Scientists Monday reported the first strong physiological evidence that lesbian and bisexual women may be biologically different from heterosexual women.
The researchers at the University of Texas in Austin found that, compared with heterosexual women, the hearing of homosexual and bisexual women tends to be a bit more like that of men.
The findings suggest that homosexual and bisexual women develop in subtly different ways than heterosexual women. Therefore, their brains may also form differently, accounting for their sexuality, the researchers said.
“It’s an indication that other brain sites have also been masculinized,” said Dennis McFadden, a professor of experimental psychology who led the study in today’s Proceedings of the National Academy of Sciences.
The study is the latest to come from the controversial investigation into whether homosexuality has a biological basis. Previous research has found, for example, that a part of the brain believed involved in sexuality appears smaller in homosexual men than in heterosexual men. But no such findings have been reported about homosexual or bisexual women.
Like much of the previous research, the new findings immediately sparked criticism.
“All of this research perpetuates stereotypes - in this case that lesbians are more masculine than heterosexual women,” said John De Cecco, a professor of psychology and human sexuality at San Francisco State University.
But Sandra Witelson of McMaster University, who studies the relationship of brain anatomy to sexual orientation, said, “The results support the theory that differences in the central nervous system exist between homosexual and heterosexual individuals and that the differences are possibly related to early factors in brain development.”
Researchers have long known that the inner ear produces “click-evoked otoacoustic emissions” - imperceptible echoes made in response to weak clicking sounds. Used to diagnose potential hearing problems in infants, these emissions generally are slightly louder in women than in men for reasons that remain unclear.
Women who have twin brothers tend to produce weaker echoes, suggesting that exposure to hormones from male twins in the womb may alter female development in subtle ways that make them slightly more “masculine” than other women.
So McFadden and his colleagues inserted into the ears of 237 subjects tiny acoustic equipment that produced a very low clicking sound and measured the echo. As a group, the emissions of 61 homosexual and bisexual women were weaker than those of 57 heterosexual women, though still stronger than men, the researchers found. No differences were found between homosexual and bisexual women, or between homosexual and heterosexual men.
“The interpretation is that the auditory system has been masculinized along with whatever brain sites are controlling sexual preference,” McFadden said in a telephone interview.
Low levels of male hormones, which circulate in all pregnant women, could masculinize some girls in the womb, perhaps affecting parts of their anatomy, such as those involved in hearing and sexual preference, he said.
Critics, such as De Cecco, were skeptical. “It’s unbelievable that this stuff can have the sponsorship of prestigious scientific organizations,” said De Cecco, editor of the Journal of Homosexuality. “Homosexuality is not a biological characteristic. It’s a psychological, social, personal, cultural thing.”
McFadden stressed that the findings were for groups of women overall and could not differentiate the sexuality of individual women.