Thursday, February 22, 2007

Lady Fingers, Empty Calories?

I’ve gone over one of the possible mechanisms in making men gay involving fetal hormone exposure or even maternal antibodies in discussing the Fraternal Birth Order Effect (here). Here I want to look at a possibly different, though equally well-publicized mechanism based on different observations; not the number of siblings but the dimensions of our bodies.

We certainly don’t experiment on humans in such a way, but it’s fairly well established that, in animal models, in utero manipulation of androgens (hormones with a masculizing effect) is capable of altering sexual behavior, orientation, and so on (1-3). In addition to behavioral differences, of course, exposure to such chemicals leave the associated physical form altered.

Humans have numerous such physical differences between the average male and female form (of course, else this whole topic would be pointless as no one would know if they were gay or not ;-)). In humans, as with other mammals, without the masculizing effects of our hormones, the genetic male would turn out somewhere in-between the two forms or even apparently female, even by behavior. One of these differences between men and women, though slight, is the ratio of the lengths of our 2nd to our 4th finger (index to ring finger).

Forget broad shoulder, cut and firm bodies… perhaps being gay is all about subtle attractions to finger length ratios?

Anyway, the lengths are measured sometimes by image analysis but one could less accurately measure themselves with a ruler. Simply take the distance between the tip of your finger to the basal crease (I gather that’s the line at the base of your finger where your skin bends when you bring the finger to the palm). To get this 2D:4D ratio, divide the length of the index by the ring finger. Not that it would tell you much more than palmistry would, by measuring a single person :-).

I’ve seen in the literature this 2D:4D ratio range among humans from between 0.8 and 1.3, and there is great overlap between men and women in their distributions. But women average at a ratio of about 1 (their index and ring finger are the same length), and men average at about 0.98 (Say your index finger were 7.5 cm long; the average man would have a ring finger 0.2 cm longer). Though a small difference, it is outside the error bars with significantly large samples.

The thinking goes that whatever makes men physically different, such as index fingers relatively shorter than ring fingers, should be the effect of these androgens, these hormones that make us male. And there is evidence to back that up here.

From a study by Manning et al., for example, it was found this ratio is established at least as early as the age of two and “is probably established in utero” (4). Furthermore, it was found that only in the right hand this ratio was negatively correlated with sperm production and testosterone concentrations. Curiously enough the ratio also positively correlates with propensity for depression in men; testosterone makes us happy… (5). I’ve also seen reference to effect on sports ability, disease susceptibility, and “attractiveness”. In short though, it’s thought that low 2D:4D ratios indicate high prenatal testosterone and low estrogen, and High 2D:4D ratios indicate low prenatal testosterone and high estrogen.

The genetics in play with this ratio are apparent. It was pretty much established that it ran in families, even in races and continents (6). But a recent twin study found that the cause behind this ratio was approximately 60% in the genes (7) (add that to my other twin facts). The correlation for Monozygotic twins was 0.66 and 0.71 for the right and left hand respectively and, for Dizygotic Twins, 0.35 and 0.37 respectively. It’s important to note this asymmetry of the body here. I have posted before, tongue in cheek, about handedness (Fraternal Birth Order Effect.

They also found the 2D:4D ratio negatively correlated with the number of older brothers only, not sisters. But, for gay men, they were unable to escape their error bars. On the other hand, lesbians, were found to have significantly masculinized right hands (with, again, the left hand measurement not escaping the likely error).

So, some hints, but, clearly, the samples were too small and not matched for age and race. More needed to be done and soon after Robinson and Manning (who did the study on sperm production and testosterone concentrations above), published their own study (9). But again with relatively small samples, and, this time, predominately in one racial category. Also problematically, the control group of “heterosexuals” was chosen “without regard to sexual orientation.”

They found no reliance of the ratio on birth order (though once again they did find the Fraternal Birth Order Effect for gay men; it’s pretty well established by now :-)). Oddly enough, and keeping in mind their sample size, they found their gay men were significantly more mascilinized than straight men… But they also found bisexual men were the most masculinized of all. So, some odd, suspicious results to be sure. Again though, it’s early in the game, still in 2000.

Next up we have a study by McFadden in 2002 (10). Again, this was a study with too small of samples (35 gays, 29 lesbians and about twice the number of heterosexuals). Quickly, they found gays were more feminized and lesbians more masculinized in the 2D:4D ratio, more along the lines of common sense, and similar to the Williams study (8)… But with the numerical significance and disagreement with Robinson, it’s not anything to hang your hat on.

Next we have a study by Brown et al. (11). They went about a gay pride festival and asked lesbians to rate themselves as "butch" of "femme". They found about 90 in each group, and measured their hands. The self-identified “femme” lesbians did not show a diminished 2D:4D ratio from heterosexuals, but “butch” lesbians did. Now, I do find this to be a bit of a humorous study for some reason, but they have a point. Women are, in general, more plastic and bisexual in their sexual orientation than men, and such segregation may be needed to find an effect…

Finally, Lippa, in 2003 (12), used a much larger sample: 351 heterosexual men, 461 gay men, 707 heterosexual women, and 472 lesbian women. He also used many races and corrected for race in his calculations (this 2D:4D ratio is a function of race). His findings confirmed McFadden’s results and bolstered, in part, Williams, though contradicted Robinson's again. He found gay men were more feminized in this ratio, and in the length of the ring finger alone. These results were found for all races, save for Asia (for which he had only a small number of participants, 19). But, he found no significant effect with lesbians, compared to straight women; they were still within the anticipated error (possibly because he didn’t correct for the Butch-Femme effect? :-)).

So what’s the point? I guess the point is to never follow the progress of research and expect to have a good narrative (or a good blog post :-)). There’s no smoking gun here; still merely hints and suggestions and uncertainty. And there may be nothing to find; what makes this subtle ratio different in men and women may have nothing to do with orientation. For now, we wait for more measurements, and ignore those in the media who claimed proof was in on this one, and got everyone to measure their fingers for hints of the attraction they experience ;-).

1. Baum, M.J., et al., Prenatal and Neonatal Testosterone Exposure Interact to Affect Differentiation of Sexual Behavior and Partner Preference in Female Ferrets. Behavioral Neuroscience, 1990. 104(1): p. 183-198.
2. McCarthy, M.M., E.H. Schlenker, and D.W. Pfaff, Enduring Consequences of Neonatal Treatment with Antisense Oligodeoxynucleotides to Estrogen Receptor Messenger Ribonucleic Acid on Sexual Differentiation of Rat Brain. Endocrinology, 1993. 133(2): p. 433-439.
3. Brand, T., et al., Adult partner preference and sexual behavior of male rats affected by perinatal endocrine manipulations. Hormones and Behavior, 1991. 25(3): p. 323-341.
4. Manning, J.T., et al., The ration of 2nd to 4th digit length: a predictor of sperm numbers and concentrations of testosterone, luteinizing hormone and oestrogen. Human Reproduction, 1998. 13(11): p. 3000-3004.
. Woodson, J.C., B.W. Balleine, and R.A. Gorski, Sexual Experience Interacts with Steroid Exposure to Shape the Partner Preferences of Rats. Hormones and Behavior, 2002. 42: p. 148-157.
5. Bailey, A.A. and P.L. Hurd, Depression in men is associated with more feminine finger length ratios. Personality and Individual Differences, 2005. 39: p. 829-836.
6. Loehlin, J.C., et al., Population Differences in Finger-Length Ratios: Ethnicity or Latitude? Archives of Sexual Behavior, 2006. 35(6): p. 739-742.
7. Paul, S.N., et al., Heritability of the Second to Fourth Digit Ratio (2d:4d): A Twin Study. Twin Research and Human Genetics, 2006. 9(2): p. 215-219.
8. Williams, T.J., C.L. Jordan, and S.M. Breedlove, Finger length patterns indicate an influence of fetal androgens on human sexual orientation. Nature, 2000. 404: p. 455-456.
9. Robinson, S.J. and J.T. Manning, The ratio of 2nd to 4th digit length and male homosexuality. Evolution and Human Behavior, 2000. 21: p. 333-345.
10. McFadden, D. and E. Schubel, Relative lengths of fingers and toes in human males and females. Hormones and Behavior, 2002. 42(492-500).
11. Brown, W.M., et al., Differences in Finger Length Ratios Between Self-Identified Butch and Femme Lesbians. Archives of Sexual Behavior, 2002. 31(1): p. 123-127.
12. Lippa, R.A., Are 2D:4D Finger-Length Ratios Related to Sexual Orientation? Yes for Men, No for Women. Journal of Personality and Social Psychology, 2003. 85(1): p. 179-188.


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