The development of human sexual orientation remains a complex and multifaceted subject. It is often studied but its origins continue to elude us. In this preregistered study, our primary objective was to demonstrate the fraternal birth order effect (FBOE), which assumes a higher prevalence of older brothers in gay men than in their straight counterparts and which has also been recently recorded in lesbian women. Our second aim was to explore any potential impact of the FBOE on anal-erotic role orientation (AERO), both in gay and straight men. Our study sample included 693 gay men, 843 straight men, 265 lesbian women, and 331 straight women from Czechia and Slovakia. Employing a conventionally parameterized logistic regression model, we substantiated the FBOE among both gay men (OR = 1.35 for maternal older brothers) and lesbian women (OR = 1.71). These outcomes were confirmed by a more nuanced parameterization recently proposed by Blanchard (2022). Nonmaternal older brothers did not exhibit a significant influence on their younger brothers' sexual orientation. Contrary to some earlier reports, however, our data did not establish the FBOE as exclusive to gay men with the receptive AERO. Furthermore, our observations indicated a lower offspring count for mothers of gay men compared to mothers of straight men. Emphasizing the positive FBOE outcomes, we acknowledge the need for caution regarding the various options that can be used to estimate the familial influences on sexual orientation.

Department of Experimental Psychology Institute of Psychology University of Sao Paulo Sao Paulo Brazil

Department of Neurosciences Faculty of Medicine KU Leuven Leuven Belgium

Department of Psychology and Life Sciences Faculty of Humanities Charles University Prague Czechia

Department of Psychology Faculty of Arts Charles University Prague Czechia

Department of Zoology Faculty of Science Charles University Viničná 7 128 00 Prague Czechia

National Institute of Mental Health Klecany Czechia

Zobrazit více v PubMed

Ablaza, C., Kabátek, J., & Perales, F. (2022). Are sibship characteristics predictive of same sex marriage? An examination of fraternal birth order and female fecundity effects in population-level administrative data from the Netherlands. PubMed DOI

Apostolou, M. (2020). Does fraternal birth order predict male homosexuality, bisexuality, and heterosexual orientation with same-sex attraction? Evidence from a Greek-speaking sample from Greece. PubMed DOI

Bartlett, N. T., Morin, J. R., & Hurd, P. L. (2024). Does the fraternal birth order effect influence handedness? PubMed DOI

Blanchard, R. (2018a). Fraternal birth order, family size, and male homosexuality: Meta-analysis of studies spanning 25 years. PubMed DOI

Blanchard, R. (2018b). Response to commentaries: Meta-analysis of probability samples and other new evidence. PubMed DOI

Blanchard, R. (2022). A novel method for studying the effect of older brothers on sexual orientation and its robustness to stopping rule distortions. PubMed DOI

Blanchard, R. (2023). Studying fraternal birth order in homosexual women and bisexual men. PubMed DOI

Blanchard, R., Beier, K. M., Gómez Jiménez, F. R., Grundmann, D., Krupp, J., Semenyna, S. W., & Vasey, P. L. (2021). Meta-analyses of fraternal and sororal birth order effects in homosexual pedophiles, hebephiles, and teleiophiles. PubMed DOI

Blanchard, R., & Bogaert, A. F. (1996). Homosexuality in men and number of older brothers. PubMed DOI

Blanchard, R., & Klassen, P. (1997). H-Y antigen and homosexuality in men. PubMed DOI

Blanchard, R., Krupp, J., VanderLaan, D. P., Vasey, P. L., & Zucker, K. J. (2020). A method yielding comparable estimates of the fraternal birth order and female fecundity effects in male homosexuality. PubMed DOI PMC

Blanchard, R., & Lippa, R. A. (2021). Reassessing the effect of older sisters on sexual orientation in men. PubMed DOI

Blanchard, R., & Skorska, M. N. (2022). New data on birth order in homosexual men and women and a reply to Vilsmeier et al. (2021a, 2021b). PubMed

Bogaert, A. F. (2006). Biological versus nonbiological older brothers and men’s sexual orientation. PubMed DOI PMC

Bogaert, A. F., Skorska, M. N., Wang, C., Gabrie, J., MacNeil, A. J., Hoffarth, M. R., VanderLean, D. P., Zucker, K. J., & Blanchard, R. (2018). Male homosexuality and maternal immune responsivity to the Y-linked protein NLGN4Y. PubMed DOI PMC

Camperio Ciani, A., Battaglia, U., & Zanzotto, G. (2015). Human homosexuality: A paradigmatic arena for sexually antagonistic selection? PubMed DOI PMC

Camperio Ciani, A., Corna, F., & Capiluppi, C. (2004). Evidence for maternally inherited factors favouring male homosexuality and promoting female fecundity. PubMed DOI PMC

Camperio Ciani, A., & Pellizzari, E. (2012). Fecundity of paternal and maternal non-parental female relatives of homosexual and heterosexual men. PubMed DOI PMC

Camperio Ciani, A., Iemmola, F., & Blecher, S. R. (2009). Genetic factors increase fecundity in female maternal relatives of bisexual men as in homosexuals. PubMed DOI

Cohain, J. S., Buxbaum, R. E., & Mankuta, D. (2017). Spontaneous first trimester miscarriage rates per woman among parous women with 1 or more pregnancies of 24 weeks or more. PubMed DOI PMC

Cómitre-Mariano, B., Martínez-García, M., García-Gálvez, B., Paternina-Die, M., Desco, M., Carmona, S., & Gómez-Gaviro, M. V. (2022). Feto-maternal microchimerism: Memories from pregnancy. PubMed DOI PMC

Ganna, A., Verweij, K. J. H., Nivard, M. G., Maier, R., Wedow, R., Busch, A. S., Abdellaoui, A., Guo, S., Sathirapongsasuti, J. F., 23 and Me Research Team16, Lichtenstein, P., Lundström, S., Långström, N., Auton, A., Harris, K. M., Beecham, G. W., Martin, E. R., Sanders, A. R., Perry, J. R. B., & Zietsch, B. P. (2019). Large-scale GWAS reveals insights into the genetic architecture of same-sex sexual behavior. PubMed DOI PMC

Gómez Jiménez, F. R., Semenyna, S. W., & Vasey, P. L. (2020). The relationship between fraternal birth order and childhood sex-atypical behavior among the Istmo Zapotec PubMed DOI

Haig, D. (2014). Interbirth intervals: Intrafamilial, intragenomic and intrasomatic conflict. PubMed DOI PMC

Hamer, D., Hu, S., Magnuson, V., Hu, N., & Pattatucci, A. (1993). A linkage between DNA markers on the X-chromosome and male sexual orientation. PubMed DOI

Hernández, A., Hoffman, K., Reyes, R., & Fernández-Guasti, A. (2024). Multiparity favors same-sex partner preference in male rats. PubMed

Iemmola, F., & Camperio Ciani, A. (2009). New evidence of genetic factors influencing sexual orientation in men: Female fecundity increase in the maternal line. PubMed DOI

King, M., Green, J., Osborn, D. P. J., Arkell, J., Hetherton, J., & Pereira, E. (2005). Family size in white gay and heterosexual men. PubMed DOI

Lang, T. (1940). Studies on the genetic determination of homosexuality. DOI

Luoto, S., Krams, I., & Rantala, M. J. (2019). A life history approach to the female sexual orientation spectrum: Evolution, development, causal mechanisms, and health. PubMed DOI

Nakagawa, S. (2004). A farewell to Bonferroni: The problems of low statistical power and publication bias. DOI

Posit Team. (2023).

Raymond, M., Turek, D., Durand, V., Nila, S., Suryobroto, B., Vadez, J., Barthes, J., Apostolou, M., & Crochet, P.-A. (2023). Increased birth rank of homosexual males: Disentangling the older brother effect and sexual antagonism hypothesis. DOI

Raymond, M., Turek, D., & Crochet, P.-A. (2024). Testing fraternal birth order effects and antagonistic effects for homosexual men: Power comparison of various methods. PubMed DOI

Rice, W. R., Friberg, U., & Gavrilets, S. (2012). Homosexuality as a consequence of epigenetically canalized sexual development. PubMed DOI

Semenyna, S. W., Gómez Jiménez, F. R., & Vasey, P. L. (2022). Confounds, causality, and sexual orientation: The implications of a population-level study of sibling characteristics and same-sex attraction. PubMed DOI

Semenyna, S. W., Gómez Jiménez, F. R., VanderLaan, D. P., & Vasey, P. L. (2023). Male androphilia, fraternal birth order, and female fecundity in Samoa: A 10-y retrospective. PubMed DOI PMC

Song, S., & Zhang, J. (2023). Contraception ends the genetic maintenance of human same-sex sexual behavior. PubMed DOI PMC

Swift-Gallant, A., Coome, L. A., Aitken, M., Monks, D. A., & VanderLaan, D. P. (2019). Evidence for distinct biodevelopmental influences on male sexual orientation. PubMed DOI PMC

Swift-Gallant, A., Coome, L. A., Monks, D. A., & VanderLaan, D. P. (2018). Gender nonconformity and birth order in relation to anal sex role among gay men. PubMed DOI

Swift-Gallant, A., Di Rita, V., Major, C. A., Breedlove, C. J., Jordan, C. L., & Breedlove, S. M. (2021). Differences in digit ratios between gay men who prefer receptive versus insertive sex roles indicate a role for prenatal androgen. PubMed DOI PMC

Tasos, E. (2022). To what extent are prenatal androgens involved in the development of male homosexuality in humans? PubMed DOI

The Jamovi Project. (2022).

Valentova, J. V., Fořt, J., Freudenfeld, P., Varella, M. A. C., Amaral, B. H., & Havlíček, J. (2023). Different subgroups of homosexuality: Great ideas, little evidence, promising future [Commentary]. PubMed DOI

VanderLaan, D. P., & Vasey, P. L. (2011). Male sexual orientation in independent Samoa: Evidence for fraternal birth order and maternal fecundity effects. PubMed DOI

Wampold, C. H. (2013). Redesigning fraternal birth order studies from top to bottom. PubMed DOI

Wampold, C. H. (2018a). The association between fraternal birth order and anal-erotic roles of men who have sex with men. PubMed DOI

Wampold, C. H. (2018b). Volvivity, femininity, fraternity: Breaching the sexual orientation fortress [Commentary]. PubMed DOI

Wampold, C., H., Aime, C., Turner, T., & Grosskopf, N. (2018).

Xu, Y., & Zheng, Y. (2017). Fraternal birth order, handedness, and sexual orientation in a Chinese population. PubMed DOI

Zheng, L., Hart, T. A., & Zheng, Y. (2012). The relationship between intercourse preference positions and personality traits among gay men in China. PubMed DOI

Zietsch, B. P., Sidari, M. J., Abdellaoui, A., Maier, R., Långström, N., Guo, S., Beecham, G. W., Martin, E. R., Sanders, A. R., & Verweij, K. J. H. (2021). Genomic evidence consistent with antagonistic pleiotropy may help explain the evolutionary maintenance of same-sex sexual behaviour in humans. PubMed DOI

Crusaders, monks and family fortunes: evolutionary models of male homosexuality and related phenomena