How could same-sex sexual behavior persist in evolution even though homosexual individuals have fewer children? A large genome association study now provides a possible explanation: According to this, the same gene variants that favor same-sex sexual behavior could lead to heterosexual individuals having more sexual partners – and thus greater reproductive success.
Around two to ten percent of men and women worldwide report having sex with partners of the same sex, either exclusively or in addition to sex of the opposite sex. Previous studies have indicated that this sexual behavior – which is not synonymous with a particular sexual identity or orientation – is associated with a number of genetic variants and thus has a hereditary component. But how could gene variants that are associated with reduced reproduction persist in evolution?
Opposite influence on evolutionary success
A possible explanation for this apparent paradox has been found by a team led by Brendan Zietsch from the University of Queensland in Australia with the help of a large genome association study. “We have shown that genetic effects associated with same-sex sexual behavior are related to a greater number of sexual partners of the opposite sex in people who have never had a same-sex partner,” the researchers report.
The same gene variants that lead in some individuals to having sex with partners of the same sex and not fathering children could therefore increase evolutionary success in others, as they have more reproductive partners of the opposite sex and thus greater chances of having a lot of offspring. In this case, researchers speak of a so-called antagonistic pleiotropy. This means that a gene variant has several opposing effects – and prevails, provided that the positive effects on reproduction outweigh the negative ones.
Tendency towards more partners among heterosexuals
For their study, the researchers used the data of almost half a million people in the UK and the US who had made their genetic, health and lifestyle information available for the UK Biobank or the American Add Health Study – two large collections of data that many various research questions can be used. Zietsch and his colleagues evaluated which genetic variants are associated with same-sex sexual behavior and how these are related to the number of sexual partners in people who only have sex with partners of the opposite sex.
From the point of view of the researchers, the number of sexual partners in this case is more indicative of possible reproductive success than the number of children. “Modern contraception and in vitro fertilization have decoupled sexual behavior from reproduction in a way that has not been the case in our evolutionary history,” they write. So whoever has many partners today does not necessarily have many offspring. In our evolutionary history, however, the propensity to procreate with many partners may have increased reproductive success. In this way, variants that are also associated with homosexuality could have “paid off” evolutionarily.
However, it is still unclear why gene variants that are associated with same-sex sexual behavior also have a reproductive advantage for heterosexual individuals. “We found that the personality traits willingness to take risks and openness to experience can explain part, but not all, of the genetic correlation,” the researchers report. They also found indications that individuals with the corresponding gene variants may be classified as more physically attractive and thus have greater success with possible reproductive partners.
Strong social influences
However, Zietsch and his colleagues also point out that their study is subject to numerous restrictions. Since the data only included people from the UK and the US, they only represent a small portion of the genetic and behavioral diversity of humanity. In addition, both homosexual behavior and sex with many different partners are heavily regulated by society – many test subjects were born at a time when homosexuality was still considered illegal in their countries. So what is due to genetic and what to social influences is difficult to determine.
The genetic effects are also complex: “Both for same-sex sexual behavior and for the number of sexual partners, these variations are not based on individual genes with a great effect, but on a very large number of genes with very little effect that are distributed over the entire genome are ”, say the researchers. So there is no such thing as a “homosexuality gene”, but also numerous different influencing factors at the genetic level.
Consider the potential for abuse
In an accompanying contribution to the study, which was also published in the journal “Nature Human Behavior”, ethicists working with Julian Savulescu from the University of Oxford point out that research such as this can have unwanted consequences. To this day, people who have sex with partners of the same sex are persecuted in many countries and sometimes threatened with death. Genetic traits that could indicate possible behaviors become a risk in this context.
“It would be conceivable that technologically advanced repressive regimes, in which homosexuality is forbidden, would require genetic testing on embryos and fetuses and have them aborted if the genes indicate a corresponding tendency,” write Savulescu and his colleagues. Even if the current study does not provide a basis for this, the potential for abuse must also be considered in future research. “We need to create a solid ethical framework for the responsible use of this knowledge,” the authors say.
Source: Brendan Zietsch (University of Queensland, Australia) et al., Nature Human Behavior, doi: 10.1038 / s41562-021-01168-8