Homosexuality in men and number of older brothers.
Level 4 - case-series / case-control
Matched case-control study
PubMed 8540587 · doi:10.1176/ajp.153.1.27
What was done
A matched case-control design was used to evaluate whether male sexual orientation is specifically related to the number of older brothers versus siblings in general or parental age. Participants included 302 homosexual men matched by birth year with 302 heterosexual controls (the abstract text inadvertently states 302 heterosexual men were matched with an equal number of heterosexual men). Participants completed an anonymous, self-administered questionnaire detailing biodemographic information and family background. Predictors of sexual orientation were evaluated using logistic regression analysis.
What was found
Homosexuality was positively correlated with the proband's number of older brothers. Each additional older brother increased the odds of homosexuality by 33%. Homosexuality was not correlated with the number of older sisters, younger brothers, younger sisters, or parental age at the time of the proband's birth.
Why it matters
This paper demonstrates that the fraternal birth order effect on male sexual orientation is specific to older brothers rather than overall sibship size, older sisters, or parental age.
Limits
The abstract contains an apparent typo stating heterosexual men were matched with heterosexual men. Data rely on self-reported retrospective questionnaires. Confidence intervals, exact p-values, sample recruitment details, and potential confounding variables are not reported in the abstract.
Cited by
- supports Gay men have an older brother to older sister ratio of roughly 140 to 100, compared to approximately 105 to 100 in heterosexual men.
- supports The fraternal birth order effect on male sexual orientation was first identified and reported by Ray Blanchard.