Researchers say they have found the first strong evidence of a physical difference between lesbians and straight women - a finding that the inner ears of gay women work more like those of men.
The discovery adds new support to the theory that sexual orientation may be predisposed before birth.
The origin of homosexuality has long been a matter of contention. Some believe it to be a matter of choice, but others - including many gay people - say it is not choice but biology.
Previous research has found that two parts of the male brain are different in gay and heterosexual men. Other studies have found that some genes differ between gay and straight men.
In the study to be published today in the Proceedings of the National Academy of Sciences, researchers at the University of Texas, Austin, said they found the inner ears of female homosexuals have undergone “masculinization,” probably from hormone exposure before birth.
“Their auditory centers have been masculinized and the presumption is that so have the sites in the brain that direct sexual preference,” said Dennis McFadden, the lead author of the study.
It has yet to be proven, he said, that there is a specific site in the brain that directs women to be lesbians.
Dr. Michael Bailey of Northwestern University, said the research is “compelling” and may be “consistent with the biological origin of lesbianism.”
“The most likely interpretation,” he said, “is that this represents some kind of effect of early hormones on the developing fetus.”
Bailey cautioned, however, that the research will not be accepted as valid until others replicate the experiment.
Sandra Witelson, an expert on brain anatomy and sexual orientation at McMaster University in Ontario, Canada, said the study supports the theory that lesbianism may be “related to early factors in brain development.”