The relatives of gay individuals do not have more offspring. The old evolutionary enigma unresolved?
– by Jakub Fořt & Jan Havlíček
Why do some individuals direct their sexual behavior exclusively toward individuals of the same sex? This is often viewed as an “evolutionary conundrum” or “evolutionary enigma” because same-sex attraction does not lead to reproduction – and yet it persists. Robust evidence from all around the world shows that individuals attracted only to same-sex persons have less offspring than other-sex attracted individuals. In fact, in some samples their reproductive outcome is close to zero1,2. It is also known that sexual orientation is partly determined by the genetic makeup, with the contribution of heritability estimated at around 30%. Genome-wide association studies indicate that the number of genes involved is large and each has just a small effect. In short, there is no single gene responsible for sexual orientation3. The question is then: how come homosexuality-associated alleles are maintained in human population if they decrease the fitness of gay individuals?
In recent decades, scientists have proposed several evolutionary theories to explain this apparent evolutionary paradox. The sexually antagonistic genes hypothesis proposed by Camperio Ciani and colleagues4 is based on the principle of sexually antagonistic selection. In other words, they claim that the same alleles that lower fitness in one sex may confer a reproductive advantage to the other sex. Specifically, their hypothesis predicts that the same alleles that cause male homosexual orientation increase the fertility of gay men’s heterosexual female relatives. In their paper, they recorded higher fertility rates in the mothers, maternal aunts, and maternal grandparents of Italian gay men (compared to relatives of heterosexual men). These results were replicated, or partly replicated, not only by the same team but also by other researchers and in other populations including, for instance, the islanders from Samoa5.
In spite of the promising nature of these early findings, more recent studies did not find evidence in support of this hypothesis6. Further doubts emerged when meta-analyses7 revealed no higher reproductive outcomes in the mothers of gay, as opposed to heterosexual, men. A re-examination of the earlier studies had moreover shown that some of the previous findings may have been an artifact of an uncontrolled variable. In particular, they may have been due to the fact that, as compelling evidence shows, gay men have more older brothers than heterosexual men do6. This is called the fraternal birth order effect. When the early data that seemingly confirmed a higher fertility of the mothers of Samoan androphilic (male-attracted) males were reanalyzed while controlling for the fraternal birth order effect, it turned out that the mothers of androphilic males have fertility rates comparable with those of heterosexual men’s mothers8. In other words, part of the original evidence that seemed to support the sexually antagonistic genes hypothesis was just an artifact of androphilic males having more older brothers.
The principle of the sexually antagonistic genes hypothesis can also be applied to lesbian women and their male relatives. Women have been too often overlooked by research into sexual orientation, its biological underpinnings, and possible ultimate causes. As a result, the abovementioned fraternal birth order effect has been mostly investigated only in the context of the biological development of male sexual orientation. New studies that include female samples have nevertheless shown that the effect is present not only in gay men but also in lesbian women. Still, few studies so far have examined the pattern of familial fertility in lesbian women: the possible role of sexually antagonistic selection in maintaining female homosexuality is thus underexplored. In our study, we have therefore included a female sample.
In a recent issue of the Evolution of Human Behavior, we have published a study where we tested the hypothesis in a new Central European sample. In an online survey, we asked our participants how many biological offspring, siblings, maternal and paternal cousins, aunts, and uncles they have. We have collected data from 693 gay men, 265 lesbian women, 843 heterosexual men, and 331 heterosexual women. After controlling for sociodemographic variables, we have confirmed that gay men (M = 0.11) had less offspring than heterosexual men did (M = 0.72, OR = 0.17) and the same held of lesbian women (M = 0.11) compared to heterosexual women (M = 0.74, OR = 0.26). This was not surprising: it is an essential assumption of all evolutionary theories of exclusive homosexuality.
In men, we recorded a slightly (but significantly) higher fertility in gay men’s paternal grandparents (OR = 1.09) but no other significant fertility differences. Most notably, neither the mothers nor the maternal aunts of gay men had higher fertility than those of heterosexual men. In women, we found a slightly higher fertility only in lesbian women’s mothers (OR = 1.09) – nonetheless, this effect disappeared when the fraternal birth order effect (present in our sample for both men and women) was controlled for.
In line with other recent studies from WEIRD societies, our findings thus do not support the sexually antagonistic genes hypothesis. One could object that in Western societies, sexual behavior had, in the course of the 20th century, become largely decoupled from reproduction. That is why it is important to note that some results from high-fertility non-WEIRD societies likewise fail to support the sexually antagonistic genes hypothesis8.
Does it mean the hypothesis should be abandoned? Our answer is cautiously negative. In particular, we believe that focus on the reproductive outcomes of homosexuality-associated alleles in the direct kin of homosexual individuals is not the optimal way of testing the sexually antagonistic effects on the genetic level. Based on genetic data, Zietsch and colleagues9 found that alleles associated with homosexuality actually do confer reproductive advantages. Specifically, women who have alleles that predispose men to homosexual behavior tend to have more opposite-sex sexual partners and, vice versa, men who have alleles that predispose women to homosexual behavior tend to have more opposite-sex sexual partners and more children.
From an evolutionary viewpoint, exclusive homosexuality remains an unresolved question. Our results, which are based on fertility data of the direct kin of homosexual and heterosexual individuals, are not consistent with the sexually antagonistic genes hypothesis but, based on genetic data, we would like to argue that sexual antagonism plays a role in maintaining homosexuality in human population, probably jointly with some other factors that have been proposed. Perhaps exclusive homosexuality is just an extreme case of an otherwise adaptive phenomenon of partial same-sex attraction and bisexuality, that is, behaviors whose advantageousness has been demonstrated in non-human animals10.
Read the original article: Fořt J., Valentova, J., Havlíček, J. (2025). An evolutionary perspective on homosexuality: testing the sexually antagonistic genes hypothesis through familial fertility analysis. Evolution & Human Behavior, 46(1), 106649.
