A method yielding comparable estimates of the fraternal birth order and female fecundity effects in male homosexuality.
Level 4 - case-series / case-control
Pooled retrospective analysis of archival observational and case-control dataset samples.
PubMed 32183625 · doi:10.1098/rspb.2019.2907
What was done
The authors applied a method proposed by Khovanova to estimate the fraternal birth order effect and female fecundity effect in male homosexuality using a common metric. The method restricts analysis to families with one or two sons and excludes daughters. The performance of this method was evaluated using archived data from 10 studies, comprising 14 samples totalling 5,390 homosexual and heterosexual participants.
What was found
An increase from zero to one older brother was associated with a 38% increase in the odds of homosexuality. By contrast, an increase from zero to one younger brother was not associated with any increase in the odds of homosexuality (exact numerical values for the null effect were not reported in the abstract).
Why it matters
This study provides evidence supporting the maternal immune hypothesis for male homosexuality via older brothers while failing to find evidence for the balancing selection hypothesis via increased maternal fecundity.
Limits
The analytical method is restricted strictly to families with one or two sons and excludes daughters, limiting generalizability across family structures. The study relies on archival data from earlier studies, and the abstract does not report confidence intervals, exact effect sizes for female fecundity, or demographic characteristics.
Cited by
- supports A baby boy with no older brothers has about a 2% baseline probability of being gay, and each older brother increases the odds by roughly 33% (to approximately 2.6% for one older brother and 3.5% for two older brothers).