Quantitative and theoretical analyses of the relation between older brothers and homosexuality in men.
Level 3 - non-randomized controlled study
Meta-analysis of observational studies combined with theoretical modeling (by design analogy, not clinical CEBM)
PubMed 15302549 · doi:10.1016/j.jtbi.2004.04.021
What was done
Meta-analysis of aggregate data from 14 samples comprising 10,143 male subjects examining the association between sibling composition (older brothers, older sisters, younger brothers, younger sisters) and male sexual orientation, alongside theoretical evaluation of prenatal mechanisms.
What was found
Homosexuality in human males was predicted by higher numbers of older brothers, but not by higher numbers of older sisters, younger brothers, or younger sisters. This relation held only for males. Calculations assuming causality estimated the proportion of homosexual men attributing their sexual orientation to fraternal birth order at 15% in one study and 29% in another. Specific odds ratios and confidence intervals were not reported in the abstract.
Why it matters
This synthesis provides aggregate evidence for the fraternal birth order effect and formalizes the maternal immune hypothesis as a candidate biological mechanism for prenatal sexual differentiation.
Limits
The abstract lacks numerical effect sizes, test statistics, and confidence intervals. The primary data are observational and retrospective, and the proposed maternal immune mechanisms (anti-male antibodies, cytokine transfer, placental immune reactions) are theoretical rather than directly measured.
Cited by
- supports The probability of a male being gay increases with the number of older biological brothers he has.
- supports Statistical modeling estimates that approximately one in seven gay men are homosexual due to the fraternal birth order effect.
- supports Having older or younger sisters, or younger brothers, does not change a male's odds of being gay.