Evidence of Androgen Effect On Digit Ratio
- Women with congenital adrenal hyperplasia (CAH), which results in elevated androgen levels before birth, have lower, more masculinized 2D:4D on average. Other possible physiological effects include an enlarged clitoris and shallow vagina.
- Males with CAH have more masculine (smaller) digit ratios than control males, which also suggests that prenatal androgens affect digit ratios, since amniocentesis samples show that prenatal levels of testosterone are in the high normal range in males with CAH, while levels of the weaker androgen androstenedione are several fold higher than in control males. These measures indicate that males with CAH are exposed to greater prenatal concentrations of total androgens than are control males.
- Digit ratio in men correlates with genetic variation in the androgen receptor gene. Men with genes that produce androgen receptors that are more sensitive to testosterone have lower, more masculine, digit ratios.
- XY individuals with androgen insensitivity syndrome (AIS) due to a dysfunctional gene for the androgen receptor present as women and have feminine digit ratios on average, as would be predicted if androgenic hormones affect digit ratios. This finding also demonstrates that the sex difference in digit ratios is unrelated to the Y chromosome per se.
- The sex difference in 2D:4D is present before birth in humans, which rules out any social influences that might affect digit growth differentially in the two sexes. Because all somatic sex differences in mammals to date have been found to be due to either androgenic masculinization or effects of the sex chromosomes, and as the AIS finding rules out a role for sex chromosomes in the sex difference in digit ratios, the prenatal sexual dimorphism also indicates that androgens act before birth to affect digit ratios.
- The ratio of testosterone to estradiol measured in 33 amniocentesis samples correlates with the child's subsequent 2D:4D ratio.
- In pheasants, the ratio of the 2nd to 4th digit of the foot has been shown to be influenced by manipulations of testosterone in the egg.
- Studies in mice indicate that prenatal androgen acts primarily by promoting growth of the fourth digit.
There is evidence that this reflects fetal exposure to the hormones testosterone and estrogen.
Several studies present evidence that digit ratios are heritable.
The level of estrogen in the amniotic fluid is not correlated with higher 2D:4D, and when examined researchers found no difference in estrogen levels between males and females.
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