The fraternal birth order effect (FBOE) is a well documented preponderance of older brothers among gay men. Until now, it has lacked a proper formal treatment in light of evolutionary theory. In this article, we propose two related mathematical models based on the rigorous application of the inclusive fitness concept. The models examine sibling rivalry and resource allocation within families. One of the models assumes that the laterborn's reproductive ability is not compromised by the manipulation that leads to the exclusive allocation of family resources to the firstborn. The other model posits that this manipulation may lower the laterborn's direct fitness to zero. Both models suggest that the FBOE arises primarily from the older brother's manipulation, as the firstborn's inclusive fitness may increase, while the mother's decreases, but not vice versa. However, under certain conditions, the mother should support an activity that discourages division of family resources, and in extreme cases, the younger brother's homosexuality may be the best available reproductive strategy for his genes. The models also provide insight into the cultural evolution of primogeniture and explain how manipulation between relatives could evolve before the emergence of substantial economic inequality.

Center for Theoretical Study Charles University and Czech Academy of Sciences Jilská 1 Prague 1 110 00 Czechia

Department of Philosophy and History of Sciences Faculty of Science Charles University Viničná 7 Prague 128 00 Czechia

Department of Psychology Faculty of Arts Charles University Celetná 20 Prague 116 42 Czechia

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

Zobrazit více v PubMed

Apostolou M. 2022. The direct reproductive cost of same-sex attraction: evidence from two nationally representative U.S. samples. Arch. Sex. Behav. 51, 1857–1864. (10.1007/s10508-021-02199-y) PubMed DOI

Ciani AC, Battaglia U, Zanzotto G. 2015. Human homosexuality: a paradigmatic arena for sexually antagonistic selection? Cold Spring Harb. Perspect. Biol. 7, a017657. (10.1101/cshperspect.a017657) PubMed DOI PMC

Coome LA, Skorska MN, VanderLaan DP. 2020. Direct reproduction and sexual orientation and gender diversity in Thailand. Arch. Sex. Behav. 49, 2449–2460. (10.1007/s10508-020-01830-8) PubMed DOI

Camperio-Ciani A, Corna F, Capiluppi C. 2004. Evidence for maternally inherited factors favouring male homosexuality and promoting female fecundity. Proc. R. Soc. Lond. B 271, 2217–2221. (10.1098/rspb.2004.2872) PubMed DOI PMC

Miller EM. 2000. Homosexuality, birth order, and evolution: toward an equilibrium reproductive economics of homosexuality. Arch. Sex. Behav. 29, 1–34. (10.1023/a:1001836320541) PubMed DOI

Zietsch BP, et al. . 2021. Genomic evidence consistent with antagonistic pleiotropy may help explain the evolutionary maintenance of same-sex sexual behaviour in humans. Nat. Hum. Behav. 5, 1251–1258. (10.1038/s41562-021-01168-8) PubMed DOI

Kirkpatrick RC. 2000. The evolution of human homosexual behavior. Curr. Anthropol. 41, 385–413. (10.1086/300145) PubMed DOI

Dewar CS. 2003. An association between male homosexuality and reproductive success. Med. Hypoth. 60, 225–232. (10.1016/s0306-9877(02)00363-8) PubMed DOI

Ruse M. 1988. Homosexuality: a philosophical inquiry. New York, NY: Blackwell.

Trivers RL. 1974. Parent-offspring conflict. Am. Zool. 14, 249–264. (10.1093/icb/14.1.249) DOI

Flegr J. 2023. Xenoadaptations. In Encyclopedia of sexual psychology and behavior (ed. Shackelford TK), pp. 1–5. Cham, Switzerland: Springer International Publishing. (10.1007/978-3-031-08956-5_389-1) DOI

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. J. Sex Res. 59, 671–683. (10.1080/00224499.2021.1974330) 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. Arch. Sex. Behav. 49, 575–579. (10.1007/s10508-019-01466-3) PubMed DOI

Fořt J, Kunc B, Valentova JV, Bártová K, Hudáčová K. 2024. Examining the fraternal birth order effect and sexual orientation: insights from an East European population. Arch. Sex. Behav. 53, 2905–2922. (10.1007/s10508-024-02892-8) PubMed DOI PMC

Gómez Jiménez FR, Semenyna SW, Vasey PL. 2020. The relationship between fraternal birth order and childhood sex-atypical behavior among the Istmo Zapotec muxes. Dev. Psychobiol. 62, 792–803. (10.1002/dev.21987) PubMed DOI

Semenyna SW, Gómez Jiménez FR, VanderLaan DP, Vasey PL. 2023. Male androphilia, fraternal birth order, and female fecundity in Samoa: a 10-y retrospective. Proc. Natl Acad. Sci. 120, e2313284120. (10.1073/pnas.2313284120) PubMed DOI PMC

Blanchard R. 2018. Fraternal birth order, family size, and male homosexuality: meta-analysis of studies spanning 25 years. Arch. Sex. Behav. 47, 1–15. (10.1007/s10508-017-1007-4) PubMed DOI

Blanchard R, Krupp J, VanderLaan DP, Vasey PL, Zucker KJ. 2020. A method yielding comparable estimates of the fraternal birth order and female fecundity effects in male homosexuality. Proc. R. Soc. B 287, 20192907. (10.1098/rspb.2019.2907) PubMed DOI PMC

Jensch K. 1941. Zur genealogie der homosexualität. Arch. Psychiatr. Nervenkrankh. 112, 527–540. (10.1007/BF01813997) DOI

Slater E. 1962. Birth order and maternal age of homosexuals. Lancet 279, 67–71. (10.1016/s0140-6736(62)91719-1) PubMed DOI

Blanchard R, Bogaert AF. 1996. Homosexuality in men and number of older brothers. Am. J. Psychiatry 153, 27–31. (10.1176/ajp.153.1.27) PubMed DOI

Blanchard R, Beier KM, Gómez Jiménez FR, Grundmann D, Krupp J, Semenyna SW, Vasey PL. 2021. Meta-analyses of fraternal and sororal birth order effects in homosexual pedophiles, hebephiles, and teleiophiles. Arch. Sex. Behav. 50, 779–796. (10.1007/s10508-020-01819-3) PubMed DOI

Bogaert AF. 2006. Biological versus nonbiological older brothers and men’s sexual orientation. Proc. Natl Acad. Sci. USA 103, 10771–10774. (10.1073/pnas.0511152103) PubMed DOI PMC

Blanchard R, Klassen P. 1997. H-Y antigen and homosexuality in men. J. Theor. Biol. 185, 373–378. (10.1006/jtbi.1996.0315) PubMed DOI

Bogaert AF, Skorska MN, Wang C, Gabrie J, MacNeil AJ, Hoffarth MR, VanderLaan DP, Zucker KJ, Blanchard R. 2018. Male homosexuality and maternal immune responsivity to the Y-linked protein NLGN4Y. Proc. Natl Acad. Sci. USA 115, 302–306. (10.1073/pnas.1705895114) PubMed DOI PMC

Flegr J. 2022. Adaptations, by-products, and spandrels. In The Cambridge handbook of evolutionary perspectives on sexual psychology (ed. Shackelford TK), pp. 87–113. Cambridge, UK: Cambridge University Press. (10.1017/9781108943529.007) DOI

Apostolou M. 2013. Interfamily conflict, reproductive success, and the evolution of male homosexuality. Rev. Gen. Psychol. 17, 288–296. (10.1037/a0031521) DOI

Betzig L. 2012. Means, variances, and ranges in reproductive success: comparative evidence. Evol. Hum. Behav. 33, 309–317. (10.1016/j.evolhumbehav.2011.10.008) DOI

Wilson EO. 1975. Sociobiology: the new synthesis. Cambridge, MA: Belknap Press of Harvard University Press.

Flegr J. 2023. Sibling manipulation hypothesis of male homosexuality. In Encyclopedia of sexual psychology and behavior (ed. Shackelford TK), pp. 1–5. Cham, Switzerland: Springer International Publishing. (10.1007/978-3-031-08956-5_2307-1) DOI

Ross CT, et al. . 2018. Greater wealth inequality, less polygyny: rethinking the polygyny threshold model. J. R. Soc. Interface 15, 20180752. (10.1098/rsif.2018.0752) PubMed DOI PMC

Khovanova T. 2020. On the mathematics of the fraternal birth order effect and the genetics of homosexuality. Arch. Sex. Behav. 49, 551–555. (10.1007/s10508-019-01573-1) PubMed DOI

Micheletti AJC, Ge E, Zhou L, Chen Y, Zhang H, Du J, Mace R. 2022. Religious celibacy brings inclusive fitness benefits. Proc. R. Soc. B 289, 20220965. (10.1098/rspb.2022.0965) PubMed DOI PMC

Zhou L, Ge E, Micheletti AJC, Chen Y, Du J, Mace R. 2022. Monks relax sibling competition over parental resources in Tibetan populations. Behav. Ecol. 33, 1070–1079. (10.1093/beheco/arac059) DOI

Scheidel W. 2009. Sex and empire, a Darwinian perspective. In The dynamics of ancient empires: state power from Assyria to Byzantium (eds Morris I, Scheidel W), pp. 255–324. Oxford, UK: Oxford University Press. (10.1093/oso/9780195371581.003.0007) DOI

Haig D. 2014. Interbirth intervals: intrafamilial, intragenomic and intrasomatic conflict. Evol. Med. Public Health 2014, 12–17. (10.1093/emph/eou002) PubMed DOI PMC

O’Donoghue K. 2008. Fetal microchimerism and maternal health during and after pregnancy. Obstet. Med. 1, 56–64. (10.1258/om.2008.080008) PubMed DOI PMC

Schepanski S, et al. . 2022. Pregnancy-induced maternal microchimerism shapes neurodevelopment and behavior in mice. Nat. Commun. 13, 4571. (10.1038/s41467-022-32230-2) PubMed DOI PMC

Fujimoto K, Nakajima A, Hori S, Tanaka Y, Shirasaki Y, Uemura S, Irie N. 2022. Whole-embryonic identification of maternal microchimeric cell types in mouse using single-cell RNA sequencing. Scient. Rep. 12, 18313. (10.1038/s41598-022-20781-9) PubMed DOI PMC

Johnson BN, et al. . 2021. Male microchimerism in females: a quantitative study of twin pedigrees to investigate mechanisms. Hum. Reprod. 36, 2529–2537. (10.1093/humrep/deab170) PubMed DOI PMC

Yan Z, Lambert NC, Guthrie KA, Porter AJ, Loubiere LS, Madeleine MM, Stevens AM, Hermes HM, Nelson JL. 2005. Male microchimerism in women without sons: quantitative assessment and correlation with pregnancy history. Am. J. Med. 118, 899–906. (10.1016/j.amjmed.2005.03.037) PubMed DOI

Alexander RD. 1974. The evolution of social behavior. Annu. Rev. Ecol. Syst. 5, 325–383. (10.1146/annurev.es.05.110174.001545) DOI

Dawkins R. 1982. The extended phenotype. Oxford, UK: Oxford University Press.

Kulich T, Flegr J. 2011. Positive effects of multiple gene control on the spread of altruism by group selection. J. Theor. Biol. 284, 1–6. (10.1016/j.jtbi.2011.05.017) PubMed DOI

Dawkins R, Krebs JR. 1979. Arms races between and within species. Proc. R. Soc. Lond. B 205, 489–511. (10.1098/rspb.1979.0081) PubMed DOI

O’Connor LJ, Schoech AP, Hormozdiari F, Gazal S, Patterson N, Price AL. 2019. Extreme polygenicity of complex traits is explained by negative selection. Am. J. Hum. Genet. 105, 456–476. (10.1016/j.ajhg.2019.07.003) PubMed DOI PMC

Zietsch BP. 2024. Genomic findings and their implications for the evolutionary social sciences. Evol. Hum. Behav. 45, 106596. (10.1016/j.evolhumbehav.2024.106596) DOI

Blanchard R. 2022. A novel method for studying the effect of older brothers on sexual orientation and its robustness to stopping rule distortions. J. Sex Res. 59, 684–689. (10.1080/00224499.2021.1984379) PubMed DOI

Veller C, Haig D, Nowak MA. 2016. The Trivers–Willard hypothesis: sex ratio or investment? Proc. R. Soc. B 283, 20160126. (10.1098/rspb.2016.0126) PubMed DOI PMC

Hernández A, Hoffman K, Reyes R, Fernández-Guasti A. 2024. Multiparity favors same-sex partner preference in male rats. Behav. Brain Res. 461, 114842. (10.1016/j.bbr.2023.114842) PubMed DOI

Swift-Gallant A, Coome LA, Aitken M, Monks DA, VanderLaan DP. 2019. Evidence for distinct biodevelopmental influences on male sexual orientation. Proc. Natl Acad. Sci. USA 116, 12787–12792. (10.1073/pnas.1809920116) PubMed DOI PMC

Dostoyevsky F. 2017. Crime and punishment. (Transl. by Garnett C.). New York, NY: Dover Publications. (Original work published 1866.).

Vasey PL, VanderLaan DP. 2010. Avuncular tendencies and the evolution of male androphilia in Samoan Fa’afafine. Arch. Sex. Behav. 39, 821–830. (10.1007/s10508-008-9404-3) PubMed DOI

Gómez Jiménez FR, Vasey PL. 2022. Kin-directed altruism and the evolution of male androphilia among Istmo Zapotec muxes. Evol. Hum. Behav. 43, 224–233. (10.1016/j.evolhumbehav.2022.02.002) DOI

Hawkes K, O’Connell JF, Jones NGB, Alvarez H, Charnov EL. 1998. Grandmothering, menopause, and the evolution of human life histories. Proc. Natl Acad. Sci. USA 95, 1336–1339. (10.1073/pnas.95.3.1336) PubMed DOI PMC

Healey MD, Ellis BJ. 2007. Birth order, conscientiousness, and openness to experience. Evol. Hum. Behav. 28, 55–59. (10.1016/j.evolhumbehav.2006.05.003) DOI

Felesina T, Zietsch BP. 2025. Emerging insights into the genetics and evolution of human same-sex sexual behavior. Trends Genet 2025, S0168-9525(24)00300-7. (10.1016/j.tig.2024.12.005) PubMed DOI

Henrich J, Gil-White FJ. 2001. The evolution of prestige: freely conferred deference as a mechanism for enhancing the benefits of cultural transmission. Evol. Hum. Behav. 22, 165–196. (10.1016/s1090-5138(00)00071-4) PubMed DOI

Tureček P, Fořt J, Flegr J. 2025. Supplementary material from: Crusaders, monks, and family fortunes: evolutionary models of male homosexuality and related phenomena. Figshare (10.6084/m9.figshare.c.7682068) PubMed DOI PMC

The Evolutionary Roots of Familial Altruism: Paternity Uncertainty Shapes Patterns of Kindness

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