Increased birth rank of homosexual males: disentangling the older brother effect and sexual antagonism hypothesis

Male homosexual orientation remains a Darwinian paradox, as there is no consensus on its evolutionary determinants. One intriguing feature of homosexual men is their higher male birth rank compared to heterosexual men. This can be explained by two non-exclusive mechanisms: an antagonistic effect (AE), implying that more fertile women have a higher chance of having a homosexual son, or an older brother effect (OBE), where each additional older brother increases the chances for a male embryo to develop a homosexual orientation due to an immunoreactivity process. However, there is no consensus on whether both OBE and AE are present in human populations, or if only one of these mechanisms is at play and its effect mimicking the signature of the other mechanism. An additional older sister effect (OSE) has also recently been proposed. To clarify this situation, we developed theoretical and statistical tools to study OBE and AE independently or in combination, taking into account all known sampling biases. These tools were applied on new individual data, and on various available published data (two individual datasets, all relevant aggregated data, and twin data). Support for OBE was apparent in aggregated data, with the OBE increasing linearly with fertility. The OBE was also supported in two individual datasets. An OSE seems to result from a sampling bias in presence of OBE, and is likely to be artefactual. AE was not supported, either in individual datasets, including the analysis of the extended maternal family, nor in twin data. The evolutionary implications of these findings are discussed. ### Competing Interest Statement The authors have declared no competing interest.