Homosexuality does not have a reproductive benefit, then how did it evolve in humans?- this question has been posed as a paradox time and again. Homosexual individuals alone cannot reproduce and for it to be genetic, the genes that code for it must have evolved as they passed on through generations who derived an evolutionary advantage from them. Thus, it is reasoned that homosexuality should not have evolved. Multiple theories, however, say otherwise. Although it has become scientific orthodoxy that homosexuality is not a decision and that it cannot be cured or corrected with treatment or by religious gurus, research continues to be supplemented with scientific papers and theories ever so often.
Alfred Kinsey’s original work (Kinsey et al., 1948, 1953), however, remains one of the most important sources of information on sexual orientation in humans. (So much so that a well-received movie based on his life was released in the United States in 2004 but owing to his controversial findings, many theatres refused to screen it, and still do!). Kinsey reported his data, collected over 10 years from surveys, on the frequency of behavior such as masturbation, anal sex, and premarital sex. Needless to say, these reports of people engaging in alternative sexual behaviors created a furor and Kinsey concluded that sexuality exists on a spectrum, simultaneously formulating the Kinsey Scale (1948).
But since homosexual individuals reproduce far less often than heterosexuals, it gives rise to a Darwinian Puzzle. As Paul Vasey from the University of Lethbridge in Canada puts it, “How can a trait like homosexuality, which has a genetic component, persist over evolutionary time if the individuals that carry the genes associated with that trait are not reproducing?”
Several theories manage to explain why homosexuality has persisted for eons, even if partially. A striking one is the Kin Selection Hypothesis (Iemmola & Camperio Ciani, 2008; Rahman & Hull, 2005). This means that by enhancing the survival prospects of the kin, homosexuality serves an indirect benefit i.e., homosexual men enhance their genetic prospects by being “helpers in the nest”-acting altruistically towards their younger nieces and nephews. Evolutionary psychologists Paul Vasey and Doug VanderLaan (Vasey & VanderLaan, 2010) studied the fa’afafine in Samoa, people who identify as transgender, dressing as women, and having sex with men. (Interestingly, they do not like to be called ‘gay’ or ‘homosexual’). He established that ‘gay’ Samoan men spend more time fulfilling their responsibilities as uncles such as babysitting or tutoring than ‘straight men’, passing the genes for their sexual orientation down the generations. If this hypothesis holds then it partly explains why the genes that account for same-sex attraction have not been selected away through evolution, putting together some pieces of the Darwinian puzzle. But this gives rise to another debate.
We put a lot of emphasis on the reproductive functions of sex. For a trait to be inherited, reproduction through sex is the necessary condition. Is it though? What if sex is not just about reproduction but also bonding and other social functions? Indeed, it is. For many animals as well as Humans, sex serves major social functions. In Bonobos, our closest primate relative, gay sex and same-sex sexual attractions play an essential role in social relationships, barter of food, and same-sex social bonding (They share 99.6% of their DNA with us!).
In a paper published by Andre Barron from Macquarie University and Brian Hare from Duke University titled ‘Prosociality and a Sociosexual Hypothesis for the Evolution of Same-Sex Attraction in Humans’, it is proposed that homosexuality evolved as an outcome of the evolution of increased sociability in humans. Their hypothesis- called the ‘sociosexual’ hypothesis for the evolution of homosexuality- focuses on the adaptive social consequences of same-sex sexual attraction. Accordingly, human evolution was driven by natural selection favoring traits that promoted better social relationships and integration. Thus, homosexuality evolved because it gave individuals who experienced same-sex attraction the benefit of social mobility and stronger social bonds. This theory seems to be the most plausible in the light of the fact that sexual activity has multiple other social functions than just reproduction.
So far, I have talked about how homosexuality has evolved even though it does not have a direct reproductive benefit. There is another possibility that the genes that code for homosexuality might have other strong benefits that ensure their transfer from one generation to the other. For instance, ‘male-loving genes’ in a man may predispose him to have a same-sex sexual attraction but the same genes in a woman may predispose her to mate earlier than usual and have more children. Similarly, ‘female-loving genes’ in a lesbian may cause her to be attracted to other women but may make straight women more attractive to men, leading them to mate earlier and have more children. In the light of this theory, Andrea Camperio-Ciani, from the University of Padova in Italy, also found that maternal female relatives of gay men reproduce far more often than maternal female relatives of straight men. Even though ever so often a person receives a larger amount of these genes that may change their sexual orientation but the genes still have a strong benefit when seen from the lens of reproduction and evolution. This compensates for the lack of reproduction by homosexuals and ensures that their genes are not selected away, as heterosexual individuals who mate with these genes pass it down the generations.
These theories expound on the evolution of homosexuality in humans from social and historical perspectives. There is little support for the Kin Selection hypothesis as little evidence suggests that families gain an advantage from the offspring care provided by homosexual members. The sociosexual hypothesis claims that sexual activity has other social benefits that need to be focused on rather than just reproduction, which seems reasonable along with the fact that the genes that code for homosexuality are also responsible for other reproductive benefits, ensuring their evolution.
As you might have noticed, many of the theories I have quoted have been based on male homosexuals. Unfortunately, the literature on female homosexuality is notable for its paucity. Even the studies that have been performed are inadequate in terms of explaining the evolutionary purpose of sexual orientation in women and thus, an urgent need arises for studies that focus on female homosexuality from the lens of evolutionary psychology.
To conclude, homosexuality and its evolutionary purpose is a Darwinian Puzzle. Since homosexuals cannot reproduce, we have every reason to believe that it should not have persisted. However, many theories and studies have shown that homosexuality serves other social and reproductive benefits that have helped it to persist for eons. Undoubtedly, as Aristotle said, ‘Nature does nothing without a purpose’, and looking at the prevailing theories and the heap of studies that try to solve this Darwinian Puzzle, I am inclined to believe him.
Vasey et al.(2010) An Adaptive Cognitive Dissociation Between Willingness to Help Kin and Nonkin in Samoan Fa’afafine. Psychological Science, 2010; DOI: 10.1177/0956797609359623
Barron AB and Hare B (2020) Prosociality and a Sociosexual Hypothesis for the Evolution of Same-Sex Attraction in Humans. Frontiers in Psychology. 10:2955. doi: 10.3389/fpsyg.2019.02955