Genetic Component Found in Lesbianism, Study Says : Science: Research on twins shows that environment also plays a role, although unclear, in sexual orientation.
The first large study of female sexual orientation has found a strong genetic component to homosexuality, researchers at Boston University and Northwestern University report today.
Previous studies of male homosexuals have obtained the same result, but those studies have been highly criticized because the groups examined did not include women.
Some scientists have questioned whether genetics would play as strong a role in female homosexuality because the biological mechanisms proposed seemed to be unique to males.
But the new study, published in the Archives of General Psychiatry, reports that when one of a set of identical twins was a lesbian, 48% of the second twins were also lesbians. In contrast, only 16% of non-identical twin sisters of lesbians were gay and only 6% of adoptive sisters of lesbians were gay.
But the fact that not all the identical twins were gay indicates that environment also plays a role, said Northwestern psychologist J. Michael Bailey. “It presently is unclear what those environmental factors are, though possible candidates include (birth stresses) affecting twins differently or differential treatment by parents,” he said.
“It’s a well designed, carefully controlled study--really a terrific study,” said molecular biologist Dean Heimer of the National Institutes of Health. The most important aspect, he said, “is that they have begun to study sexual orientation in women, which is good because so much research has concentrated on men.”
While commending the study in general, Dr. Kenneth Kendler, an epidemiologist at the Medical College of Virginia, said: “To say it is a hard fact that can be permanently entered in our scientific database is just not tenable.”
Kendler, who has conducted studies of twins, said the researchers may have unintentionally recruited a skewed population of women who were more likely to have gay sisters. “It is an important first step along the road, however,” he added.
Reaction from the lesbian community was restrained. While hailing the fact that researchers are finally studying women because “it is awful to be invisible,” Liz Hendrickson of the National Center for Lesbian Rights in San Francisco said she found some implications of such studies troubling. She said she worries that recognition and protection of the lesbian community will be enacted only if lesbianism proves to be genetically based.
“Lesbianism needs to be recognized and protected as a valid associational and lifestyle choice, whether it is genetically based or not,” she said.
The last two years have produced a minor explosion of research on the genetic basis of sexual orientation. Two studies have shown small, but distinct, differences between specific areas of the brain in gay and heterosexual males. A third study, conducted by Bailey and Dr. Richard C. Pillard, a psychiatrist at the Boston University School of Medicine, showed the presence of a strong genetic link among male homosexuals.
But few studies have examined lesbians, in part, researchers say, because it is often difficult to identify lesbians in a population. Also, there are fewer of them, perhaps representing only 1.5% to 5% of the population by various estimates, contrasted with 4% to 10% for males.
Bailey and psychologist Deana Benishay, now at USC, reported last month that lesbianism does run in families, but that study was unable to show whether the cause was genetic or environmental. The new study was designed to distinguish between the two possible influences.
As in their previous study on male homosexuals, Bailey and Pillard advertised in gay publications to locate female homosexuals, especially twins, who had at least one female sibling, no matter what the latter’s orientation. They carefully evaluated that orientation, primarily through personal interviews.
They located 71 sets of identical twins in which at least one twin was gay. Identical twins have the same genetic makeup and, if female homosexuality has a genetic basis, many of the second twins should also be gay. That is what they found: In 48% of the cases, the identical twin sister was also gay, compared to 52% of twin brothers in the previous study.
They also found 37 sets of fraternal twin sisters, who share some of their genes, but not all. Among this group, 16% of sisters were gay, compared to 22% of brothers.
Finally, they identified 35 adoptive sisters who were raised in the same household but shared no genes. Only 6% of this group was gay, compared to 11% among males.
The largest potential problem with the study, authors and critics agree, is that lesbians with a twin sister who was gay might have been more likely to respond to the advertisements. “Who knows what was going through their minds when they responded,” Kendler said. But a statistical analysis, Bailey argued, indicates that the results are valid even if such twins were two or three times as likely to respond.
Bailey and Pillard now intend to look at the identical-twin sets--male and female--in which only one is gay to try to identify the environmental factors that influenced sexual orientation. Other researchers are attempting to identify specific genes.
“Whatever the factors are, whether it is genes or something else, we think it is likely they will be different in males than in females, which emphasizes why it is important to study both,” Heimer said.
