How many gay men owe their sexual orientation to fraternal birth order?
Level 4 - case-series / case-control
Epidemiological statistical modeling applied to observational data
PubMed 11910793 · doi:10.1023/a:1014031201935
What was done
The authors developed generalized formulations of attributable fraction and population attributable fraction to handle multi-level exposures (number of older brothers) using odds ratios from logistic regression. They applied this noniterative method to an unspecified large contemporary sample of gay men to estimate the proportion of male homosexuality attributable to the fraternal birth order effect, based on the established ~33% increase in odds per additional older brother.
What was found
Roughly 1 in 7 gay men owes his sexual orientation to the fraternal birth order effect. In subgroups of gay men with 3 or more older brothers, the fraternal birth order effect exceeded all other causes combined, and equaled all other causes at a theoretical threshold of 2.5 older brothers.
Why it matters
This study provides a quantitative estimate of the population-level impact of the fraternal birth order effect, offering a statistical method to evaluate how birth order interacts with other potential genetic or biological factors in male sexual orientation.
Limits
The abstract does not report the exact sample size, demographic characteristics, or recruitment source for the analyzed cohort. The findings depend on causal assumptions required for attributable fraction calculations and the precision of the baseline ~33% odds ratio per older brother.
Cited by
- supports Statistical modeling estimates that approximately one in seven gay men are homosexual due to the fraternal birth order effect.
- supports Each additional older biological brother increases the probability of a male being homosexual by approximately 33%.