Proportion of homosexual men who owe their sexual orientation to fraternal birth order: An estimate based on two national probability samples.
Level 4 - case-series / case-control
Secondary analysis of cross-sectional survey data
PubMed 14994314 · doi:10.1002/ajhb.20006
What was done
Calculated the population attributable fraction (PAF) of male homosexuality attributable to fraternal birth order (number of older brothers) under the assumption of a causal relationship. The analysis used combined archival data from probability surveys of sexual behavior in the UK and the USA, including 2,256 heterosexual men and 71 homosexual men.
What was found
The estimated PAF of male homosexuality attributable to fraternal birth order was 28.6% (95% CI, 14.8% to 48.0%). This range encompassed a prior estimate of 15.1% from a Canadian sample.
Why it matters
It provides a quantitative population-level estimate suggesting fraternal birth order may account for a non-negligible minority of male homosexuality cases, assuming the observed statistical link is causal.
Limits
The calculation relies entirely on an unproven assumption of causality. The subgroup of homosexual men was small (n = 71), leading to wide confidence intervals. The hypothesized maternal immune mechanism was not directly measured.
Cited by
- supports Statistical modeling estimates that approximately one in seven gay men are homosexual due to the fraternal birth order effect.