How Many Gay Men Owe Their Sexual Orientation to Fraternal Birth Order?

Abstract

In men, sexual orientation correlates with the number of older brothers, each additional older brother increasing the odds of homosexuality by approximately 33%. However, this phenomenon, the fraternal birth order effect, accounts for the sexual orientation of only a proportion of gay men. To estimate the size of this proportion, we derived generalized forms of two epidemiological statistics, the attributable fraction and the population attributable fraction, which quantify the relationship between a condition and prior exposure to an agent that can cause it. In their common forms, these statistics are calculable only for 2 levels of exposure: exposed versus not-exposed. We developed a method applicable to agents with multiple levels of exposure—in this case, number of older brothers. This noniterative method, which requires the odds ratio from a prior logistic regression analysis, was then applied to a large contemporary sample of gay men. The results showed that roughly 1 gay man in 7 owes his sexual orientation to the fraternal birth order effect. They also showed that the effect of fraternal birth order would exceed all other causes of homosexuality in groups of gay men with 3 or more older brothers and would precisely equal all other causes in a theoretical group with 2.5 older brothers. Implications are suggested for the gay sib-pair linkage method of identifying genetic loci for homosexuality.

This is a preview of subscription content, access via your institution.

REFERENCES

  1. ACSF Investigators. (1992). AIDS and sexual behaviour in France. Nature, 360, 407-409.

    Google Scholar 

  2. Babinszki, A., Kerenyi, T., Torok, O., Grazi, V., Lapinski, R. H., & Berkowitz, R. L. (1999). Perinatal outcome in grand and great-grand multiparity: Effects of parity on obstetric risk factors. American Journal of Obstetrics and Gynecology, 181, 669-674.

    Google Scholar 

  3. Billy, J. O. G., Tanfer, K., Grady, W. R., & Klepinger, D. H. (1993). The sexual behavior of men in the United States. Family Planning Perspectives, 25, 52-60.

    Google Scholar 

  4. Blanchard, R. (1997). Birth order and sibling sex ratio in homosexual versus heterosexual males and females. Annual Review of Sex Research, 8, 27-67.

    Google Scholar 

  5. Blanchard, R. (2001). Fraternal birth order and the maternal immune hypothesis of male homosexuality. Hormones and Behavior, 40, 105-114.

    Google Scholar 

  6. Blanchard, R., Barbaree, H. E., Bogaert, A. F., Dickey, R., Klassen, P., Kuban, M. E., et al. (2000). Fraternal birth order and sexual orientation in pedophiles. Archives of Sexual Behavior, 29, 463-478.

    Google Scholar 

  7. 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.

    Google Scholar 

  8. Blanchard, R., & Bogaert, A. F. (1996b). Homosexuality in men and number of older brothers. American Journal of Psychiatry, 153, 27-31.

    Google Scholar 

  9. Blanchard, R., & Bogaert, A. F. (1997a). Additive effects of older brothers and homosexual brothers in the prediction of marriage and cohabitation. Behavior Genetics, 27, 45-54.

    Google Scholar 

  10. Blanchard, R., & Bogaert, A. F. (1997b). Drs. Blanchard and Bogaert reply [Letter to the editor]. American Journal of Psychiatry, 154, 137.

    Google Scholar 

  11. 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.

    Google Scholar 

  12. Blanchard, R., & Ellis, L. (2001). Birth weight, sexual orientation, and the sex of preceding siblings. Journal of Biosocial Science, 33, 451-467.

    Google Scholar 

  13. Blanchard, R., & Klassen, P. (1997). H-Y antigen and homosexuality in men. Journal of Theoretical Biology, 185, 373-378.

    Google Scholar 

  14. 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.

    Google Scholar 

  15. 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.

    Google Scholar 

  16. Bogaert, A. F., Bezeau, S., Kuban, M., & Blanchard, R. (1997). Pedophilia, sexual orientation, and birth order. Journal of Abnormal Psychology, 106, 331-335.

    Google Scholar 

  17. Dawson, D. V., Kaplan, E. B., & Elston, R. C. (1990). Extensions to sibpair linkage tests applicable to disorders characterized by delayed onset. Genetic Epidemiology, 7, 453-466.

    Google Scholar 

  18. Denman, D.W., & Schlesselman, J. J. (1983). Interval estimation of the attributable risk for multiple exposure levels in case-control studies. Biometrics, 39, 185-192.

    Google Scholar 

  19. 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.

    Google Scholar 

  20. Epstein, C. J., Smith, S., & Travis, B. (1980). Expression of H-Y antigen on preimplantation mouse embryos. Tissue Antigens, 15, 63-67.

    Google Scholar 

  21. Fay, R. E., Turner, C. F., Klassen, A. D., & Gagnon, J. H. (1989). Prevalence and patterns of same-gender sexual contact among men. Science, 243, 338-348.

    Google Scholar 

  22. Flanders, W. D., & Khoury, M. J. (1991). Extensions to methods of sib-pair linkage analyses. Genetic Epidemiology, 8, 399-408.

    Google Scholar 

  23. Flannery, K. A., & Liederman, J. (1994). A test of the immunoreactive theory of the origin of neurodevelopmental disorders: Is there an antecedent brother effect? Developmental Neuropsychology, 10, 481-492.

    Google Scholar 

  24. Gefeller, O. (1992). An annotated bibliography on the attributable risk. Biometrics Journal, 34, 1007-1012.

    Google Scholar 

  25. Green, R. (2000). Birth order and ratio of brothers to sisters in transsexuals. Psychological Medicine, 30, 789-795.

    Google Scholar 

  26. Greenland, G., & Robbins, J. M. (1988). Conceptual problems in the definition and interpretation of attributable fractions. American Journal of Epidemiology, 128, 1185-1197.

    Google Scholar 

  27. Greenwood, C. M., & Bull, S. B. (1999). Analysis of affected sib pairs, with covariates-with and without constraints. American Journal of Human Genetics, 64, 871-885.

    Google Scholar 

  28. Hamer, D. H., Hu, S., Magnuson, V. L., Hu, N., & Pattatucci, A. M. L. (1993). A linkage between DNA markers on the X-chromosome and male sexual orientation. Science, 261, 321-327.

    Google Scholar 

  29. Hu, S., Pattatucci, A. M. L., 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.

    Google Scholar 

  30. Johnson, A. M., Wadsworth, J., Wellings, K., Bradshaw, S., & Field, J. (1992). Sexual lifestyles and HIV risk. Nature, 360, 410-412.

    Google Scholar 

  31. Jones, M. B., & Blanchard, R. (1998). Birth order and male homosexuality: An extension of Slater's Index. Human Biology, 70, 775-787.

    Google Scholar 

  32. Krco, C. J., & Goldberg, E. H. (1976). H-Y (male) antigen: Detection on eight-cell mouse embryos. Science, 193, 1134-1135.

    Google Scholar 

  33. Laumann, E. O., Gagnon, J. H., Michael, R. T., & Michaels, S. (1994). The social organization of sexuality: Sexual practices in the United States. Chicago: University of Chicago Press.

    Google Scholar 

  34. Ott, J. (1991). Analysis of human genetic linkage (2nd ed.). Baltimore, MD: Johns Hopkins University Press.

    Google Scholar 

  35. Park, C. B. (1981). Attributable risk for recurrent events: An extension of Levin's measure. American Journal of Epidemiology, 113, 491-493.

    Google Scholar 

  36. Purcell, D. W., Blanchard, R., & Zucker, K. J. (2000). Birth order in a contemporary sample of gay men. Archives of Sexual Behavior, 29, 349-356.

    Google Scholar 

  37. Raschka, L. B. (1995). On older fathers [Letter to the editor]. American Journal of Psychiatry, 152, 1404.

    Google Scholar 

  38. Rice, G., Anderson, C., Risch, N., & Ebers, G. (1999). Male homosexuality: Absence of linkage to microsatellite markers at Xq28. Science, 284, 665-667.

    Google Scholar 

  39. 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.

    Google Scholar 

  40. Schimmel, M. S., Eidelman, A. I., Zadka, P., Kornbluth, E., & Hammerman, C. (1997). Increased parity and risk of trisomy 21: Review of 37 100 live births. British Medical Journal, 314, 720-721.

    Google Scholar 

  41. Shelton, J. A., & Goldberg, E. H. (1984). Male-restricted expression of H-Y antigen on preimplantation mouse embryos. Transplantation, 37, 7-8.

    Google Scholar 

  42. Tuomilehto, J., Podar, T., Tuomilehto-Wolf, E., & Virtala, E. (1995). Evidence for importance of gender and birth cohort for risk of IDDM in offspring of IDDM parents. Diabetologia, 38, 975-982.

    Google Scholar 

  43. Walter, S. D. (1978). Calculation of attributable risk from epidemiological data. International Journal of Epidemiology, 7, 175-182.

    Google Scholar 

  44. White, K. L., Anderson, G. B., & BonDurant, R. H. (1987). Expression of a male-specific factor on various stages of preimplantation bovine embryos. Biology of Reproduction, 37, 867-873.

    Google Scholar 

  45. White, K. L., Lindner, G. M., Anderson, G. B., & BonDurant, R. H. (1983). Cytolytic and fluorescent detection of H-Y antigen on preimplantation mouse embryos. Theriogenology, 19, 701-705.

    Google Scholar 

  46. Williams, T. J., Pepitone, M. E., Christensen, S. E., Cooke, B. M., Huberman, A. D., Breedlove, N. J., et al. (2000). Finger-length ratios and sexual orientation. Nature, 404, 455-456.

    Google Scholar 

  47. Yang, Q., & Khoury, M. J. (1997). Evolving methods in genetic epidemiology. III: Gene-environment interaction in epidemiologic research. Epidemiologic Reviews, 19, 33-43.

    Google Scholar 

  48. Zucker, K. J., Green, R., Coates, S., Zuger, B., Cohen-Kettenis, P. T., Zecca, G. M., et al. (1997). Sibling sex ratio of boys with gender identity disorder. Journal of Child Psychology and Psychiatry, 38, 543-551.

    Google Scholar 

Download references

Author information

Affiliations

Authors

Rights and permissions

Reprints and Permissions

About this article

Cite this article

Cantor, J.M., Blanchard, R., Paterson, A.D. et al. How Many Gay Men Owe Their Sexual Orientation to Fraternal Birth Order?. Arch Sex Behav 31, 63–71 (2002). https://doi.org/10.1023/A:1014031201935

Download citation

  • attributable fraction
  • attributable risk
  • birth order
  • homosexuality
  • H-Y antigen
  • logistic regression
  • sexual orientation
  • sib-pair linkage method