Some studies that
show evidence about
the relationship between
The evidence of the possible relationship between male homosexuality and genes comes from several lines of research, among which we will cite three. It´s about predispositions that interact with the environment.
1- Comparison between monozygotic (identical) twins, fraternal twins and the average population (e.g., Bailey, Dunne, & Martin, 2000; Bailey & Pillard, 1991; for a review, see Ngun, Ghahramani, Sánchez, Bocklandt, & Vilain, 2011). All these studies consistently show that male homosexuality is more highly inheritable among identical twins than in the average population.
2-Almost without exception in the national states (whatever their form of government is) male homosexuality represents between 2 and 4% of the population (Gates, 2011; Vasey & VanderLaan, 2014; Whitam, 1983).
3- Maternal immunity hypothesis: in a comparative study conducted in a variety of nations with 20,000 subjects, males with older siblings are 47% more likely to have homosexual orientation. (Blanchard, 1997). The hypothesis that explains it is that with each male that is born, the immune system of the mother responds more strongly to counteract the androgens produced by the previous one. It´s estimated that 15% of cases of homosexuality could be attributed to this effect. (Cantor, Blanchard, Paterson, & Bogaert, 2002). This study was consistently replicated in modern societies and in a traditional one. (Vasey & VanderLaan, 2007; VanderLaan & Vasey, 2011).
Bailey, J. M., Dunne, M. P., & Martin, N. G. (2000). Genetic and environmental influences on sexual orientation and its correlates in an Australian twin sample. Journal of Personality and Social Psychology.
Bailey, J. M., & Pillard, R. C. (1991). A genetic study of male sexual orientation. Archives of General Psychiatry.
Blanchard, R. (2018). Fraternal birth order, family size, and male homosexuality: Meta-analysis of studies spanning 25 years. Archives of sexual behavior, 47(1), 1-15.
Cantor, J. M., Blanchard, R., Paterson, A. D., & Bogaert, A. F. (2002). How many gay men owe their sexual orientation to fraternal birth order? Archives of Sexual Behavior.
Gates, G. J. (2011). How many people are lesbian, gay, bisexual, and transgender? The Williams Institute. Retrieved from https://escholarship.org/uc/item/09h684x2.
Ngun, T. C., Ghahramani, N., Sánchez, F. J., Bocklandt, S., & Vilain, E. (2011). The genetics of sex differences in brain and behavior. Frontiers in Neuroendocrinology.
Vasey, P., & VanderLaan, D. (2007). Birth order and male androphilia in Samoan fa’afafine. Proceedings of the Royal Society
Vasey, P. L., & VanderLaan, D. P. (2009). Maternal and avuncular tendencies in Samoa. Human Nature
VanderLaan, D. P., & Vasey, P. L. (2011). Male sexual orientation in Independent Samoa: Evidence for fraternal birth order and maternal fecundity effects. Archives of Sexual Behavior, 40(3), 495-503.
Vasey, P. L., & VanderLaan, D. P. (2014). Evolving research on the evolution of male androphilia. Canadian Journal of Human Sexuality.
Whitam, F. L. (1983). Culturally invariable properties of male homosexuality: Tentative conclusions from cross-cultural research. Archives of sexual behavior, 12(3), 207-226.