Start of summary – The study is poorly designed as it assumes a certain digit ratio is clearly male or female when there is simply a correlation; it uses an unsatisfactory control group and ignores that other factors such as ethnicity have a greater impact on digit ratio than biological sex.
The study ignores research into intersex people, which demonstrates that unusual levels of androgen do not lead to an increase in Transsexualism. Schneider does not offer an adequate explanation of why exposure to androgens pre natally would lead to a different digit ratio for transitioning men; and not to different digit ration for transitioning women – end of summary.
Schneider undertook research into the lengths of index and ring fingers in transitioningnmen and women. This research says that:
1. A greater proportion of biological men have shorter index fingers than ring fingers and vice versa in females.
2. That this is caused by the levels of androgen that foetuses are exposed to in the womb
3. That the ratio of the digit length of index and ring fingers was similar in transitioning men as biologicalwomen .
4. That the ratio of the digit length for transitioning women was similar to biological women.
This study is quoted to demonstrate that pre natal exposure to androgens causes Transsexualism.
It is true that there is extensive evidence that pre natal exposure to androgens appears to cause the difference in digit ratios. Nobody knows exactly why this is, but a common theory is that this is linked to bone growth.
However this study makes a number of assumptions that invalidates its findings. These are:
- That a particular digit ratio is clearly male or female. Examining both the research carried out by Schneider and other research, it is clear that there is a wide variation between individual men and women in terms of digit ratio. When Schneider refers to a male or female digit ratio he means that there is a correlation in particular digit ratios with the sex men and women, not that a particular digit ratio is clear evidence that an individual is male or female.
To explain this further consider height. In general men are taller than women. Thus if I measured the height of a number of men I could come up with a height range that I could label as typically male. But obviously men who fell outside this range would still be men and women who fell into this height range would still be women.
- 2. That a group of biological females can act as a control group. A control group is a group where the factor being tested does not apply, so that it can serve as a standard for comparison. In this research a control group would be one where biological females clearly have a digit ratio that is female. Overall the females as a mass in the control group have a digit ratio consistent with females, but individually there is great variation.
- 3. That variation in digit ratio is only or most pronounced between the sexes. In reality digit ratio varies considerably between different ethnicities. In his paper “Finger Length Ratios and Sexual Orientation”, Manning says “There’s more difference between a Pole and a Finn than a man and a woman.”
However, even if you discount the assumptions made and thus the very low standard to which this research has been designed, the research still does not provide any evidence that digit length ratio is linked to Transsexualism. This is because in his research Schneider ignores two key points.
- That intersex people whose conditions mean that they are exposed to unusual levels of androgen pre-natally do not develop Transsexualism. If Schneider’s research had any validity we would expect to see adults with certain intersex conditions exhibit a significantly higher level of Transsexualism than the general population. However, Transsexualism is extremely rare amongst this group.
- Schneider ignores the results of his own research that digit ratios in transitioning females are consistent with biological females. It is not adequate to claim that pre natal exposure to androgens appears to cause Transsexualism in transitioning men and ignore that there is no evidence to support a similar explanation for transitioning women.
In summary, Schenider’s study is poorly designed, makes assumptions without any evidence and ignores research, particularly into intersex conditions, that provide evidence that his conclusions are wrong.
Schneider HJ, Pickel J, Stalla GK (February 2006). “Typical female 2nd-4th finger length (2D:4D) ratios in male-to-female transsexuals-possible implications for prenatal androgen exposure”. Psychoneuroendocrinology 31 (2): 265–9. DOI:10.1016/j.psyneuen.2005.07.005. PMID 16140461
Churchchill AJG, Manning JT, Peters M (2007). “The effects of sex, ethnicity, and sexual orientation on self-measured digit ratio (2D:4D)”. Archives of Sexual Behavior 36 (2): 251–260. DOI:10.1007/s10508-006-9166-8. PMID 17394056.