Research has shown that male homosexuality tends to cluster in families and that homosexual males have, on average, a greater number of older brothers than do heterosexual males. This study investigated whether the former, between-families effect and the latter, within-families effect are additive. The subjects were 717 full siblings over age 40 reported by 343 heterosexual and homosexual male probands examined in Southern Ontario in 1994-1995. The sibling's history of legal marriage or cohabitation in a heterosexual relationship was taken as a proxy variable for sexual orientation. There were no significant findings for the female siblings. As expected, the never-married male siblings were more likely to come from the sibships of the homosexual probands, and they had a greater average number of older brothers. A bootstrapped logistic regression analysis showed that an additive model best explained the male siblings' data. The results suggest that the familial aggregation of male homosexuality cannot be explained by the birth order effect and that older brothers and family membership reflect separate influences on sexual orientation or sexual orientation-correlated behavior.
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