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."
"What we found is that men with more older brothers showed a more masculine finger pattern than men without older brothers, whether they were straight or gay," says Breedlove. "So, we know that those men with more older brothers are more likely to be gay, and now we know that they seem to have been seeing more prenatal androgen [because of their finger pattern]. We think the two are related; that is, that one of the reasons later-born men are more likely to be gay is because they saw more prenatal androgen."
As in the earlier studies, older sisters were found to have no apparent influence on male sexual orientation, and older siblings of either gender had no effect on female sexual orientation.
This isn't the first time researchers have found differences between homosexuals and heterosexuals in structures and behavior influenced by androgen. Animal and human studies have shown that gay males have a greater mean number of lifetime sexual partners, more circulating androgen, and larger penises than straight males, according to Breedlove and colleagues.
"If anything, the data indicate that some gay men are gay because they saw more androgen before birth than other men did, not because they saw less," says Breedlove.
"All of the evidence is kind of fitting together to suggest that male homosexuals are getting exposed to excess levels of androgens at some point during development, presumable prenatal development - but that is, of course, still circumstantial evidence," says Dennis McFadden, PhD, who reviewed the study for WebMD.
In 1998, McFadden, a professor of experimental psychology at the University of Texas in Austin, published his own article supporting the theory that lesbians appear to receive a larger than normal dose of prenatal androgen. He found that certain characteristics of the auditory system of bisexual and homosexual women fall between that of straight men and straight women. Like the second finger to fourth finger ratio, these characteristics are influenced by prenatal androgen exposure.
"So the implication is -- and it is just an implication at this point -- that these people were different at birth and that the differences occurred because of differences in exposure to androgens prenatally," says McFadden.
"Any human behavior as complex as sexual orientation is going to involve multiple contributing factors," says McFadden. "What the recent work of Breedlove ... and our work on [the auditory system] reveal is that there are in fact physiological concomitants to homosexuality. They represent confirmations of beliefs ... that many people had for a long period of time ... but you need the scientific documentation in order to move forward. Eventually the goal would be to identify those brain centers that seem to be responsible for sexual orientation."
As for the finger pattern, Breedlove cautions not to point any fingers based on the ratio. "Just remember," he says, "you can't classify individuals on this."
- Scientists know that the ratio of the index finger to the ring finger is different in men and women, and is probably indicative of the level of exposure to the male hormone androgen in the womb.
- In a new study, researchers found that lesbian women have a more masculine ratio, suggesting that androgen exposure could be partially responsible for sexual orientation.
- Gay men, however, showed no difference in this ratio compared to straight men, but men with older brothers showed a more masculine finger pattern than men without older brothers, whether they were straight or gay. Since other research has shown that later-born men with older brothers are more likely to be gay, researchers surmise that they may have been exposed to more prenatal androgen.