Biological versus nonbiological older brothers and men's sexual orientation.
Level 4 - case-series / case-control
Comparative observational / case-control study comparing homosexual and heterosexual men
PubMed 16807297 · doi:10.1073/pnas.0511152103
What was done
Four samples of homosexual and heterosexual men (total n = 944), including a cohort raised in nonbiological and blended families (adoptees, step-siblings, or half-siblings), were analyzed to evaluate whether prenatal factors or postnatal/rearing environments account for the fraternal birth-order effect on male sexual orientation.
What was found
The abstract reports no numerical values, odds ratios, or confidence intervals. It states that only the number of biological older brothers predicted men's sexual orientation, whereas nonbiological older brothers and other sibling characteristics showed no predictive association, regardless of time spent reared together.
Why it matters
By differentiating biological from nonbiological siblings and accounting for rearing time, the findings demonstrate that the fraternal birth-order effect is unlikely to be driven by postnatal psychosocial factors, pointing instead toward prenatal biological mechanisms.
Limits
The abstract reports no quantitative effect sizes or statistical test statistics. The design relies on observational and retrospective sibship reports and does not directly measure candidate prenatal biological factors (such as maternal antibodies or hormonal markers).
Cited by
- supports Older stepbrothers do not increase a male's odds of being gay, whereas biological older brothers from the same mother increase the odds regardless of whether they were raised together.