Darwinian Paradox: How Has Homosexuality Persisted During Evolution?

Same-sex sexual behaviour may seem to present a Darwinian paradox. It provides no obvious reproductive or survival benefit, and yet same-sex sexual behavior is fairly common — around 2-10% of individuals in diverse human societies — and is clearly influenced by genes.

These observations raise the question: why have genes associated with same-sex sexual behavior been maintained over evolutionary time? Given that evolution depends on genes being passed down through the generations via reproduction, how and why were these genes passed down too?

In a new paper published in Nature Human Behaviour, my colleagues and I tested one possible explanation: that the genes associated with same-sex sexual behavior have evolutionarily advantageous effects in people who don’t engage in same-sex sexual behavior.

Specifically, we tested whether those genes are also associated with having more opposite-sex partners, which might therefore confer an evolutionary advantage.

To investigate this, we used genetic data from more than 350,000 people who had participated in the UK Biobank, a huge database of genetic and health information.

These participants reported whether they had ever had a same-sex partner, and also how many opposite-sex partners they had had in their lifetime.

We analyzed the association of millions of individual genetic variants with each of these self-reported variables. For both variables, there were not only one or a few associated genetic variants, but very many, spread throughout the genome. Each had only a tiny effect, but in aggregate, their effects were substantial.

We then showed that the aggregate genetic effects associated with ever having had a same sex partner were also associated — among people who had never had a same-sex partner — with having had more opposite-sex partners.

This result supported our main hypothesis.

Further exploration

We then tried replicating and extending our findings.

First, we successfully replicated the main finding in an independent sample.

Second, we tested whether our results still held true if we used different definitions of same-sex sexual behavior.

For example, did it still hold true if we tightened the definition of same-sex sexual behavior to cover only those individuals with predominantly or exclusively same-sex partners (rather than including anyone who has ever had one)?

Our results remained largely consistent, although statistical confidence was lower due to the smaller sub-samples used.

Third, we tested whether physical attractiveness, risk-taking propensity, and openness to experience might help to account for the main result.

In other words, could genes associated with these variables be associated with both same-sex sexual behavior and with opposite-sex partners in heterosexuals?

In each case, we found evidence supporting a significant role for these variables, but most of the main result remained unexplained.

So we still don’t have a solid theory on exactly how these genes confer an evolutionary advantage. But it might be a complex mix of factors that generally make someone “more attractive” in broad terms.

Simulating evolution

To investigate how the hypothesized evolutionary process might unfold, we also constructed a digital simulation of a population of reproducing individuals over many generations. These simulated individuals had small “genomes” that affected their predispositions for having same-sex partners and opposite-sex reproductive partners.

These simulations showed that, in principle, the kind of effect suggested by our main result can indeed maintain same-sex sexual behavior in the population, even when the trait itself is evolutionarily disadvantageous.

The study involved Western participants – so the next step will be to look at other populations. Credit: Stanley Dai

Crucially, our simulations also showed that if there were no countervailing benefit to genes associated with same-sex sexual behavior, the behavior would likely disappear from the population.

These findings give us intriguing clues about the evolutionary maintenance of same-sex sexual behavior, but there are important caveats too.

An important limitation is that our results are based on modern, Western samples of white participants – we cannot know to what extent our findings apply to other ethnicities or cultures in different places and times. Future studies using more diverse samples may help clarify this.

On a final note, I am aware some people believe it is inappropriate to study sensitive topics such as the genetics and evolution of same-sex sexual behavior. My perspective is that the science of human behavior aims to shine a light on the mysteries of human nature and that this involves understanding the factors that shape our commonalities and our differences.

Were we to avoid studying sexual preference or other such topics due to political sensitivities, we would be leaving these important aspects of normal human diversity in the dark.

Written by Brendan Zietsch, Associate Professor, The University of Queensland.

This article was first published in The Conversation.

Reference: “Genomic evidence consistent with antagonistic pleiotropy may help explain the evolutionary maintenance of same-sex sexual behaviour in humans” by Brendan P. Zietsch, Morgan J. Sidari, Abdel Abdellaoui, Robert Maier, Niklas Långström, Shengru Guo, Gary W. Beecham, Eden R. Martin, Alan R. Sanders and Karin J. H. Verweij, 23 August 2021, Nature Human Behaviour.
DOI: 10.1038/s41562-021-01168-8

Behavioral ScienceEvolutionHomosexualityPopularThe ConversationUniversity of Queensland
Comments ( 10 )
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  • Sf. Ramon Careaga, founder EPEMC

    This is a long (and interesting so thank you) way to go test the hypothesis that homosexuals are more promiscuous on average. It would seem yes but there are some heterosexual superspreaders to borrow a modern term. It would seem Nature favors a small but healthy and gene dominant mixture of the two.
    Shame you had to add that bit about politics at the end. Shows how far down from the Enlightenment the standards of liberty and liberalism have fallen. In an ideal world you could havr said all this flatly or with humor and not worried about your job!
    This is the one time I would encourage a scientist to be sure to include diversity for its own sake. I am sure you will do a follow-up.
    Also findings of male vs female would be nice to see a breakout and also traits of infidelity (especially with 50 or more years of data to see if morals change the genetic behavior).
    We ascribe far too much self control to social conventions to be realistic about human behavior, whatever individual choices are made. I suspect the signals in white populations are stronger on those extreme ends, biologically, and this accounts for much of the supposed collapse of traditional families. As the population slows in birthrate that is.

  • Clyde Spencer

    “Crucially, our simulations also showed that if there were no countervailing benefit to genes associated with same-sex sexual behavior, the behavior would likely disappear from the population.”

    The unstated assumptions are that homosexual behavior is controlled entirely by genes, and that these are otherwise-normal genes that get passed down to offspring.

    The fact that you are considering looking at other human populations suggests that you have considered cultural influence on the behavior. One might call it the “Tik Tok” effect.

    An alternative hypothesis is that, assuming the behavior is controlled entirely by genetics, it is the result of random mutations and/or imperfect replication of genes that select for heterosexuality. That would explain persistence.

    Another hypothesis is that bisexual behavior is more common than homosexual behavior, and therefore there is still adequate opportunity for the hypothetical ‘homo-gene’ to be passed on.

  • Bernard Lobo

    I am not sure this explains the persistence of homosexuality in our species – though it does provide data that heterosexuals are more randy.

    Could it be that homosexuality is preserved/favored in a similar manner to the ‘grandmother’ effect – more adults caring/protecting offspring in the clan?

  • D M

    From Clyde Spencer comment: “The unstated assumptions are that homosexual behavior is controlled entirely by genes, and that these are otherwise-normal genes that get passed down to offspring.”

    I agree with Clyde that this is the assumption of most of the article, even though early on (in the second sentence, actually) it states that the behavior is “influenced by genes.” There is no mention of the fact that this conclusion is debated among geneticists, and others who study such things.

    From article: “These [350,000] participants reported whether they had ever had a same-sex partner, and also how many opposite-sex partners they had had in their lifetime.”

    The other assumption is that the sexual practices of those participants were entirely “at will” … that they always acted, and had the opportunity to act, on their sexual desires. If one wants more sex partners, all one has to do is walk up to a random passerby of the desired gender and tell them: “I’d like to have sex with you. How about it?” I predict you wouldn’t get 100% participation. But 100% participation seems to be what is implied/assumed in the research. Not to mention that 350K is a miniscule sample size relative to global population. I don’t see any way this research could be deemed valid.

  • Douglas Furtek

    I object to the notion “even when the trait itself is evolutionarily disadvantageous” with regard to homosexuality. This implies that homosexuality is analogous to sickle cell anemia in malaria-endemic regions – a balanced lethal genetic system where heterozygotes have a reproductive advantage. Evolution works at many levels, from specks of genetic material all the way to societies. I posit that homosexuality itself can be evolutionary advantageous. Who can deny the contributions homosexuals have made to cultural advancement? So how does this contribute to H-alleles – my unofficial designation – being passed through generations? Because the alleles of all of us are scattered throughout our population. So populations – societies – with a higher frequency of H-alleles have a higher frequency of homosexuals. And a higher frequency of homosexuals leads to a superior society that leads to greater reproductive success for its members. By the way, what makes us US is not only the alleles we possess but also how these alleles are combined and of course environmental effects. So non-identical siblings may possess many of the same H-alleles but differ in sexual preference. (I am aware of the power of epigenetics in trait expression, but I am just trying to make a simple point here.)

    • Clyde Spencer

      “I posit that homosexuality itself can be evolutionary advantageous.”

      Yes, homosexuals have made significant contributions to society. Someone like Alan Turing would be a good example. However, genes are carried by individuals, not by society in general. Therefore, everyone benefits from the cultural contributions of everyone else, and homosexuals don’t receive special benefits to make up for a lower reproductive rate. Additionally, the contributions aren’t continuous, but periodic, and probably less frequently than a single generation. Therefore, the benefits would be periodic and there would be many individuals that would die before another benefit showed up. It would seem to me that genetic control of sexual preference would be, at best, a benign trait for the individuals carrying the gene(s).

      The question was why the genes for homosexuality haven’t become extinct. Unless the cultural contributions from homosexuals tended to convey survival/reproductive advantages primarily to homosexuals, the general contributions don’t compensate for a lower frequency of being passed on.

      It would seem to me that the more obvious explanation is that the gene pool is continually being refreshed with mutations, or that genes are of less importance than cultural imprinting. The latter is politically incorrect. But, then, sometimes reality is unpopular.

  • The Stone

    I would have thought it quite obvious that nature is reserving the homosexual gene to eliminate “bad” genes. (No reproduction – no passing on of bad genes)…..The Darwin effect in operation!

    • Clyde Spencer

      An interesting suggestion. However, one would have to posit an explanation of how the human organism would detect and evaluate a congenital defect that was dangerous enough long-term to warrant triggering a mechanism that would suppress reproduction. Also, one would have to demonstrate how the gene(s) favoring homosexuality would be activated. Commonly, serious congenital defects lead to early death of individual organisms.

      Actually, the way that Darwinian Evolution works is NOT by the individual organism making changes, but by a ‘decision’ made through the environment, which eliminates sub-optimal individuals through predation or starvation. Those organisms that die early don’t have an opportunity to pass on their genes. Death is the engine of evolution.

  • Tim Stone

    I would have thought it quite obvious that nature is reserving the homosexual gene to eliminate “bad” genes. (No reproduction – no passing on of bad genes)…..The Darwin effect in operation!

  • Tommy B

    “I would have thought it quite obvious that nature is reserving the homosexual gene to eliminate “bad” genes. (No reproduction – no passing on of bad genes)…..The Darwin effect in operation!”

    Why hasn’t the Darwin effect gotten rid of the cancer genes or dwarfism genes or down syndrome genes or hereditary heart defects? For some reason you think nature is less concerned with the genes that lower the chances of survival & that make humans weaker because it’s focus is squarely on eliminating sodomy?

    Sorry to have to be the one to say it to you but I don’t think evolution is as worried about “the gays” as you think! In fact, I would bet heavily that you’re projecting your insecurities and fears into your analysis. Like evolution is waiting to eliminate the genes that cause prostate cancer until it’s sure there’s no men left that might proposition you to a night of poppers & KY? You can’t be serious!!