On the Science of Changing Sex

Step right up, Let Madame Sciencia read your fortune, etched on your hand…

Posted in Science Criticism by Kay Brown on January 24, 2010

Or Scientific Chiromancy.

critical-thinkingScientists, and especially, science textbooks, don’t always get things right the first time around.  This erroneous information can have personal consequences.

When I was in a biology class back in college in the mid-70s, we were taught about sex linked, sex determined, sex influenced, and sex limited genetic traits.  Two of them struck me personally.

First, it was thought that the relative length of the pointing finger (Digit Two, starting from the thumb, also called 2D) verses the ring finder (Digit Four, or 4D) was a sex influenced trait in which for XY karyotyped (normally male) individuals the 4D Long gene was dominant and the 4D Short gene was recessive, but for XX karyotyped (normally female) individuals, the reverse was true, thus explaining the observation that women usually have noticeably longer pointing finger to ring finger ratios than men.  Second, the gene for Hypertrichosis of the Pinea (hairy outer ears) is a Y chromosome linked gene that would only and always express in individuals with a Y chromosome.  This second item bothered me, since my father, and his father had both had such hairy ears.  As a teenager recently transitioned to living as a woman, I sure as anything didn’t want to have hairy ears!

But, it was the strange thing about my finger length ratio (2D:4D) that had me most concerned.  I had much longer pointing fingers than ring fingers!  As an exercise, I looked at my parents and my siblings hands and did some family tree analysis, and found a strange paradox.  I couldn’t explain the phenotype I expressed, given that from the analysis, I should have been heterozygous for the trait.  Thus, as a presumed XY karyotyped individual, I should have had longer ring fingers than pointing fingers!  You can forgive me for then entertaining the suspicion that I might have an… umm… “interesting” alternative karyotype.  Such things have happened.  This also gave me hope that I wouldn’t develop those much to be dreaded, hairy ears!

But, in 1981, I was informed that, though I had not authorized it, the tissue that had been removed during my SRS had been provided to  researchers trying to test / duplicate Wolf Eicher’s work on HY antigen in transsexuals.  My cells had expressed HY antigen, which, along with many other TS folk, disproved Eicher’s hypothesis that the absence of HY antigen, normally found on the cell surfaces of those cells with a Y chromosome, had led to the development of MTF transsexuality.  This also put to rest any hope that I might be the rare XX male!  (It does happen, the SRY gene very occasionally crosses from the Y to the X chromosome during meiosis, in spermatogenesis.)  But… how to explain the unlikely long pointing finger to ring finger ratio?  Could it be that I was XXY?  That’s not so rare… occuring in one out of 500 males.  However, the usual phenotypical sequelae is for above average height, longish arms and legs, an unattractive appearance, low energy due to hypogonadism, and low IQ, first described by, and named after, Dr. Klinefelter.  Well, at 5’7″, several inches shorter than my two brothers, I was considerably below average male height, I was well proportioned and considered reasonably attractive, and at least three standard deviations above normal IQ… but at least I could have the hypogonadism, as I had a distinctly hypomasuline appearance even before starting feminizing HRT at age 18.  So… maybe I could just possibly be the lucky XXY karytype individual that doesn’t look like one?  But, that still left me with real possibility of developing those much to be dreaded, hairy ears.

But… as I mentioned before, I needn’t have worried, as the text books got it wrong on both counts.

First, the 2D:4D ratio is not sex influenced, merely mildly sexually dimorphic based on the androgen/estrogen ratio during fetal development.  Second, hairy ears is an autosomal dominant trait that is sex limited in the same fashion as male pattern baldness.  Now that I was taking feminizing HRT, the much to be dreaded hairy ears would simply not develop (and never did).  I needn’t worry about unusual karyotypes as well.  And now, I have only to explain how it is that I still have significantly higher 2D:4D ratio than most males; in fact, have significantly higher 2D:4D ratio than most females!!!



The conventionally approved method of measuring the 2D:4D ratio is from the middle of the crease between the finger and the palm to the middle of the tip of the finger.  When I measure mine, I find that for the left hand, they measure 67mm:62mm giving a 2D:4D ratio of 1.08.  For my right hand, I measure 67mm:63mm, giving a 2D:4D ratio of 1.06.  When we do this measurement for a large population and graph it as a histogram, we see that the average male hand has a 2D:4D ratio of 0.975, and the average female hand has a 2D:4D ratio  of 0.995.  One also notices that there is a range, a classic bell curve, of ratios.  Note that they are highly overlapping.  Thus, for any given individual, the ratio is essentially meaningless.  I don’t need to explain my ratio at all, right?  Well, maybe.  Looking at the data, It would appear that my ratio is at the very edge of the range.  But, still… in range.  (The graph goes to 1.05, but has not quite gone to zero individuals.)  It is only when we look at large numbers, can we average out the noise, the random factors that push the measurement one direction or the other, that we can see a real signal that might give us interesting clues to scientific questions.

Given that the 2D:4D ratio is now known to be sensitive to the ratio of androgens/estrogens in the developing fetus, researchers are looking at our fingers in an attempt to test hypotheses such as does the prenatal and/or neonatal hormonal environment effect behavior?  Does it effect sexual orientation?  Does it effect gendered behavior or gender identity?  There have been many other indications that this is in fact the case, but this ratio seemed to be an ideal probe to further test these hypotheses.

The first studies to test the hypothesis that homosexuality is influenced by the fetal hormonal environment seemed to answer in the positive, that gay men have hypomasculinized 2D:4D ratios.  But later studies showed just the opposite!  This required re-examination of the data.  Interestingly, the problem was not in the variability of the homosexual population, but of the heterosexual controls!

One would think that perhaps the transsexual population would exhibit a stronger signal… In fact, I sure felt that way, given that my own 2D:4D ratio would suggest an extremely hypomasculinized signal should be found in others like me, right?  But, we see the same pattern of confusion in the studies, with some studies saying that there is a more female like 2D:4D ratio found in MTF TS and no difference between FtM TS and heterosexual females, while other studies show just the opposite!

Thus, we have a quandary.  Although I strongly believe that one day, studies of the 2D:4D ratio will yield useful results, for the moment, the researchers are, like Tantalus, forever reaching for definitive results that slip away, just out of reach.  It would seem that for the moment, scientific studies of our hands gives no better results than those found at a carnival palm reader.

Further Reading:

Update on 2D:4D research (2020)


On the expression of H-Y antigen in transsexuals
Stephen Wachtel, Richard Green, Neal G. Simon, Alison Reichart, Linda Cahill, John Hall, Dean Nakamura, Gwendolyn Wachtel, Walter Futterweit, Stanley H. Biber, and Charles Ihlenfeld

Transsexualism and the H-Y antigen
Eicher W, Spoljar M, Murken JD, Richter K, Cleve H, Stengel-Rutkowski S.

Molecular evidence for absence of Y-linkage of the Hairy Ears trait
Andrew C Lee, Angamuthu Kamalam, Susan M Adams and Mark A Jobling

Fetal development of the hand, digits and digit ratio (2D:4D)
Mehmet Ali Malasa, Sevkinaz Doganb, E. Hilal Evcila, Kadir Desdicioglu

Sexual Dimorphism in the Prenatal Digit Ratio (2D:4D)
Frietson Galis, Clara M. A. Ten Broek, Stefan Van Dongen, Liliane C. D. Wijnaendts

Finger-length ratios and sexual orientation
Terrance J. Williams, Michelle E. Pepitone, Scott E. Christensen, Bradley M. Cooke, Andrew D. Huberman, Nicholas J. Breedlove, Tessa J. Breedlove, Cynthia L. Jordan,S. Marc Breedlove

A Reanalysis of Five Studies on Sexual Orientation and the Relative Length of the 2nd and 4th Fingers (the 2D:4D Ratio)
Dennis McFadden, Ph.D., John C. Loehlin, Ph.D., S. Marc Breedlove, Ph.D.,
Richard A. Lippa, Ph.D., John T. Manning, Ph.D., and Qazi Rahman, Ph.D.

Typical female 2nd–4th finger length (2D:4D) ratios in male-to-female transsexuals—possible implications for prenatal androgen exposure
Harald J. Schneider, Johanna Pickel, Günter K. Stalla

Finger length ratio (2D:4D) in adults with gender identity disorder.
Kraemer B, Noll T, Delsignore A, Milos G, Schnyder U, Hepp U.

2D:4D finger-length ratios in children and adults with gender identity disorder.
Wallien MS, Zucker KJ, Steensma TD, Cohen-Kettenis PT.

The relationship between second-to-fourth digit ratio and female gender identity.
Hisasue et al., Journal of Sexual Medicine, doi: 10.1111/j.1743-6109.2012.02815.x.


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