Genetic clues to female homosexuality.

Genes substantially influence the development of homosexuality among women, according to a preliminary study of female twins and adoptive sisters.

"Genes don't account for all individual differences in sexual orientation, but pairs of female identical twins report homosexuality significantly more often than pairs of female fraternal twins or biologically unrelated sisters," says psychiatrist Richard C. Pillard of the Boston University School of Medicine, who conducted the study with psychologist J. Michael Bailey of Northwestern University in Evanston, Ill.

The two researchers presented their findings last week at the annual meeting of the American Psychological Association in Washington, D.C.

Pillard and Bailey recently used the same research strategy to identify a moderate to strong genetic influence on male homosexuality (SN: 1/4/92, p.6).

The researchers recruited homosexual women with an identical, fraternal, or adoptive sister through advertisements in lesbian, gay, and feminist publications throughout the United States. They then interviewed pairs of sisters, usually over the telephone. Additional interviews with relatives of most participants revealed a high degree of agreement within families about each woman's sexual orientation.

A total of 115 twin pairs, about equally divided between identical and fraternal twins, participated in the study, as did 32 pairs of adoptive sisters. Identical twins share all the same genes, whereas fraternal twins share some of the same genes.

Homosexuality or bisexuality occurred among both sisters in nearly half of the identical twin pairs, Bailey asserts. That figure drops to about one-quarter of the fraternal twin pairs and one in six adoptive-sister pairs.

Using estimates of female homosexuality in the general population and the assumption that several genes influence sexual orientation, Pillard and Bailey calculate that genes account for half of the individual differences in women's sexual orientation. Scientists refer to this as a heritability estimate.

Heritability studies, including those concerning sexual preferences, have recently sparked considerable controversy (SN: 12/7/91, p.376).

Some investigators consider heritability a hazy statistic that can change depending on the phrasing of interview questions and the way in which researchers recruit volunteers. Moreover, heritability estimates may tap into any number of genetically influenced traits, critics add.

Bailey acknowledges the problems in interpreting individual heritability studies and says independent research teams should conduct further twin studies of male and female homosexuality He also notes an important problem with the new study: Lesbians with a lesbian twin may have been more willing to volunteer for the research than lesbians with a heterosexual twin.

However, heritability estimates for male sexual orientation recently reported by University of Minnesota researchers, based on identical twins reared apart since shortly after birth, closely match those calculated by Pillard and Bailey. Current heritability figures appear to reflect multiple genes involved somehow in male and female sexual orientation, Bailey contends.

Genes act in concert with the biological, psychological, and social environment, Pillard cautions. For example, he cites the case of two male identical twins separated at birth, raised in different families, and reunited as adults by the Minnesota researchers.

These men displayed striking similarities in virtually all aspects of their lives. They even discovered a shared proclivity for wearing leather garments during sex and employing various devices to heighten stimulation. Yet one man was gay, the other heterosexual.