What’s the evolutionary explanation for homosexuality? Ironically, genes that help people make more babies may be involved

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On the face of it, homosexual behavior and Darwin’s theory of evolution don’t match. Genes have to be passed to offspring otherwise they die out, hence any genes that will make an animal more likely to engage in same-sex mating ought to quickly be eliminated from the population. Yet same-sex behavior is quite prevalent among human populations across the globe.

In a new study published in Nature Human Behavior, researchers led by Brendan Zietsch, Associate Professor at the School of Psychology at the University of Queensland, have found compelling clues in our genomes that may resolve this paradox. According to the findings, the same genes that may drive homosexuality in some individuals may enhance the reproductive success of heterosexual individuals.

In other words, genes that offer evolutionary advantageous effects to some people may result in homosexual offspring in subsequent generations as an unintended effect.

For their study, the researchers analyzed the genetic effects associated with same-sex sexual behavior in a dataset of 477,522 people from the UK and the US that contains a wealth of genetic and health information. They performed the same analysis for opposite-sex sexual behavior in a sample of 358,426 people from the same countries.

Participants in the opposite-sex dataset reported how many sexual partners they had in their lifetime. The number of opposite-sex sexual partners is an indicator of mating success, which during evolution would have led to more children

The researchers scoured millions of individual genetic variants that were associated with two variables: whether people ever had a same-sex partner and how many partners they had in their lifetime.

Each variable had many associated genetic variants spread through the genome. And although each of these variants had a tiny effect, in aggregate their effects were substantial.

Ultimately, this analysis showed that the genetic effects associated with ever having had a same-sex partner were also associated with having had more opposite-sex partners among people who never engaged in same-sex behavior.

In order to verify the confidence of their results, the researchers replicated their findings by narrowing the study conditions. Specifically, they performed the same analysis on a sample of individuals with predominantly or exclusively same-sex partners. The results remained largely consistent.

Lastly, the researchers tested whether physical attractiveness, risk-taking propensity, and openness to experience may also influence the results.

“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 results 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,” explained Zietsch in an article.

These findings were also validated by an evolutionary computer simulation that crunched the numbers and found that in the lack of any countervailing benefits to genes associated with same-sex sexual behavior, these genes disappear from the gene pool.

Of course, this isn’t the last word on the matter. Important limitations include samples involving Western white participants which may not be representative of the general population. Secondly, the number of opposite-sex sexual partners reported in individuals today may not necessarily reflect the same reproductive advantage in our evolutionary past.

Even so, this hypothesis seems like the most solid explanation for same-sex behavior in humans proposed thus far.

“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,” Zietsch wrote. “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.”

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