WASHINGTON -- 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 Tuesday 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, however, that there is a specific site in the brain that directs women to be lesbians, he said.
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."
The inner ear difference between homosexual and heterosexual women was detected using a test that measures the function of the cochlea, a key sound amplifier in the inner ear, said McFadden, a professor of experimental psychology.
The cochlea amplifier in women is more sensitive than that of men, giving women an increased ability to detect very soft sounds in a very quiet room. The test measures a very slight sound that the cochlea makes when responding to a soft clicking sound.
Females, with their more sensitive cochlea, respond more powerfully to this test than do men, said McFadden. This is true even among infants.
To test for differences between the sexes, the researchers recruited more than 200 adults divided into four groups: homosexual women and men, and heterosexual women and men. Some from each of the four groups were later identified as bisexual. The sexual orientation of the subjects was determined by questionnaire.
The results, McFadden said, indicated that lesbians had click-responses that were significantly weaker than those of heterosexual women. The signal was weaker still for all males, both gay and straight. Bisexual men and women were in the middle, although McFadden said there were not enough of these to draw firm conclusions.
What is clear, he said, is that there is a dramatic difference in the development of the hearing systems of lesbians and of heterosexual women. It also is known that development of the inner ear is affected before birth by androgens, a male hormone.
Androgens, said McFadden, may also "alter the brain centers that produce sexual orientation." But he said researchers have yet to find a brain structure that determines sexual orientation in women.