Archives of Sexual Behavior

, Volume 31, Issue 1, pp 63–71

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


  • James M. Cantor
    • Centre for Addiction and Mental Health
  • Ray Blanchard
    • Centre for Addiction and Mental Health
    • Department of Psychiatry, Faculty of MedicineUniversity of Toronto
  • Andrew D. Paterson
    • Department of Psychiatry, Faculty of MedicineUniversity of Toronto
    • Department of GeneticsThe Hospital for Sick Children
  • Anthony F. Bogaert
    • Departments of Psychology and Community Health SciencesBrock University

DOI: 10.1023/A:1014031201935

Cite this article as:
Cantor, J.M., Blanchard, R., Paterson, A.D. et al. Arch Sex Behav (2002) 31: 63. doi:10.1023/A:1014031201935


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.

attributable fractionattributable riskbirth orderhomosexualityH-Y antigenlogistic regressionsexual orientationsib-pair linkage method

Copyright information

© Plenum Publishing Corporation 2002