• We recruited 51 predominantly homosexual and 50 predominantly heterosexual men as index subjects for a family study of sexual orientation. The sexual orientation of siblings (115 sisters and 123 brothers) was ascertained in two ways: (1) via interview with the index subjects and (2) via interview of and/or mailed questionnaire to the siblings themselves. Index subjects' knowledge of the sexual orientation of their siblings was, for the most part, quite accurate. Moreover, heterosexual index men had about as many homosexual brothers as would be predicted given national prevalence figures for homosexuality, but homosexual index men had about four times as many homosexual brothers, although in both cases most brothers were heterosexual. There was no significant difference between the index groups in number of homosexual sisters. We conclude that there is a significant familial component to male homosexuality.
Pillard RC, Weinrich JD. Evidence of Familial Nature of Male Homosexuality. Arch Gen Psychiatry. 1986;43(8):808–812. doi:10.1001/archpsyc.1986.01800080094012