TWINS

"Richard Pillard of the Boston University School of Medicine and Michael Bailey of Northwestern University in Evanston, Illinois collaborated on studies of twins and homosexuality. They compared the percentage of male siblings who were both gay with the amount of genetic material they shared, in an effort to find evidence for a genetic basis of homosexuality. Their most frequently cited study was published in 1991, 'A Genetic Study of Male Sexual Orientation.'

Bailey and Pillard were interested in pursuing a biological explanation of sexual orientation for a variety of reasons. They were persuaded by some hormonal research, other twin studies that had been performed, and by a belief that proving a biological component to sexual orientation could result in more positive attitudes toward gay, lesbian and bisexual people.

Bailey and Pillard recruited subjects for their study through advertisements in gay newspapers in cities across the Midwest and the Southwest. They looked for gay men, 18 years or older that had a twin, or an adoptive or otherwise 'genetically unrelated' brother. They compared the sexual orientation of the brothers and looked for childhood gender nonconformity or 'sissy-type' behavior in boys, which is often considered a common indicator of homosexuality.

Bailey and Pillard studied:

56 pairs of identical twins

54 pairs of fraternal twins

142 non-twin brothers of twins

57 pairs of adoptive brothers

Among twin pairs where one twin was gay, Bailey and Pillard found that:

52 percent of the identical twins were both gay

22 percent of the fraternal twins were both gay

9 percent of the non-twin brothers were both gay

11 percent of the adopted, or genetically unrelated brothers were both gay

Bailey and Pillard concluded that the patter of identical twins having a higher percentage rate than other siblings is consistent with the idea that sexual orientation is genetically influenced.

The findings also suggest that factors outside of genetics can guide sexual orientation:

Fraternal twins and non-twin brothers share the same percentage of genetic material --- 50 percent. If sexual orientation, however, were to have a strong genetic basis, one would expect them to share the same percentage of concordance, which they did not.

Almost half (48%) of the identical twins did not share the same sexual orientation.

Some critiques include:

Bailey and Pillard share a concern with others that men with gay brothers would be more likely to respond to an advertisement to participate in a study. Gay men may not approach a brother to participate in a study, if the heterosexual brother is not comfortable with his gay brother's sexuality.

Respondents were classified only as heterosexual or non-heterosexual -- without regard to a range of sexual behavior and orientations.

Bailey and Pillard's methodology has been described as 'painstaking' and they are careful to draw modest conclusions from their study. While their study is often used to further the idea of a genetic marker, Bailey and Pillard drew no such conclusions. Once again, they concluded that their findings are consistent with the idea that homosexuality is genetically influenced. They did not locate a gene, nor identify how genetics could influence sexual orientation. Bailey and Pillard's evidence, moreover, can be used to argue that genes are not the determining factor in sexual orientation.