The science of homosexuality

Siddartha Yerramilli
16 July 2013

Above: Image © istockphoto.com/hh5800

From parliamentary debates to dinner table conversations, homosexuality is often a hot topic of discussion. Whether it’s a politician or your uncle Jerry, people often support their arguments about homosexuality by referring to science. So what have scientists actually learned about homosexuality?

Exactly what determines a person's sexual orientation still remains a mystery. Traditionally, many have pointed to social cues, such as childhood experiences or upbringing. However, many recent studies suggest that sexual orientation is primarily determined by biological cues, especially genetics. Furthermore, these cues may be linked to the reproductive ability of the heterosexual siblings of homosexuals.

Until the mid-20th century, the scientific community generally considered homosexuality a disorder and a perversion. In more recent decades, scientists’ attitudes have changed alongside those of society. In 1990, the World Health Organization stopped listing homosexuality as a psychological disorder. It is now recognized as an acceptable sexual preference, alongside heterosexuality.

Fast fact: Bonobos indulge in casual sex with individuals of either sex. This helps them to form friendships and resolve conflicts.In nature, homosexuality is widespread. It occurs in varying degrees in hundreds of different species of insects, fish, amphibians, reptiles, birds, and mammals. Some notable examples include dolphins, penguins, and bonobos.

Homosexual behaviour has also been documented in various human cultures since antiquity. Studies show that about 8% of both men and women report some form of homosexual attraction.

There is some evidence that homosexuality may be inherited. However, researchers have been unable to identify any “gay genes”. Furthermore, some argue that a genetic basis for homosexuality violates Darwin’s theory of evolution. This theory predicts that heritable traits that increase an individual’s reproductive success (number and health of offspring) will persist or increase in a population. Since homosexual couples don’t directly produce offspring, those sceptical of a genetic explanation wonder how the genes coding for it could persist in a population.

One recent study suggests that the answer might lie in epigenetics, and the possibility that females related to homosexual men have increased fecundity (ability to reproduce). Epigenetic markers are molecules that regulate gene activity by turning genes “on” or “off”, without directly affecting the actual DNA sequence. According to the study, certain epigenetic markers influence an individual’s sexual preference during sexual differentiation. That is when an embryo begins to show male or female characteristics.

Fast fact: Male ruffs, migratory marsh birds, indulge in sexual activity with each other in order to attract females.Epigenetic changes can affect embryonic cells by altering their sensitivity or resistance to testosterone. A fetus with higher sensitivity to testosterone will develop certain physical characteristics and experience "masculinization" (development of male features) of the brain. Resistance to testosterone induces "feminization" (development of female features) of the brain. This process may influence an individual’s future sexual preference.

These epigenetic changes might help prevent males from having abnormally low testosterone levels or females from having abnormally high testosterone levels during development. If these markers are inherited in the next generation, they may have a positive effect on reproductive ability in children of the same sex. But the same markers could cause hormonal changes that lead to homosexuality in opposite–sex children. This theory is also supported by other research showing that female relatives of homosexual men have higher fertility rates.

There probably isn’t one single factor that explains homosexuality. However, recent scientific research suggests that personal choice and upbringing don’t play much of a role. Rather, homosexual attraction is likely determined by natural, especially genetic factors. And far from being a threat to human reproduction, the same epigenetic markers that contribute to homosexuality might increase the fecundity of heterosexual siblings.

References

General science and news websites

Could Scientists Have Found A Gay Switch? (Jennifer Abbasi, Popular Science) Gay Men, Straight Women Have Similar Brains (James Owen, National Geographic) Homosexuality May Start in the Womb (Elizabeth Norton, AAAS) Is Homosexuality Inherited? (Richard Horton, PBS)

Popular science publications

Olivia Judson, Dr. Tatiana's sex advice to all creation (New York: Metropolitan Books, 2002).

Scholarly publications

Berglund H, Lindstrom P, Savic I. 2006. Brain response to putative pheromones in lesbian women. Proceedings of the National Academy of Sciences of the United States of America 103:8269-8274. Camperio Ciani AS, Fontanesi L, Iemmola F, Giannella E, Ferron C, Lombardi L. 2012. Factors associated with higher fecundity in female maternal relatives of homosexual men. The journal of sexual medicine 9:2878-2887. Purves D. 2008. Neuroscience, 4th ed. Sinauer, Sunderland, Mass. Rice WR, Friberg U, Gavrilets S. 2012. Homosexuality as a consequence of epigenetically canalized sexual development. The Quarterly review of biology 87:343-368.

Siddartha Yerramilli

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