New evidences of genetic factors influencing sexual orientation in Men: Female fecundity increase in the maternal line.

There is a long-standing debate on the role of genetic factors influencing homosexuality. Recently, CamperioCiani, Corna, and Capiluppi (2004) suggested that genetic factors that are partly linked to the X-chromosome and that influence homosexual orientation in males are not selected against because they increase fecundity in female carriers, thus offering a solution to the Darwinian paradox and an explanation of why natural selection does not progressively eliminate homosexuals. Since then, newdata have emerged suggesting not only an increase inmaternal fecundity but also larger paternal family sizes for homosexuals. This study examined the family trees of 250 male probands, of which 152 were homosexuals. The results confirmed the study of Camperio Ciani et al. (2004).We observed a significant fecundity increase even in primiparous mothers, which was not evident in the previous study. No evidence of increased paternal fecundity was found; thus, our data confirmed a sexually antagonistic inheritance partly linked to the X-chromosome that promotes fecundity in females and a homosexual sexual orientation in males.