Study that links bisexuality genes to the tendency to take risks generates controversy – 01/05/2024 – Science

A new study on the genetic components of same-sex attraction suggests that bisexual men may have greater than average reproductive success — that is, the ability to father children. This would lead to this sexual orientation being favored by natural selection and remaining in the population over time.

The research, which has just been published in the specialized journal Science Advances, adds to a long series of DNA analyzes that, since 1990, have sought to understand the biological aspects of human sexual variability.

The new conclusions by researchers Siliang Song and Jianzhi Zhang, from the University of Michigan in Ann Arbor (USA), are sparking controversy because the duo identified an association between genes linked to bisexuality and genes linked to a supposed greater tendency to take risks. It is this combination, and not just the bisexual orientation, that would be behind the increased reproductive success.

“This is an empirical observation. We are not making a moral judgment about risk-taking, and we believe it has pros and cons, like almost any other trait. In part, this is a biological issue, and we should [tentar] understand it”, declared Zhang in an interview on the Science group website, which is also responsible for publishing the study.

Evolutionary biology used to see non-heterosexual behavior as something somewhat enigmatic because living beings, strictly speaking, would be reproduction machines. The main mechanism of evolution, natural selection, establishes that individuals with genetic characteristics that give them greater reproductive capacity, leaving more descendants, end up spreading these characteristics throughout the population over generations.

Thus, from a simplistic point of view, it would be expected that forms of sexuality without reproductive potential, involving individuals of the same sex, would end up quickly eliminated by natural selection. However, this form of sexuality appears in countless species of animals, from insects to primates like us. Furthermore, it is present in all human societies and at all times, in a proportion that usually ranges between 2% and 10% of individuals. Finally, identical twins, whose DNA is almost 100% the same, have a higher than average chance of not being heterosexual if their siblings are not heterosexual either, indicating the presence of a genetic component to the behavior.

These findings have led several researchers to postulate that there could be a counterintuitive link between a genetic inheritance that favors non-straight behavior in certain contexts, on the one hand, and reproductive success, on the other.

One way to achieve this could be so-called kin selection: individuals who prefer same-sex partners could somehow favor the reproduction of their close relatives. This could happen in a variety of ways, such as extra help supporting the family or taking care of the children of straight relatives, for example.

Close relatives often carry the same versions of a wide range of genes. Thus, even if non-heterosexuals reproduce less, their help to the family would increase the chance that the gene versions typical of that group will multiply more in the population, so that, in subsequent generations, non-heterosexual behavior influenced by these genes appear again.

Another possibility is what geneticists call pleiotropy, which corresponds to the influence of the same DNA variant (or a set of variants) on multiple characteristics. This happens, for example, in the domestication of animals. Giving preference to the reproduction of the most docile individuals also has effects on a series of other characteristics of the animals, such as the color of the coat and the shape of the ears and snouts.

In short, Zhang and Song claim to have identified a case of pleiotropy involving precisely genetic contributions to risk-taking behavior that would also be associated with bisexual behavior. They reached this conclusion after sifting through a huge public database, the UK Biobank, in which around 500,000 Britons of European origin are registered, with genetic information and also on various aspects of their health and behavior, including sexuality.

Basically, what they did was sift through the Biobank’s DNA data and its association with different types of sexual behavior, divided into three large groups: people who had only heterosexual relationships throughout their lives, those who had partners of both sexes and those who only had same-sex partners.

In general, they found, first, that self-declared straight people had more children (1.86 children per person, on average, versus 1.32 children for each bisexual and 0.47 for each homosexual). The data does not specify the details of these averages, but it is possible to assume that even people who declared themselves exclusively homosexual had biological children through assisted reproduction methods.

This average, however, hides the fact that there is a statistically significant correlation between being a bisexual male and having more children than expected (when bisexual women are added, the correlation is negative, as they have fewer children, which nullifies the effect for both sexes). And this appears to be mediated or influenced by the presence of genetic variants associated with taking more risks (also according to participants’ self-declaration); in bisexual men without these variants, the association with greater reproductive success disappears.

How exactly could this happen? In theory, the tendency to take more risks could correspond to a more intense search for sexual partners in both sexes, or greater behavioral disinhibition, elements that could result in a greater capacity to have children. But it is not simple to clarify the exact mechanism.

This uncertainty, as well as the possible stigma associating bisexuality with a tendency to take risks, has led to much of the criticism of the University of Michigan duo’s work.

For Steven Reilly, a geneticist at the Yale University School of Medicine (USA), pointed out in an interview with Science, for example, that the association with “taking risks” comes from answers to a single question in the Biobank questionnaires — “are you a person who likes to take risks?”, without detailing which ones—, so that it is not clear what this behavior could mean.

Robbee Wedow, an expert in sociogenomics at Purdue University, also in the USA, told the publication that “weak analyzes like this can be used to create a narrative about bisexuality and evolution that is not true” and fuel prejudice.

At the end of the article, the authors of the new study highlighted that this was not their intention, in a very unusual paragraph in publications of this type. “We want to make it clear that our results predominantly contribute to the diversity, richness and better understanding of human sexuality. Their intention is in no way to suggest or support discrimination made on the basis of sexual behavior,” the pair wrote.