Twin and family tree studies are based on the principle that genetically influenced traits run in families. The first modern study on the patterns of homosexuality within families was published in 1985 by Richard C. Pillard and James D. Weinrich of Boston University. Since then, Þve other systematic studies on the twins and siblings of gay men and lesbians have been reported.
The pooled data for men show that about 57 percent of identical twins, 24 percent of fraternal twins and 13 percent of brothers of gay men are also gay. For women, approximately 50 percent of identical twins, 16 percent of fraternal twins and 13 percent of sisters of lesbians are also lesbian. When these data are compared with baseline rates of homosexuality, a good amount of family clustering of sexual orientation becomes evident for both sexes. In fact, J. Michael Bailey of Northwestern University and his co-workers estimate that the overall heritability of sexual orientation -that proportion of the variance in a trait that comes from genes- is about 53 percent for men and 52 percent for women. (The family clustering is most obvious for relatives of the same sex, less so for male-female pairs.)
To evaluate the genetic component of sexual orientation and to clarify its mode of inheritance, we need a systematic survey of the extended families of gay men and lesbians. One of us (Hamer), Stella Hu, Victoria L. Magnuson, Nan Hu and Angela M. L. Pattatucci of the National Institutes of Health have initiated such a study. It is part of a larger one by the National Cancer Institute to investigate risk factors for certain cancers that are more frequent in some segments of the gay population.
Hamer and his colleagues initial survey of males confirmed the sibling results of Pillard and Weinrich. A brother of a gay man had a 14 percent likelihood of being gay as compared with 2 percent for the men without gay brothers. (The study used an unusually stringent deÞnition of homosexuality, leading to the low average rate.) Among more distant relatives, an unexpected pattern showed up: maternal uncles had a 7 percent chance of being gay, whereas sons of maternal aunts had an 8 percent chance. Fathers, paternal uncles and the three other types of cousins showed no correlation at all.
Although this study pointed to a genetic component, homosexuality occurred much less frequently than a single gene inherited in simple Mendelian fashion would suggest. One interpretation, that genes are more important in some families than in others, is borne out by looking at families having two gay brothers. Compared with randomly chosen families, rates of homosexuality in maternal uncles increased from 7 to 10 percent and in maternal cousins from 8 to 13 percent. This familial clustering, even in relatives outside the nuclear family, presents an additional argument for a genetic root to sexual orientation.
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