through the influence of prenatal hormones (e.g., testosterone).

The evidence for postnatal social factors affecting sexual orientation after birth is much more speculative, although these may also interact with genetic or prenatal factors at the psychological level.

For example, an abundance of female psychological or hormonal traits in men (or male traits in women) may increase one’s susceptibility to becoming gay as a result of psychological or social pressures (e.g., ridicule or rejection) to establish a personal identity that is consistent with one's biology.

Indeed, during late adolescence, nearly everyone experiences an identity crisis. According to Erik Erikson, during this crisis we strive to understand ourselves by searching for and strengthening our core attributes to create the basis for our adult selves. Grey-areas in our thoughts become black and white, which may be why this is often a time in which gay individuals “come out."

This position is bolstered by the work of Richard Green, who followed the development of 78 male children, 44 of whom were identified as exhibiting feminine behaviors. Two-thirds of these boys became gay, as opposed to only one of the non-feminine boys.

Could predisposed preferences (e.g., in toy choice) affect how children later identify?

Could predisposed preferences (e.g., in toy choice) affect how children later identify?

Can Homosexuality Be Naturally Selected?

The existence of a gay gene would appear to present a dilemma for evolutionary biologists: if gay sex doesn't produce children, why haven't gay genes died out? Various theories have addressed the dilemma.

Kin Selection

One potential solution is kin selection. Without children of their own, gay individuals can provide assistance and protection to the offspring of their brothers and sisters, ensuring that those offspring stand a better chance of survival. By sharing some of the same genetic material, gay genes may survive in these offspring. For example, feminine male Samoans, or fa'afafine, are known for their laborious dedication to family.

Although Westerners no longer live in large familial units (e.g., with uncles and aunties present), it may be that kin selection occurred before this societal change took place. If this is true, then the gay orientation should be in decline, although this could be offset by increased rates of gay people becoming parents through sperm donors or surrogate mothers.

Polymorphism Theory

A second possibility is polymorphism theory, which argues that a moderate amount of "gay" genetic traits (e.g., less aggressive, more empathic and committal, etc.) can actually make males more attractive to the opposite sex than highly-heterosexual "alpha" males.

The theory suggests that the straight brothers of gay men will have several of these attractive "gay" genetic traits, making them more likely to have families (and thus offspring that have the genes). However, a test of the theory found that straight men with gay brothers had no statistically significant reproductive advantage over straight men with straight brothers.

Sexually Antagonistic Selection

A third alternative, called sexually antagonistic selection, may be better supported. Rather than the brothers of gay males enjoying a reproductive advantage, the theory suggests that sisters do instead. For example, if gay genes increase same-sex attraction, then siblings of the opposite sex who share this genetic material will enjoy greater heterosexual attraction. This would mean that sisters of gay men (or brothers of gay women) will have more children, leading to the propagation of gay genes.

Studies conducted in Britain and Italy have found that women with gay male relatives have significantly more children, and this occurs even if the relative is from a different generation (discounting kin selection as an explanation). The effect is seen in the maternal pedigree line, suggesting that gay genes exist in the X-chromosome, and supporting earlier research about the importance of the Xq28 genetic marker.

Gay Animals Are Common

Nature or Nurture?

In sum, the sexually antagonistic selection theory allows gay genes to propagate through natural selection because of the benefits they bestow on siblings of the opposite sex. This opens the door for researchers to ask if being gay is genetic.

Although current research supports the existence of genetic causes for being gay, there is no basis for the belief in one controlling “gay gene.” Rather, there appear to be hundreds or even thousands of gay genes that collectively contribute 8%–50% of the likelihood that someone will be gay.

While there are other factors that affect sexual orientation, such as whether these genes are "activated," and the effect of the prenatal hormonal environment on this process, there is insufficient evidence to suggest that social factors can independently cause someone to be gay.

Thus, the conclusion is that nature determines homosexuality (as supported by the widespread evidence for homosexuality in the animal kingdom; see video above), but that nurture and developmental processes often interact with nature to create variation and inconsistent results.

Given that people cannot control the genetics they are dealt, the prenatal environment they develop in, or even the postnatal environment they grow up in, it is highly unlikely that being gay is a lifestyle choice for anyone.

Further Reading

  • LeVay, S. (2017). Gay, Straight, and the Reason Why: The Science of Sexual Orientation. Oxford University Press.
  • Bailey, J. M., Vasey, P. L., Diamond, L. M., Breedlove, S. M., Vilain, E., & Epprecht, M. (2016). Sexual Orientation, Controversy, and Science. Psychological Science in the Public Interest, 17(2), 45–101.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2012 Thomas Swan

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