Fecundity of Paternal and Maternal Non-Parental Female Relatives of Homosexual and Heterosexual Men

Abstract

A variety of social, developmental, biological and genetic factors influence sexual orientation in males. Thus, several hypotheses have attempted to explain the sustenance of genetic factors that influence male homosexuality, despite decreased fecundity within the homosexuals. Kin selection, the existence of maternal effects and two forms of balancing selection, sexually antagonistic selection and overdominance, have been proposed as compensatory mechanisms for reduced homosexual fecundity. Here, we suggest that the empirical support for kin selection and maternal effects cannot account for the low universal frequency and stability of the distribution of homosexuals. To identify the responsible compensatory mechanism, we analyzed fecundity in 2,100 European female relatives, i.e., aunts and grandmothers, of either homosexual or heterosexual probands who were matched in terms of age, culture and sampling strategy. Female relatives were chosen to avoid the sampling bias of the fraternal birth order effect, which occurs when indirectly sampling mothers though their homosexual sons. We observed that the maternal aunts and grandmothers of homosexual probands were significantly more fecund compared with the maternal aunts and maternal grandmothers of the heterosexual probands. No difference in fecundity was observed in the paternal female lines (grandmothers or aunts) from either of the two proband groups. Moreover, due to the selective increase in maternal female fecundity, the total female fecundity was significantly higher in homosexual than heterosexual probands, thus compensating for the reduced fecundity of homosexuals. Altogether, these data support an X-linked multi-locus sexually antagonistic hypothesis rather than an autosomal multi-locus overdominance hypothesis.

Evolving research on the evolution of male androphilia

“…If the Samoan population is relatively free of susceptibility to anomalous reproductive patterns compared to Western populations, then the study by indicates that male androphilia is associated with elevated reproductive output in both the maternal and paternal lines. This is not the case for some of the research that has been presented from certain Western populations (e.g., Camperio Ciani et al, 2004;Iemmola & Camperio Ciani, 2009;Camperio Ciani & Pellizzari, 2012;Rahman, Collins, Morrison, Orrells, Cadinouche, Greenfield et al, 2008). On the basis of our Samoan research, it seems reasonable to argue that sexually antagonistic genetic factors may be present on the autosomal chromosomes because androphilic males share genetic factors on these chromosomes with both paternal and maternal relatives.…”

A method yielding comparable estimates of the fraternal birth order and female fecundity effects in male homosexuality

“…Homosexual and heterosexual subjects are compared with regard to the number of offspring produced by various classes of relatives, for example, mothers, grandmothers, sisters, brothers, aunts, uncles and cousins. Several such studies have found evidence of greater fecundity in the relatives of homosexual subjects, but they differ in finding this primarily on the mother's side [6][7][8][41][42][43], on the father's side [36,38] or about equally on both sides [44,45]. These studies might be useful for investigating patterns of transmission of fecundity-promoting genes (e.g.…”

Common Genetic Factors among Sexual Orientation, Gender Nonconformity, and Number of Sex Partners in Female Twins: Implications for the Evolution of Homosexuality

“…Sexually antagonistic mechanisms propose that alleles inducing male homosexuality may increase female fitness but be detrimental (or indifferent) to male fecundity. This mechanism is supported by two mathematical models [14,24] and data showing that female maternal relatives (or both maternal and paternal line relatives) of homosexual men have increased fecundity compared with relatives of heterosexual men [11–13,25]. These models and behavioral data, such as fecundity rates, are only available for hypotheses regarding male homosexuality.…”