New Evidence of Genetic Factors Influencing Sexual Orientation in Men: Female Fecundity Increase in the Maternal Line
- 2.5k Downloads
There is a long-standing debate on the role of genetic factors influencing homosexuality because the presence of these factors contradicts the Darwinian prediction according to which natural selection should progressively eliminate the factors that reduce individual fecundity and fitness. Recently, however, Camperio Ciani, Corna, and Capiluppi (Proceedings of the Royal Society of London, Series B: Biological Sciences, 271, 2217–2221, 2004), comparing the family trees of homosexuals with heterosexuals, reported a significant increase in fecundity in the females related to the homosexual probands from the maternal line but not in those related from the paternal one. This 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, new data have emerged suggesting not only an increase in maternal fecundity but also larger paternal family sizes for homosexuals. These results are partly conflicting and indicate the need for a replication on a wider sample with a larger geographic distribution. 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.
KeywordsMale homosexuality Fecundity Evolution Sexually antagonistic genetic model Birth order
We are grateful to the participants of our study for sharing with us their most personal information. We thank Luca Carniello for his support and help in data analysis. This work was not funded by any public funds and has been supported only by the interests of the authors in the scientific progress of this field.
- Bell, A. P., & Weinberg, M. S. (1978). Homosexuality: A study of diversity among men and women. New York: Simon & Schuster.Google Scholar
- Camperio Ciani, A., Cermelli, P., & Zanzotto, G. (2008). Sexually antagonistic selection in human male homosexuality. Plos One, in press.Google Scholar
- Camperio Ciani, A., Iemmola, F., & Blecher, S. (2008). Bisexuals and not exclusive homosexuals show evidence of the same genetic factors that promote a female fecundity increase on the maternal line. Manuscript submitted for publication.Google Scholar
- Camperio Ciani, A., Iemmola, F., & Lombardi, L. (2008). Male homosexuality partly correlates with an increased androphilia and fecundity in females from maternal line. Manuscript submitted for publication.Google Scholar
- Churchill, W. (1967). Homosexual behavior among males: A cross-cultural and cross-species investigation. Englewood Cliffs, NJ: Prentice-Hall.Google Scholar
- Enquist, M., Ghirlanda, S., Wachtmeister, C. A., & Lundqvist, D. (2002). An ethological theory of attractiveness. In G. Rhodes & L. A. Zebrowitz (Eds.), Advances in visual cognition: Facial attractiveness (pp. 127–151). Westport, CT: Ablex Press.Google Scholar
- Kinsey, A. C., Pomeroy, W. B., & Martin, C. E. (1948). Sexual behavior in the human male. Philadelphia: W.B. Saunders Company.Google Scholar
- Rahman, Q., Collins, A., Morrison, M., Orrells, J. C., Cadinouche, K., Greenfield, S., et al. (2008). Maternal inheritance and familial fecundity factors in male homosexuality. Archives of Sexual Behavior, 37. doi: 10.1007/s10508-007-9191-2.
- Wilson, E. O. (1975). Sociobiology: The new synthesis. Cambridge, MA: Harvard University Press.Google Scholar