CHICAGO (UPI) -- Lesbian women are at least three times as likely as heterosexual women to have a sister who is also a lesbian, suggesting female homosexuality may run in families, researchers said Monday.
In a study being published in the American Journal of Psychiatry, Northwestern University researchers interviewed 85 homosexual women and 79 heterosexual women who responded to advertisements seeking study participants.
Between 12 percent and 35 percent of the sisters of the homosexual women were homosexual while only between 2.4 percent and 13.8 percent of the sisters of the heterosexual women were homosexual, the researchers found.
``The study shows a familial relationship to the causes of female homosexuality and paves the way for further research to determine whether sexual orientation results from genetics or family environment,'' said J. Michael Bailey, a psychologist who led the study.
The study could not distinguish between possibly inherited factors and environmental factors, such as religion, he said. Bailey added he had begun another, larger study involving female twins examining the same issue.
Previous studies have suggested that inherited factors may play a role in a person's sexual orientation.
Bailey said he was surprised that the study found that homosexual women were more likely to have a homosexual sister than a homosexual brother.
``What we found was that lesbians were twice as likely to have a gay sister as a gay brother, which is particularly interesting given that the rate of female homosexuality in the general population is probably half that of males,'' he said.
``If anything, we would have expected them to have more gay brothers. That suggests to me that there may be separate factors that cause male and female homosexuality,'' Bailey said.