A family history study of male sexual orientation using three independent samples

Available evidence suggests that male homosexuality is both familial and so mewhat heritable and that some cases may be caused by an X-linked gene. How ever, most studies have recruited subjects in a relatively unsystematic man ner, typically via advertisements, and hence suffer from the potential meth odological flaw of ascertainment bias due to volunteer self-selection. In t he present study we assessed the familiality of male homosexuality using tw o carefully ascertained samples and attempted to replicate findings consist ent with X-linkage in three samples. The percentage of siblings of the prob ands rated as either homosexual or bisexual, with a high degree of certaint y, ranged from 7 to 10% for brothers and 3 to 4% for sisters. These estimat es are higher than recent comparable population-based estimates of homosexu ality, supporting the importance of familial factors for male homosexuality . Estimates of lambda(s) for male homosexuality ranged from 3.0 to 4.0. Non e of the samples showed a significantly greater proportion of maternal than paternal homosexual uncles or homosexual male maternal first cousins. Alth ough our results differed significantly with those of some prior studies, t hey do not exclude the possibility of moderate X-linkage for male sexual or ientation.