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Pointing the Finger at Androgen as a Cause of Homosexuality

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WebMD Health News

March 29, 2000 (Atlanta) -- There is more evidence that what happens inside the womb may affect the future sexual orientation of a fetus, according to a "brief communication" appearing in the March 30 issue of the journal Nature. And this time, the finger of evidence points to a person's finger pattern and how it is affected by androgen, or male hormones.

Previous studies have shown that, in general, a woman's index finger, or second finger (2D), is almost the same length as her ring finger, or fourth finger (4D), while a man's index finger is more often shorter than his ring finger. Since this ratio between the index and ring fingers (2D:4D) is a gender difference that can be measured very early on -- by age two -- researchers believe that prenatal androgens are the main factor influencing it.

"We used an index of prenatal androgens -- the length of the index finger to the length of the ring finger -- and then we asked a question that people have asked for a long time and that is, 'is human sexual orientation being influenced by prenatal androgen?'" lead author S. Marc Breedlove, PhD, tells WebMD. Breedlove is a professor of psychiatry at the University of California, Berkeley.

To answer the question, the researchers conducted an anonymous survey of 720 adults who attended public street fairs in the San Francisco area. Subjects were asked their age, gender, sexual orientation, handedness, and the number and gender of children their mother had carried before them. Finger lengths were measured from photocopies of subjects' hands.

The Breedlove survey confirmed that women's index and ring fingers were closer to equal size, meaning that their ratio is closer to one. When comparing men's left hands to women's left hands and men's right hands to women's right hands, the study showed that the difference between the men's and women's ratios is greater on the right hand than on the left This indicates "that the right-hand [second to fourth finger ratio] is more sensitive to fetal androgens than the left-hand ratio," according to Breedlove.

When researchers looked at the right-hand index to ring finger ratio of lesbians, they found it was significantly more masculine than that of heterosexual women; in fact, it didn't differ significantly from the ratio of heterosexual men.

"The relationship is simplest for women," says Breedlove. "We found that the 2D:4D ratio -- which tends to be smaller in men than women -- tends to be smaller in lesbians than in heterosexual women. We take that as evidence that, as a group, lesbians see slightly higher prenatal androgen levels than do heterosexual women and that the prenatal androgen is responsible for some of those cases of lesbianism."

"In men it is more complicated. If you just compare straight and gay men, you don't see a difference - that of course is interesting in itself. People would like to have predicted that you'd see a more feminine ratio in gay men than in straight men," says Breedlove. "But when we classified our male subjects based on factors known to influence sexual orientation, we saw a relationship. That factor is birth order: the more older brothers a man has, the more likely that man is to be gay in adulthood."

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