Genetic variants that increase the likelihood that someone will have sex with people of the same sex are also present in heterosexuals who find partners more easily, a new study points out.
If the research conclusions are correct, a paradoxical relationship between homosexuality and human reproduction will be demonstrated. According to the analysis, homosexual behaviors would continue to manifest themselves in the population because the genetic variants associated with them also help heterosexuals to reproduce more.
The paradox and the data that support the idea are described in an article that has just come out in the specialized journal Nature Human Behavior. The work, coordinated by Brendan Zietsch of the University of Queensland in Australia, is a continuation of a 2019 study that had used some of the same databases to investigate the broadly defined genetic bases of homosexuality—were included in the sample DNA variants of people who reported having had a relationship with someone of the same sex at least once in their lives.
Studies like these have grown in recent decades because, when viewed from a strictly biological point of view, homosexual behavior is somewhat enigmatic.
Homosexuality is present in all human cultures and at all times, even when it is brutally repressed. It has also been identified in a wide range of animal species—mammals, birds, fish and even insects. Furthermore, it is possible to infer that there is a genetic component that influences its manifestation.
Research on identical (identical) twins has shown that the probability of the two being homosexual is much higher than that of being homosexual when the twins are not identical or between siblings who were not conceived at the same time. Since identical twins carry almost the same DNA, this suggests that there are genetic factors influencing this propensity.
However, the stricter logic of natural selection, which governs the evolution of living beings, indicates that a characteristic only persists in a species when it favors the reproductive success of individuals or, at the very least, does not hinder it. As homosexual behaviors do not directly favor reproduction, a more simplistic view would suggest that, over time, they would disappear from the population.
Of course, that’s not what happens. On the contrary: everything indicates that homosexuality is always present in human populations, in a minority but significant proportion, which is between 2% and 10% of people. This finding led to the emergence of a series of hypotheses that propose an indirect positive effect of homosexual behavior, or the genetic variants associated with it, on the success of reproduction.
The new research by Zietsch and his colleagues adds weight to this type of hypothesis. For starters, they were able to mine a very large amount of information, using public genomic databases on nearly 400,000 people in the UK and the US.
The DNA of each of these people has been mapped to millions of variants known as SNPs (pronounced “snips”). SNPs correspond to exchanges of a single chemical “letter” of DNA (the human genome has 3 billion pairs of them). They can be associated with the most different characteristics of the organism, although they are rarely the only cause of them.
As the same volunteers who donated their DNA also anonymously answered questionnaires about their behavior and personality, it was possible to cross-reference the data from the SNPs with these other variables. The result is a kind of map of associations between mutations and the behavior that one wants to study.
In the new article, researchers compared the SNPs associated with homosexual behavior with those linked to the number of sexual partners an exclusively heterosexual person had over a lifetime. They decided to use this second measure as a possible indicator of reproductive success, rather than something more obvious, such as number of children, since modern technology is able to control this variable through contraceptive methods.
After all the calculations and statistical analysis, the result was that there is a positive correlation between the SNPs of those who had homosexual relationships throughout their lives and the SNPs of straights with many partners. The correlation is not 100%, however. It actually corresponds to 31% for men and 73% for women, or 44% for both sexes. Trans people were not included in the sample.
The research team tried to explore further correlations between genes, sexual behavior and personal characteristics. There seems to be some statistical link between the two groups of SNPs and personality traits such as openness to new experiences and willingness to take risks.
Considering how research of this type can be manipulated for political or discriminatory ends, the journal Nature Human Behavior has prepared an editorial and a series of supporting texts to scrutinize the implications of the findings, a very rare measure in publications of this type.
In one of these articles, researchers led by Julian Savulescu, from the Center for Ethics and Humanides at Oxford University, point out that a huge amount of genes (probably in the thousands), acting together with very small individual effects, seems to explain the genetic component of homosexuality. This means that there is no justification for using genomic data to modify the sexual propensities of an unborn child, for example—the effects would most likely be unpredictable.
Furthermore, they say, while the idea that innate factors permeate homosexuality seems to increase acceptance of it, ethical issues should not be tied to a scientific landscape that may eventually change. “The civil rights of LGBTQIA+ people cannot depend on the latest scientific data or theories about human sexuality,” they write.