Bailey and Pillard recruited 110 pairs of male twins, half identical, half fraternal. In each case, they knew that one twin was gay. They then sent a questionnaire to the other brother in each pair, to determine his sexual orientation. Among the identical twins, 52 percent of the brothers were gay. Among the fraternals, the number was 22 percent, high enough above the background population rate to suggest that there was something distinctive in those families. The researchers found a very similar pattern with lesbians.
52% of identical (monozygotic) twins of homosexual men were likewise homosexual
22% of fraternal (dizygotic) twins were likewise homosexual
11% of adoptive brothers of homosexual men were likewise homosexual
J.M. Bailey and R.C. Pillard, “A genetic study of male sexual orientation,” Archives of General Psychiatry, vol. 48:1089-1096, December 1991.
Bailey and Pillard (1993): occurrence of homosexuality among sisters
48% of identical (monozygotic) twins of homosexual women were likewise homosexual (lesbian)
16% of fraternal (dizygotic) twins were likewise homosexual
6% of adoptive sisters of homosexual women were likewise homosexual
Bailey, J. M. and D. S. Benishay (1993), “Familial Aggregation of Female Sexual Orientation,” American Journal of Psychiatry 150(2): 272-277.
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