Genetic Factors Increase Fecundity in Female Maternal Relatives of Bisexual Men as in Homosexuals Andrea Camperio Ciani, PhD,* Francesca Iemmola, MS,* and Stan R. Blecher, MD, FCCMG † *University of Padova, General Psychology, Padova, Italy; † University of Guelph, Molecular Biology and Genetics, Guelph, ON, Canada DOI: 10.1111/j.1743-6109.2008.00944.x ABSTRACT Introduction. Recent studies on male homosexuals showed increased fecundity of maternal female relatives of homosexual probands, compared to those of heterosexual controls. We have suggested that these data could be explained by the transmission, in the maternal line, of an X-linked genetic factor that promotes androphilic behavior in females and homosexuality in males. Aim. Our original studies were on relatives of male subjects who declared themselves to be exclusively homosexual. However, the relationship between homosexuality and bisexuality, including the possibility of shared genetic factors, is complex and largely unexplored. To cast light on this issue, in the present study we examined whether relatives of bisexuals show the same indirect fitness advantage as previously demonstrated for homosexuals. Main Outcome Measures. Subjects completed a questionnaire on their sexual orientation, sexual behavior, and their own and their relatives’ fecundity. Methods. We studied 239 male subjects, comprising 88 who were exclusively or almost exclusively heterosexual (pooled to comprise our “heterosexual” group), 86 who were bisexual, and 65 exclusively or almost exclusively homosexual individuals (pooled in our “homosexual” group). Bisexuals were here defined on the basis of self- identification, lifetime sexual behavior, marital status, and fecundity. Results. We show that fecundity of female relatives of the maternal line does not differ between bisexuals and homosexuals. As in the previous study on homosexuals, mothers of bisexuals show significantly higher fecundity, as do females in the maternal line (cumulated fecundity of mothers, maternal grandparents, and maternal aunts), compared to the corresponding relatives of heterosexual controls.This study also shows that both bisexuals and homosexuals were more frequently second and third born. However, only homosexuals had an excess of older male siblings, compared to heterosexuals. Conclusions. We present evidence of an X-chromosomal genetic factor that is associated with bisexuality in men and promotes fecundity in female carriers. Camperio Ciani A, Iemmola F, and Blecher SR. Genetic factors increase fecundity in female maternal relatives of bisexual men as in homosexuals. J Sex Med 2009;6:449–455. Key Words. Male Homosexuality; Male Bisexuality; Fecundity; Androphilia; Birth Order; X-Chromosomal Genetic Factor Introduction D ifferent lines of research have suggested the existence of genetic factors in the causation of male homosexuality [1–4]. Recent studies [5,6] have shown that females in the maternal line of homosexual males are significantly more fecund than females in the maternal line of heterosexual males, producing almost one third more offspring. No significant differences emerged when paternal relatives of homosexuals and heterosexuals were compared. We proposed that these results could be accounted for by (an) X-linked gene(s) that pro- mote(s) male homosexuality and increase(s) female fecundity [5]. More recently we have suggested that the hypothetical X-chromosomal genetic factor(s) may act by promoting sexual attraction to males (androphilia) in both male and female 449 © 2008 International Society for Sexual Medicine J Sex Med 2009;6:449–455