2D:4D Evidence Supports Transexual Taxonomy

A new paper provided both new direct evidence and a meta-analysis of measurements of 2D:4D finger ratios in transsexuals, both FtM and MTF.  Such measurements are interesting because it is known to be influenced by testosterone levels in utero and thus an indirect measure of testosterone exposure that might influence brain sexual dimorphism.

What is doubly exciting about this paper is that the authors fully comprehend the overwhelming evidence for the two type taxonomy and of the (mild) scientific controversy regarding sexual orientation vs. age of onset as the best clinical markers for the two taxons.  The study is open access so I highly recommend following the link to it and reading it for yourself.  The study is also interesting because of where it was conducted; Iran.

Iran is a Muslim country which while being extremely homophobic, both culturally and legally, treats transsexuals fairly well, at least legally and medically.  Make no mistake, culturally, it is far from truly accepting.  Further, Iran is considered a “Collectivist Society” according to the Hofstede Individualism vs. Collectivism Index.  Lawrence has shown that this index highly correlates with the percentage of non-androphilic (and thus likely autogynephilic / late onset) transwomen transitioning in a given culture.  Thus, we would expect that there were fewer such transwomen in the study and the reported data bear this out.

Let’s look at the new data they provide:

Table 1

Means (and SD) for 2D:4D in the left and right hand for transmen, transwomen, control women, and control men

	Transmen	Control women	Transwomen	Control men
Left 2D:4D	0.991 (0.034)	0.991 (0.032)	0.981 (0.033)	0.974 (0.029)
	n = 104	n = 53	n = 88	n = 56
Right 2D:4D	0.981 (0.030)	0.983 (0.033)	0.972 (0.029)	0.959 (0.033)
	n = 104	n = 53	n = 89	n = 56

Table 2

Means (and SD) for 2D:4D in transsexuals’ left and right hand as a function of early or late onset of gender dysphoria

	Transwomen		Transmen
	Early onset	Late onset	Early onset	Late onset
Left 2D:4D	0.982 (0.034)	0.975 (0.022)	0.988 (0.033)	1.009 (0.031)
	n = 80	n = 8	n = 92	n = 12
Right 2D:4D	0.973 (0.029)	0.963 (0.026)	0.977 (0.028)	1.007 (0.027)
	n = 81	n = 8	n = 92	n = 12

Before the analysis of transfolk, it would be a good idea to scale the effect by looking at the effect size between the controls.  The difference between control women and men is d= 0.56 for the left hand and d= 0.76 for the right. This is only a moderate effect size.

Although the number of late onset is small, and thus must be viewed with caution, the analysis is still very interesting and would seem to confirm (agree) with the two type hypothesis.  Consider that the two MTF types have a small but distinct difference of d= 0.24 for the left hand and d= 0.22 for the right.  When we compare early onset type to the male controls we get d= 0.25 and d= 0.45 for the right.   When we compare early onset to female controls we get d= -0.27 for the left and d= -0.32 on the right.  This shows that early onset transwomen are roughly halfway between the controls, and if anything a bit closer to the female controls.

But even more intriguing, and the reason for trusting this interpretation is that when we compare the late onset population to the male controls we see that it exactly agrees with the hypothesis that the late onset type is essentially like the majority heterosexual male population and not at all feminized, with effect sizes that are, statistically speaking, non-existent at d= 0.04 and d= 0.12 for the left and right hands respectively.

This shows that early onset MTF type has notably hypomasculine (feminized) hands while the late onset MTF type does not, and thus in agreement with other data that supports the two type MTF taxonomy.

But what about the FtM transmen?  Here we see an even more intriguing set of data.

The two FtM types have a moderate to substantial, very notable, difference of d= -0.66 for the left hand and d = -1.07 for the right, indicating that early onset transmen are far more masculine than late onset.  When we compare the early onset FtM to female controls we find effect sizes of d= -0.09 for the left hand and d= -0.20 for the right indicating a non-existent to small masculinization signal.

However when we compare the late onset FtM to female controls we see a very different pattern with effect sizes of d= 0.57 for the left and d= 0.80 for the right.  The positive sign indicates that late onset transmen have a more feminine 2D:4D ratio than control women (!!).  And the effect size difference between early and late onset transmen is far greater than the difference between control men and women (!!!).

This, if replicated, is very big news.  It would support the notion that transmen also exhibit two taxons as has long been suspected, one that is masculinized in both behavior, sexual orientation, and very mildly in appearance, the other that is very feminine, androphilic, and autoandrophilic, the mirror image of late onset transwomen.

Further Reading:

Essay on 2D:4D History

Essay on Cultural Difference in Percentage of HSTS vs. AGP Transwomen

Essay on Androphilic Transmen

Reference:

Sadr, M., Khorashad, B.S., Talaei, A. et al. “2D:4D Suggests a Role of Prenatal Testosterone in Gender Dysphoria” Archives of Sexual Behavior (2020)
https://doi.org/10.1007/s10508-020-01630-0

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