Examining the Fraternal Birth Order Effect and Sexual Orientation: Insights from an East European Population.
Level 4 - case-series / case-control
Cross-sectional case-control observational study
PubMed 38869747 · doi:10.1007/s10508-024-02892-8
What was done
This preregistered cross-sectional study evaluated the fraternal birth order effect (FBOE) on sexual orientation and anal-erotic role orientation (AERO) in Czechia and Slovakia. The sample included 693 gay men, 843 straight men, 265 lesbian women, and 331 straight women. Associations between maternal and nonmaternal older brothers and sexual orientation were analyzed using conventional logistic regression models and an alternative parameterization method.
What was found
Maternal older brothers were significantly associated with homosexual orientation in gay men (OR = 1.35) and lesbian women (OR = 1.71). Nonmaternal older brothers showed no significant effect on younger brothers' sexual orientation. The FBOE was not exclusive to gay men with receptive AERO. Mothers of gay men had lower offspring counts than mothers of straight men.
Why it matters
This study provides evidence of the fraternal birth order effect in an East European population, extends observations to lesbian women, and challenges previous findings restricting the effect to gay men with receptive anal-erotic role orientation.
Limits
The study is observational and relies on self-reported familial data from Czechia and Slovakia. Confidence intervals and exact p-values are not reported in the abstract, and the authors note sensitivity to the statistical parameterization methods used to estimate familial influences.