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Proportion of homosexual men who owe their sexual orientation to fraternal birth order: An estimate based on two national probability samples
Author(s) -
Blanchard Ray,
Bogaert Anthony F.
Publication year - 2004
Publication title -
american journal of human biology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.559
H-Index - 81
eISSN - 1520-6300
pISSN - 1042-0533
DOI - 10.1002/ajhb.20006
Subject(s) - birth order , sexual orientation , homosexuality , demography , population , heterosexuality , psychology , male homosexuality , medicine , social psychology , sociology , men who have sex with men , immunology , syphilis , human immunodeficiency virus (hiv) , psychoanalysis
Homosexuality in men correlates with an individual's number of older brothers, greater numbers of older brothers being associated with a greater probability of homosexuality. There are reasons to believe that this relationship is causal rather than merely statistical, that is, that older brothers produce the increase in the probability of homosexuality for later‐born males. It is possible, under this assumption, to estimate the proportion of homosexual men who can attribute their sexual orientation to their birth order among their brothers (fraternal birth order). This statistic, the population attributable fraction (PAF), was computed on the combined archival data of 2,256 heterosexual and 71 homosexual men examined in survey studies of sexual behavior in the UK and the USA. The PAF was 28.6%, with 95% confidence limits of 14.8% and 48.0%. These limits encompass the PAF of 15.1% previously estimated with a Canadian sample. The results indicate that the proportion of homosexual men whose sexual orientation is attributable to fraternal birth order constitutes a minority, but not a negligible minority, of all homosexual men. The fraternal birth order effect may reflect the progressive immunization of some mothers to Y‐linked antigens by each succeeding male fetus, and the concomitantly increasing effects of antimale antibodies on the sexual differentiation of the brain in each succeeding male fetus. Am. J. Hum. Biol. 16:151–157, 2004. © 2004 Wiley‐Liss, Inc.