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.
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Bailey, J. M., & Pillard, R. C. (1991). A genetic study of male sexual orientation. Archives of General Psychiatry, 48, 1089–1096.
Bailey, J. M., Pillard, R. C., Dawood, K., Miller, M. B., Farrer, L. A., Tivedi, S., et al. (1999). A family history study of male sexual orientation using three independent samples. Behavior Genetics, 29, 79–86.
Bell, A. P., & Weinberg, M. S. (1978). Homosexuality: A study of diversity among men and women. New York: Simon & Schuster.
Blanchard, R. (1997). Birth order and sibling sex ratio in homosexual versus heterosexual males and females. Annual Review of Sex Research, 8, 27–67.
Blanchard, R. (2004). Quantitative and theoretical analyses of the relation between older brothers and homosexuality in men. Journal of Theoretical Biology, 230, 173–187.
Blanchard, R., & Klassen, P. (1997). H-Y antigen and homosexuality in men. Journal of Theoretical Biology, 185, 373–378.
Blanchard, R., & Lippa, R. A. (2007). Birth order, sibling sex ratio, handedness, and sexual orientation of male and female participants in a BBC Internet research project. Archives of Sexual Behavior, 36, 163–176.
Bobrow, D., & Bailey, J. M. (2001). Is male homosexuality maintained via kin selection? Evolution and Human Behavior, 22, 361–368.
Bogaert, A. F. (2006). Biological versus nonbiological older brothers and men’s sexual orientation. Proceedings of the National Academy of Sciences, 103, 10771–10774.
Camperio Ciani, A., Cermelli, P., & Zanzotto, G. (2008). Sexually antagonistic selection in human male homosexuality. Plos One, in press.
Camperio Ciani, A., Corna, F., & Capiluppi, C. (2004). Evidence for maternally inherited factors favouring male homosexuality and promoting female fecundity. Proceedings of the Royal Society of London, Series B: Biological Sciences, 271, 2217–2221.
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.
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.
Churchill, W. (1967). Homosexual behavior among males: A cross-cultural and cross-species investigation. Englewood Cliffs, NJ: Prentice-Hall.
DuPree, M. G., Mustanski, B. S., Bocklandt, S., Nievergelt, C., & Hamer, D. H. (2004). A candidate gene study of CYP19 (aromatase) and male sexual orientation. Behavior Genetics, 34, 243–250.
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.
Gavrilets, S., & Rice, W. R. (2006). Genetic models of homosexuality: Generating testable predictions. Proceedings of the Royal Society of London, Series B: Biological Sciences, 273, 3031–3038.
Hamer, D. H., Hu, S., Magnuson, V. L., Hu, N., & Pattattucci, A. M. (1993). A linkage between DNA markers on the X chromosome and male sexual orientation. Science, 261, 321–327.
Hu, S., Pattatucci, A. M., Patterson, C., Li, L., Fulker, D. W., Cherny, S. S., et al. (1995). Linkage between sexual orientation and chromosome Xq28 in males but not in females. Nature Genetics, 11, 248–256.
King, M., Green, J., Osborn, D. P. J., Arkell, J., Hetherton, J., & Pereira, E. (2005). Family size in white gay and heterosexual men. Archives of Sexual Behavior, 34, 117–122.
Kinsey, A. C., Pomeroy, W. B., & Martin, C. E. (1948). Sexual behavior in the human male. Philadelphia: W.B. Saunders Company.
Moran, P. A. P. (1972). Familial effects in schizophrenia and homosexuality. Australian and New Zealand Journal of Psychiatry, 6, 116–119.
Muscarella, F. (2000). The evolution of homoerotic behavior in humans. Journal of Homosexuality, 40, 51–77.
Mustansky, B. S., DuPree, M. G., Nievergelt, C. M., Bocklandt, S., Schork, N. J., & Hamer, D. H. (2005). A genomewide scan of male sexual orientation. Human Genetics, 116, 272–278.
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.
Rahman, Q., & Hull, M. S. (2005). An empirical test of the kin selection hypothesis for male homosexuality. Archives of Sexual Behavior, 34, 461–467.
Rice, G., Anderson, C., Risch, N., & Ebers, G. (1999). Male homosexuality: Absence of linkage to microsatellite markers at Xq28. Science, 284, 665–667.
Savolainen, V., & Lehmann, L. (2007). Genetics and bisexuality. Nature, 445, 158–159.
Spreen, M., & Marius, H. (1992). Rare populations, hidden populations and link-tracing designs: What and why? Bulletin Methodologie Sociologique, 6, 34–58.
Vasey, P. L. (1995). Homosexual behavior in primates: A review of evidence and theory. International Journal of Primatology, 16, 173–204.
Vasey, P. L., Pocock, D., & VanderLaan, D. P. (2007). Kin selection and male androphilia in Samoan fa’afafine. Evolution and Human Behavior, 28, 159–167.
Vasey, P. L., & VanderLaan, D. P. (2007). Birth order and male androphilia in Samoan fa’afafine. Proceedings of the Royal Society of London, Series B: Biological Sciences, 274, 1437–1442.
Watters, J. K., & Biernacki, P. (1989). Targeted sampling options for the study of hidden populations. Social Problems, 36, 416–430.
Wilson, E. O. (1975). Sociobiology: The new synthesis. Cambridge, MA: Harvard University Press.
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.
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Iemmola, F., Camperio Ciani, A. New Evidence of Genetic Factors Influencing Sexual Orientation in Men: Female Fecundity Increase in the Maternal Line. Arch Sex Behav 38, 393–399 (2009). https://doi.org/10.1007/s10508-008-9381-6
- Male homosexuality
- Sexually antagonistic genetic model
- Birth order