A new study, published in the journal Evolution and Human Behavior, examining fertility rates among homosexual and heterosexual individuals in Czechia and Slovakia, has found no support for a widely debated evolutionary theory. The research, which analysed data from thousands of participants, sought to test the sexually antagonistic genes hypothesis (SAGH), a theory proposing that genetic factors linked to homosexuality are preserved through increased fertility in female maternal relatives. However, the findings did not confirm this idea.
The study involved an extensive online survey of gay and straight men and women, assessing their reproductive rates alongside those of their family members. The results indicated that both gay men and lesbian women reported significantly lower fertility compared to their heterosexual counterparts. While previous research suggested that the mothers of gay men might exhibit higher fertility as a compensatory mechanism, this study found no such evidence. Instead, it observed a slight increase in the fertility of lesbian women’s mothers but no significant differences among other family members.
One of the key findings was the stark contrast in the number of children reported by participants. Heterosexual men had nearly seven times more children than gay men, while heterosexual women also had a considerably higher fertility rate than lesbian women. This pattern, consistent with previous studies, highlights the lower reproductive rates of homosexual individuals, a fundamental element in evolutionary theories on the persistence of same-sex attraction.
The researchers also examined fertility differences in extended family members. While some studies have argued that male homosexuality may be linked to genetic factors carried on the X chromosome, leading to increased fertility among maternal relatives, this study found no clear evidence to support that claim. The fertility rates of the maternal and paternal relatives of gay men did not show statistically significant differences, contradicting the predictions of the X-linked version of the SAGH. In contrast, the study found slightly higher fertility in the paternal grandparents of gay men, a result that does not align with the expectations of an X-linked genetic mechanism.
The findings raise questions about the validity of the sexually antagonistic genes hypothesis as a universal explanation for the persistence of same-sex attraction in human populations. Some alternative explanations include the antagonistic pleiotropy hypothesis, which suggests that genetic traits associated with homosexuality may confer reproductive advantages in other contexts. However, the study’s results do not provide strong support for this theory either.
One possible reason for the lack of evidence supporting the SAGH is the impact of modern societal factors. The researchers noted that widespread contraceptive use and declining fertility rates in contemporary societies may obscure ancestral reproductive patterns. In historical contexts, genetic factors influencing same-sex attraction might have been counterbalanced by increased fertility in certain family members, but such effects may no longer be observable in modern populations.
Despite its comprehensive approach, the study has limitations. The data on family fertility relied on self-reports from participants rather than direct verification, introducing potential recall biases. Additionally, given that many participants were in their early thirties, some may not have completed their reproductive years, which could affect the accuracy of fertility estimates.
