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2000
Finger length
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Many of us may have seen the report in our local
newspapers about finger-lengths and sexual orientation. It was carried around
the world by Reuters, though it originated from a short paper published in
Nature magazine, 30 March 2000. Probably to make it more readable, the Reuters
version simplified the conclusions somewhat, and in the process, some important
nuances of the paper were lost.
What I'd like to do here is to try to explain, in plain language, what this paper, by Terrance Williams et al (University of California), discussed. The published paper is archived in the appendix. First, we have to understand the background, for without having the same understanding of the background as the scientists, we cannot grasp their conclusions. In the paper, the background was sketched in just the first sentence: "Animal models have indicated that androgenic steroids acting before birth might influence the sexual orientation of adult humans." In plain language, what it said was that previous studies, using animals as subjects, have found that certain hormones called "androgens" had an effect on foetuses and their subsequent sexual orientation. The best known androgen is testosterone. Some people may trip over the paper's opening sentence. They are those who refuse to accept that animal studies can be extended to humans. If you take this view, there is very little I can say, because your position is akin to the Creationist view. It is an assertion of belief, nothing more. Given that position, you really don't accept any science. You only want belief. Note however, that no serious scientist today dismisses animal studies as irrelevant to humans, since we share the same evolutionary origins. The second idea contained within that first sentence is also important, and has been established for a while now, though it strikes many people as a new idea: that animals can also be homosexual. It is not a trait found only in humans. Scientists have observed homosexuality in animals in the wild, and have bred homosexual animals through modifying androgen levels in foetuses. [1] Embryos are basically sexless, except that deep in the cells, some have an Y chromosome while others have an extra X chromosome. How does an embryo then develop physical features, which at birth we recognise as male? It's a two-stage process. The Y chromosome prods some embryonic cells to develop into testes, which in turn produce androgens, including testosterone. It is these hormones that affect the developing foetus' body, to make the features we know as male. The most obvious features of course, are the penis and scrotum, but as this article points out, there are other physical features that distinguishes male from female, and these are similarly thought to come from the effect of androgens as well. The one other feature this study looked at was finger length. The report recalled that other studies had shown that in women, "the index finger (2D, second digit) is almost the same length as the fourth digit (4D), although it may be slightly longer or shorter; in men, the index finger is more often shorter than the fourth." This is not something that is evident only in adults. As the paper pointed out, "The greater 2D:4D ratio in females is established in two-year-olds," and hence, "the sex difference in the 2D:4D ratio probably reflects the prenatal influence of androgen on males." Since androgens caused physical differences such as the 2D:4D ratio, then other characteristics that correlated with the 2D:4D ratio would most likely be caused by the same androgens. The researchers found that, indeed, sexual orientation correlated with 2D:4D ratio. 720 adults attending street fairs in San Francisco had their hands measured, and were also were asked their gender, age, sexual orientation, handedness, and the number and gender of children their mother had carried before them.
What they then found, I shall show diagrammatically (although this diagram
was drawn by me; it was not part of the published paper):
Homosexual women "The right-hand 2D:4D ratio of homosexual women", said the researchers, "did not differ significantly from that of heterosexual men." This suggests that "homosexual women were exposed to greater levels of fetal androgen than heterosexual women." For the men, however, the results were more complex. Taking homosexual men as a whole, "the 2D:4D ratio of homosexual men was not significantly different from that of heterosexual men for either hand." However, when the researchers segregated eldest sons from younger sons (i.e. those with at least two older brothers), they found that younger sons, straight or gay, had a significantly more "masculine" finger ratio than eldest sons. Now, previous studies had found that there was a higher proportion of homosexual men among younger sons. This study found support for that. The researchers found that homosexual men had twice as many older brothers than older sisters, when usually the chance of having an older brother would be nearly the same as having an older sister. So, the general rule that came out of this study was: the more older brothers one had, the higher chance of being homosexual. At the same time, the more older brothers one had, the more "masculinised" one's finger ratio was likely to be. Since prenatal androgens caused the masculinised finger ratio, they most likely were also a causative factor in homosexuality – among younger sons. Now why would younger sons be so affected in the womb compared to other babies? This was the mystery posed by the study's data. It would appear that a mother's body could "remember" how many previous sons (but not daughters) she had carried, such that younger sons with older brothers got more androgens. At this point, no one knows how this comes about. Even more mysterious, women did not show the same pattern. "No effect of older brothers or sisters on 2D:4D in women was observed, consonant with reports that older siblings exert no effect on female sexual orientation." Moreover, still unexplained were homosexual eldest sons, whose finger ratio was no different from heterosexual men. It would suggest, the researchers said, that androgens were not the only cause for homosexuality; genes could be playing a part too. "Although hyper-androgenization of homosexual men," the researchers wrote, "might not fit some cultural expectations, homosexual men display several hyper-masculine characteristics." For example, they listed "a greater mean number of sexual partners in a lifetime than heterosexual men, who in turn report more sexual partners than do women of either orientation." "Furthermore, reports that adult homosexual men have more circulating androgens, larger genitalia and more 'masculine' auditory evoked potentials than heterosexual men, are consistent with at least some homosexual men being hyper-androgenized." But if homosexual men are more "male" than straight men, are homosexual women more "female" than heterosexual women? No, it seems not, for as mentioned above, they had hands that tended to resemble male hands. Things get curiouser and curiouser! This study questions quite a few simple assumptions that people tend to hold in the absence of proper understanding of homosexuality (including many gay people themselves!). It suggests that homosexuality can have different origins, hormonal or genetic, for different groups of people – younger sons, older sons. Furthermore, homosexual women cannot be so easily equated with homosexual men. There is probably a different explanation for them altogether. It also suggests that contrary to popular belief, homosexual men may be more androgenised (more "male") than heterosexual men [2], but homosexual women are also more androgenised, not more "female", than straight women.
That these simple results are so fascinating only shows how little we really
know about the biology of homosexuality. In the absence of knowledge, alas, too
many people still trot out their age-old admonitions -- about sin, deviancy, immorality,
upbringing and "choice". Ignorance is dreadfully boring! © Yawning Bread
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Footnotes None Addenda None
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