Fraternal Birth Order, Family Size, and Male Homosexuality: Meta-Analysis of Studies Spanning 25 Years

Target Article

Abstract

The fraternal birth order effect is the tendency for older brothers to increase the odds of homosexuality in later-born males. This study compared the strength of the effect in subjects from small versus large families and in homosexual subjects with masculine versus feminine gender identities. Meta-analyses were conducted on 30 homosexual and 30 heterosexual groups from 26 studies, totaling 7140 homosexual and 12,837 heterosexual males. The magnitude of the fraternal birth order effect was measured with a novel variable, the Older Brothers Odds Ratio, computed as (homosexuals’ older brothers ÷ homosexuals’ other siblings) ÷ (heterosexuals’ older brothers ÷ heterosexuals’ other siblings), where other siblings = older sisters + younger brothers + younger sisters. An Older Brothers Odds Ratio of 1.00 represents no effect of sexual orientation; values over 1.00 are positive evidence for the fraternal birth order effect. Evidence for the reliability of the effect was consistent. The Older Brothers Odds Ratio was significantly >1.00 in 20 instances, >1.00 although not significantly in nine instances, and nonsignificantly <1.00 in 1 instance. The pooled Older Brothers Odds Ratio for all samples was 1.47, p < .00001. Subgroups analyses showed that the magnitude of the effect was significantly greater in the 12 feminine or transgender homosexual groups than in the other 18 homosexual groups. There was no evidence that the magnitude of the effect differs according to family size.

Keywords

Birth order Homosexuality Maternal immune hypothesis Meta-analysis RevMan Sexual orientation Transgender 

References

  1. Bailey, J. M., & Zucker, K. J. (1995). Childhood sex-typed behavior and sexual orientation: A conceptual analysis and quantitative review. Developmental Psychology, 31, 43–55.CrossRefGoogle Scholar
  2. Bearman, P. S., & Brückner, H. (2002). Opposite-sex twins and adolescent same-sex attraction. American Journal of Sociology, 107, 1179–1205.CrossRefGoogle Scholar
  3. Bell, A. P., Weinberg, M. S., & Hammersmith, S. K. (1981). Sexual preference: Its development in men and women. Bloomington, IN: Indiana University Press.Google Scholar
  4. Bieber, I., Dain, H. J., Dince, P. R., Drellich, M. G., Grand, H. G., Gundlach, R. H., … Bieber, T. B. (1962). Homosexuality: A psychoanalytic study of male homosexuals. New York, NY: Basic Books.CrossRefGoogle Scholar
  5. Blanchard, R. (1989). The classification and labeling of nonhomosexual gender dysphorias. Archives of Sexual Behavior, 18, 315–334.CrossRefPubMedGoogle Scholar
  6. Blanchard, R. (1997). Birth order and sibling sex ratio in homosexual versus heterosexual males and females. Annual Review of Sex Research, 8, 27–67.PubMedGoogle Scholar
  7. Blanchard, R. (2004). Quantitative and theoretical analyses of the relation between older brothers and homosexuality in men. Journal of Theoretical Biology, 230, 173–187.CrossRefPubMedGoogle Scholar
  8. Blanchard, R. (2007). Older-sibling and younger-sibling sex ratios in Frisch and Hviid’s (2006) national cohort study of two million Danes. Archives of Sexual Behavior, 36, 860–863.CrossRefPubMedGoogle Scholar
  9. Blanchard, R. (2012). Fertility in the mothers of firstborn homosexual and heterosexual men. Archives of Sexual Behavior, 41, 551–556.CrossRefPubMedGoogle Scholar
  10. Blanchard, R. (2014). Detecting and correcting for family size differences in the study of sexual orientation and fraternal birth order. Archives of Sexual Behavior, 43, 845–852.CrossRefPubMedGoogle Scholar
  11. Blanchard, R., Barbaree, H. E., Bogaert, A. F., Dickey, R., Klassen, P., Kuban, M. E., & Zucker, K. J. (2000). Fraternal birth order and sexual orientation in pedophiles. Archives of Sexual Behavior, 29, 463–478.CrossRefPubMedGoogle Scholar
  12. Blanchard, R., & Bogaert, A. F. (1996a). Biodemographic comparisons of homosexual and heterosexual men in the Kinsey interview data. Archives of Sexual Behavior, 25, 551–579.CrossRefPubMedGoogle Scholar
  13. Blanchard, R., & Bogaert, A. F. (1996b). Homosexuality in men and number of older brothers. American Journal of Psychiatry, 153, 27–31.CrossRefPubMedGoogle Scholar
  14. Blanchard, R., & Bogaert, A. F. (1998). Birth order in homosexual versus heterosexual sex offenders against children, pubescents, and adults. Archives of Sexual Behavior, 27, 595–603.CrossRefPubMedGoogle Scholar
  15. Blanchard, R., & Bogaert, A. F. (2004). Proportion of homosexual men who owe their sexual orientation to fraternal birth order: An estimate based on two national probability samples. American Journal of Human Biology, 16, 151–157.CrossRefPubMedGoogle Scholar
  16. Blanchard, R., Cantor, J. M., Bogaert, A. F., Breedlove, S. M., & Ellis, L. (2006). Interaction of fraternal birth order and handedness in the development of male homosexuality. Hormones and Behavior, 49, 405–414.CrossRefPubMedGoogle Scholar
  17. Blanchard, R., & Ellis, L. (2001). Birth weight, sexual orientation and the sex of preceding siblings. Journal of Biosocial Science, 33, 451–467.CrossRefPubMedGoogle Scholar
  18. Blanchard, R., & Klassen, P. (1997). H-Y antigen and homosexuality in men. Journal of Theoretical Biology, 185, 373–378.CrossRefPubMedGoogle Scholar
  19. 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.CrossRefPubMedGoogle Scholar
  20. Blanchard, R., & Sheridan, P. M. (1992). Sibship size, sibling sex ratio, birth order, and parental age in homosexual and nonhomosexual gender dysphorics. Journal of Nervous and Mental Disease, 180, 40–47.CrossRefPubMedGoogle Scholar
  21. Blanchard, R., & VanderLaan, D. P. (2015). Commentary on Kishida and Rahman (2015), including a meta-analysis of relevant studies on fraternal birth order and sexual orientation in men. Archives of Sexual Behavior, 44, 1503–1509.CrossRefPubMedGoogle Scholar
  22. Blanchard, R., & Zucker, K. J. (1994). Reanalysis of Bell, Weinberg, and Hammersmith’s data on birth order, sibling sex ratio, and parental age in homosexual men. American Journal of Psychiatry, 151, 1375–1376.CrossRefPubMedGoogle Scholar
  23. Blanchard, R., Zucker, K. J., Bradley, S. J., & Hume, C. S. (1995). Birth order and sibling sex ratio in homosexual male adolescents and probably prehomosexual feminine boys. Developmental Psychology, 31, 22–30.CrossRefGoogle Scholar
  24. Blanchard, R., Zucker, K. J., Cavacas, A., Allin, S., Bradley, S. J., & Schachter, D. C. (2002). Fraternal birth order and birth weight in probably prehomosexual feminine boys. Hormones and Behavior, 41, 321–327.CrossRefPubMedGoogle Scholar
  25. Blanchard, R., Zucker, K. J., Cohen-Kettenis, P. T., Gooren, L. J. G., & Bailey, J. M. (1996). Birth order and sibling sex ratio in two samples of Dutch gender-dysphoric homosexual males. Archives of Sexual Behavior, 25, 495–514.CrossRefPubMedGoogle Scholar
  26. Blanchard, R., Zucker, K. J., Siegelman, M., Dickey, R., & Klassen, P. (1998). The relation of birth order to sexual orientation in men and women. Journal of Biosocial Science, 30, 511–519.CrossRefPubMedGoogle Scholar
  27. Bogaert, A. F. (1998). Birth order and sibling sex ratio in homosexual and heterosexual non-white men. Archives of Sexual Behavior, 27, 467–473.CrossRefPubMedGoogle Scholar
  28. Bogaert, A. F. (2003). Interaction of older brothers and sex-typing in the prediction of sexual orientation in men. Archives of Sexual Behavior, 32, 129–134.CrossRefPubMedGoogle Scholar
  29. Bogaert, A. F. (2004). The prevalence of male homosexuality: The effect of fraternal birth order and variations in family size. Journal of Theoretical Biology, 230, 33–37.CrossRefPubMedGoogle Scholar
  30. Bogaert, A. F. (2005a). Gender role/identity and sibling sex ratio in homosexual men. Journal of Sex and Marital Therapy, 31, 217–227.CrossRefPubMedGoogle Scholar
  31. Bogaert, A. F. (2005b). Sibling sex ratio and sexual orientation in men and women: New tests in two national probability samples. Archives of Sexual Behavior, 34, 111–116.CrossRefPubMedGoogle Scholar
  32. Bogaert, A. F. (2006). Biological versus nonbiological older brothers and men’s sexual orientation. Proceedings of the National Academy of Sciences of the USA, 103, 10771–10774.CrossRefPubMedPubMedCentralGoogle Scholar
  33. Bogaert, A. F. (2010). Physical development and sexual orientation in men and women: An analysis of NATSAL-2000. Archives of Sexual Behavior, 39, 110–116.CrossRefPubMedGoogle Scholar
  34. Bogaert, A. F., Bezeau, S., Kuban, M., & Blanchard, R. (1997). Pedophilia, sexual orientation, and birth order. Journal of Abnormal Psychology, 106, 331–335.CrossRefPubMedGoogle Scholar
  35. Bozkurt, A., Bozkurt, O. H., & Sonmez, I. (2015). Birth order and sibling sex ratio in a population with high fertility: Are Turkish male to female transsexuals different? Archives of Sexual Behavior, 44, 1331–1337.CrossRefPubMedGoogle Scholar
  36. Camperio-Ciani, A., Corna, F., & Capiluppi, C. (2004). Evidence for maternally inherited factors favoring male homosexuality and promoting female fecundity. Proceedings of the Royal Society of London, Series B: Biological Sciences, 271, 2217–2221.CrossRefGoogle Scholar
  37. 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, 31, 63–71.CrossRefPubMedGoogle Scholar
  38. Currin, J. M., Gibson, L., & Hubach, R. D. (2015a). Multidimensional assessment of sexual orientation and the fraternal birth order effect. Psychology of Sexual Orientation and Gender Diversity, 2, 113–122.CrossRefGoogle Scholar
  39. Currin, J. M., Gibson, L., & Hubach, R. D. (2015b). “Multidimensional assessment of sexual orientation and the fraternal birth order effect”: Correction to Currin, Gibson, and Hubach (2015). Psychology of Sexual Orientation and Gender Diversity, 2, 265.CrossRefGoogle Scholar
  40. Deeks, J. J., Higgins, J. P. T., & Altman, D. G. (Eds.). (2011). Chapter 9: Analysing data and undertaking meta-analyses. In J. P. T. Higgins & S. Green (Eds.), Cochrane handbook for systematic reviews of interventions Version 5.1.0 (updated March 2011). The Cochrane Collaboration, 2011. http://handbook.cochrane.org/.
  41. Ellis, L., & Blanchard, R. (2001). Birth order, sibling sex ratio, and maternal miscarriages in homosexual and heterosexual men and women. Personality and Individual Differences, 30, 543–552.CrossRefGoogle Scholar
  42. Francis, A. M. (2008). Family and sexual orientation: The family-demographic correlates of homosexuality in men and women. Journal of Sex Research, 45, 371–377.CrossRefPubMedGoogle Scholar
  43. Frisch, M., & Hviid, A. (2006). Childhood family correlates of heterosexual and homosexual marriages: A national cohort study of two million Danes. Archives of Sexual Behavior, 35, 533–547.CrossRefPubMedGoogle Scholar
  44. Gebhard, P. H., Gagnon, J. H., Pomeroy, W. B., & Christenson, C. V. (1965). Sex offenders: An analysis of types. New York, NY: Harper & Row.Google Scholar
  45. Gebhard, P. H., & Johnson, A. B. (1979). The Kinsey data: Marginal tabulations of the 1938–1963 interviews conducted by the Institute for Sex Research. Philadelphia, PA: W. B. Saunders.Google Scholar
  46. Gómez-Gil, E., Esteva, I., Carrasco, R., Cruz Almaraz, M., Pasaro, E., Salamero, M., & Guillamon, A. (2011). Birth order and ratio of brothers to sisters in Spanish transsexuals. Archives of Sexual Behavior, 40, 505–510.CrossRefPubMedGoogle Scholar
  47. Green, R. (1987). The “sissy boy syndrome” and the development of homosexuality. New Haven, CT: Yale University Press.Google Scholar
  48. Green, R. (2000). Birth order and ratio of brothers to sisters in transsexuals. Psychological Medicine, 30, 789–795.CrossRefPubMedGoogle Scholar
  49. Iemmola, F., & Camperio Ciani, A. (2009). New evidence of genetic factors influencing sexual orientation in men: Female fecundity increase in the maternal line. Archives of Sexual Behavior, 38, 393–399.CrossRefPubMedGoogle Scholar
  50. Jensch, K. (1941a). Zur Genealogie der Homosexualität [Regarding the genetics of homosexuality]. Archiv für Psychiatrie und Nervenkrankheiten, 112, 527–540.Google Scholar
  51. Jensch, K. (1941b). Weiterer Beitrag zur Genealogie der Homosexualität [A further contribution regarding the genetics of homosexuality]. Archiv für Psychiatrie und Nervenkrankheiten, 112, 679–696.Google Scholar
  52. Jones, M. B., & Blanchard, R. (1998). Birth order and male homosexuality: Extension of Slater’s index. Human Biology, 70, 775–787.PubMedGoogle Scholar
  53. Kangassalo, K., Pölkki, M., & Rantala, M. J. (2011). Prenatal influences on sexual orientation: Digit ratio (2D:4D) and number of older siblings. Evolutionary Psychology, 9, 496–508.CrossRefPubMedGoogle Scholar
  54. Khorashad, B. S., Zucker, K. J., Talaei, A., Farid, F., Fayyazi Bordbar, M. R., Rajabzadeh, F., … Blanchard, R. (2017). Birth order and sibling sex ratio in a sample of Iranian transsexuals. Manuscript in preparation.Google Scholar
  55. 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.CrossRefPubMedGoogle Scholar
  56. Kishida, M., & Rahman, Q. (2015). Fraternal birth order and extreme right-handedness as predictors of sexual orientation and gender nonconformity in men. Archives of Sexual Behavior, 44, 1493–1501.CrossRefPubMedGoogle Scholar
  57. Lang, T. (1936). Beitrag zur Frage nach der genetischen Bedingtheit der Homosexualität [A contribution regarding the question of the genetic determination of homosexuality]. Zeitschrift für die gesamte Neurologie und Psychiatrie, 155, 702–713.Google Scholar
  58. Lang, T. (1940). Studies on the genetic determination of homosexuality. Journal of Nervous and Mental Disease, 92, 55–64.CrossRefGoogle Scholar
  59. Lang, T. (1960). Die Homosexualität als genetisches Problem [Homosexuality as a genetic problem]. Acta Geneticae Medicae et Gemellologiae, 9, 370–381.Google Scholar
  60. Lawrence, A. A. (2010). Sexual orientation versus age of onset as bases for typologies (subtypes) for gender identity disorder in adolescents and adults. Archives of Sexual Behavior, 39, 514–545.CrossRefPubMedGoogle Scholar
  61. McConaghy, N., Hadzi-Pavlovic, D., Stevens, C., Manicavasagar, V., Buhrich, N., & Vollmer-Conna, U. (2006). Fraternal birth order and ratio of heterosexual/homosexual feelings in women and men. Journal of Homosexuality, 51, 161–174.CrossRefPubMedGoogle Scholar
  62. Robinson, S. J., & Manning, J. T. (2000). The ratio of 2nd to 4th digit length and male homosexuality. Evolution and Human Behavior, 21, 333–345.CrossRefPubMedGoogle Scholar
  63. Schagen, S. E. E., Delemarre-van de Waal, H. A., Blanchard, R., & Cohen-Kettenis, P. T. (2012). Sibling sex ratio and birth order in early-onset gender dysphoric adolescents. Archives of Sexual Behavior, 41, 541–549.CrossRefPubMedGoogle Scholar
  64. Schwartz, G., Kim, R. M., Kolundzija, A. B., Rieger, G., & Sanders, A. R. (2010). Biodemographic and physical correlates of sexual orientation in men. Archives of Sexual Behavior, 39, 93–109.CrossRefPubMedGoogle Scholar
  65. Semenya, S. W., VanderLaan, D. P., & Vasey, P. L. (2017). Birth order and recalled childhood gender nonconformity in Samoan men and fa’afafine. Developmental Psychobiology, 59, 338–347.CrossRefGoogle Scholar
  66. Siegelman, M. (1972). Adjustment of male homosexuals and heterosexuals. Archives of Sexual Behavior, 2, 9–25.CrossRefPubMedGoogle Scholar
  67. Siegelman, M. (1973). Birth order and family size of homosexual men and women. Journal of Consulting and Clinical Psychology, 41, 164.CrossRefPubMedGoogle Scholar
  68. Siegelman, M. (1974). Parental background of male homosexuals and heterosexuals. Archives of Sexual Behavior, 3, 3–18.CrossRefPubMedGoogle Scholar
  69. Siegelman, M. (1978). Psychological adjustment of homosexual and heterosexual men: A cross-national replication. Archives of Sexual Behavior, 7, 1–11.CrossRefPubMedGoogle Scholar
  70. Siegelman, M. (1981). Parental backgrounds of homosexual and heterosexual men: A cross national replication. Archives of Sexual Behavior, 10, 505–513.CrossRefPubMedGoogle Scholar
  71. Singh, D. (2012). A follow-up study of boys with gender identity disorder (Unpublished doctoral dissertation). University of Toronto, Toronto, Canada.Google Scholar
  72. Steensma, T. D. (2013). From gender variance to gender dysphoria: Psychosexual development of gender atypical children and adolescents (Unpublished doctoral dissertation). Vrije Universiteit, Amsterdam, the Netherlands.Google Scholar
  73. Suarez, B. K., & Przybeck, T. R. (1980). Sibling sex ratio and male homosexuality. Archives of Sexual Behavior, 9, 1–12.CrossRefPubMedGoogle Scholar
  74. Swift-Gallant, A., Coome, L. A., Monks, D., & VanderLaan, D. P. (2017). Gender nonconformity and birth order in relation to anal sex role among gay men. Archives of Sexual Behavior. doi:10.1007/s10508-017-0980-y.PubMedGoogle Scholar
  75. VanderLaan, D. P., Blanchard, R., Wood, H., Garzon, L. C., & Zucker, K. J. (2015). Birth weight and two possible types of maternal effects on male sexual orientation: A clinical study of children and adolescents referred to a gender identity service. Developmental Psychobiology, 57, 25–34.CrossRefPubMedGoogle Scholar
  76. VanderLaan, D. P., Blanchard, R., Wood, H., & Zucker, K. J. (2014). Birth order and sibling sex ratio of children and adolescents referred to a gender identity service. PLoS ONE. doi:10.1371/journal.pone.0090257.PubMedPubMedCentralGoogle Scholar
  77. VanderLaan, D. P., Blanchard, R., Zucker, K. J., Massuda, R., Fontanari, A. M. V., Borba, A. O., … Lobato, M. I. (2016). Birth order and androphilic male-to-female transsexualism in Brazil. Journal of Biosocial Science. doi:10.1017/S0021932016000584.PubMedGoogle Scholar
  78. 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, 495–503.CrossRefPubMedGoogle Scholar
  79. 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.CrossRefGoogle Scholar
  80. Wallien, M. S., & Cohen-Kettenis, P. T. (2008). Psychosexual outcome of gender dysphoric children. Journal of the American Academy of Child and Adolescent Psychiatry, 47, 1413–1423.CrossRefPubMedGoogle Scholar
  81. Wampold, C. H. (2013). Redesigning fraternal birth order studies from top to bottom [Letter to the Editor]. Archives of Sexual Behavior, 42, 1387–1389.CrossRefPubMedGoogle Scholar
  82. Xu, Y., & Zheng, Y. (2014). Birth order and sibling sex ratio in relation to sexual orientation in China. Social Behavior and Personality, 42, 995–1002.CrossRefGoogle Scholar
  83. Xu, Y., & Zheng, Y. (2017). Fraternal birth order, handedness, and sexual orientation in a Chinese population. Journal of Sex Research, 54, 10–18.CrossRefPubMedGoogle Scholar
  84. Zheng, L., Hart, T. A., & Zheng, Y. (2012). The relationship between intercourse preference positions and personality traits among gay men in China. Archives of Sexual Behavior, 41, 683–689.CrossRefPubMedGoogle Scholar
  85. Zietsch, B. P., Verweij, K. J. H., Heath, A. C., Madden, P. A. F., Martin, N. G., Nelson, E. C., & Lynskey, M. T. (2012). Do shared etiological factors contribute to the relationship between sexual orientation and depression? Psychological Medicine, 42, 521–532.CrossRefPubMedGoogle Scholar
  86. Zucker, K. J., & Blanchard, R. (1994). Reanalysis of Bieber et al’.s 1962 data on sibling sex ratio and birth order in male homosexuals. Journal of Nervous and Mental Disease, 182, 528–530.CrossRefPubMedGoogle Scholar
  87. Zucker, K. J., Blanchard, R., Kim, T.-S., Pae, C.-U., & Lee, C. (2007). Birth order and sibling sex ratio in homosexual transsexual South Korean males: Effects of the male-preference stopping rule. Psychiatry and Clinical Neurosciences, 61, 529–533.CrossRefPubMedGoogle Scholar

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© Springer Science+Business Media New York 2017

Authors and Affiliations

  1. 1.Department of PsychiatryUniversity of TorontoTorontoCanada

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