Genetic variants that make someone more likely to have sex with people of the same sex also exist in heterosexuals who are more likely to find partners, a new study shows.
If the research is 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, as the associated genetic variants also help heterosexuals to reproduce more easily.
The paradox and the data supporting the idea are described in an article that just appeared in the 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 examine the broadly defined genetic basis of homosexuality that stated at least once in their life one To have had a relationship with someone of the same sex.
Studies like this have grown over the past few decades because, strictly biologically, homosexual behavior is somewhat puzzling.
Homosexuality is present in all human cultures and at all times, even when it is brutally suppressed. It has also been identified in a wide variety of animal species – mammals, birds, fish, and even insects. In addition, it is possible to infer a genetic component that affects its manifestation.
Research on identical twins has shown that the likelihood of the two being homosexual is much higher than homosexuality if the twins are not identical or between siblings who were not conceived at the same time. Since identical twins carry almost the same DNA, it suggests that genetic factors influence this propensity.
The stricter logic of natural selection, which determines the evolution of living things, indicates, however, that a characteristic persists in a species only if it favors or at least does not hinder the reproductive success of individuals. Since homosexual behaviors do not directly favor reproduction, a simplified view would suggest that they would disappear from the population over time.
Of course that doesn’t happen. On the contrary: everything indicates that homosexuality is always present in the human population, in a minority but with a significant proportion that ranges between 2% and 10% of people. This finding led to a number of hypotheses suggesting an indirect positive effect of homosexual behavior or the genetic variants associated with it on reproductive success.
New research by Zietsch and his colleagues reinforces this type of hypothesis. To start with, they were able to glean a huge amount of information using public genome databases of nearly 400,000 people in the UK and US.
Each of these people’s DNA has been mapped into millions of variants known as SNPs (pronounced “snips”). SNPs correspond to the exchange of a single chemical “letter” in DNA (the human genome has 3 billion pairs of them). They can be associated with the most varied of properties of the organism, although they are rarely the only cause.
Since the same volunteers who donated their DNA also answered questionnaires about their behavior and personality anonymously, it was possible to compare the data from the SNPs with these other variables. The result is a kind of association map between mutations and the behavior one wants to study.
In the new article, the researchers compared the SNPs related to homosexual behavior with those related to the number of sexual partners an exclusively heterosexual person had during their lifetime. They chose to use this second measure as a possible indicator of reproductive success rather than something more obvious like the number of children, as modern technology is able to control this variable through contraceptive methods.
After all the calculations and statistical analysis, it turned out that there is a positive correlation between the SNPs of those who have had homosexual relationships all their lives and the SNPs of heterosexual men with many partners. However, the correlation is not 100%. It is actually 31% for men and 73% for women and 44% for both sexes. Transsexuals were not included in the sample.
The research team tried to explore further relationships between genes, sexual behavior and personal traits. There seems to be a statistical relationship between the two groups of SNPs and personality traits such as openness to new experiences and willingness to take risks.
Given the question of how research of this nature can be manipulated for political or discriminatory purposes, the journal Nature Human Behavior has created an editorial and a number of accompanying texts to verify the impact of the results, a very rare measure in publications of this nature.
In one of these articles, researchers led by Julian Savulescu of the Center for Ethics and Humanides at Oxford University point out that a huge number of genes (probably in the thousands) that work together with very small individual effects make up the genetic component of homosexuality. This means that there is no justification for using genomic data to, for example, alter the sexual tendencies of an unborn child – the effects would most likely be unpredictable.
Furthermore, they say, ethical issues should not be tied to a scientific landscape that may eventually change, while the idea that innate factors pervade homosexuality seems to increase acceptance. “The civil rights of LGBTQIA + people cannot depend on the latest scientific data or theories about human sexuality,” they write.