An exhibit from the 1921 second international congress of eugenics shows black-white 2d:4d differences established by the third month of gestation [1].
While this bit of evidence does not definitively rule out involvement of androgens in the black-white divergence in digit ratio, it certainly does nothing to quell
my doubts about the meaningfulness of cross-racial comparisons of digit ratio for inferring cross-racial differences in prenatal testosterone exposure. The critical windows of androgen exposure for developing males seem to come with mid-gestational and postnatal surges of testosterone to near-adult levels [2]. I'm inclined to believe, rather, black-white differences in digit ratio are due largely if not entirely to genetic factors unconnected to prenatal androgenization. On an entirely unrelated note, a caption above reads:
The hand is relatively broader and slightly shorter and the thumb relatively longer in the white fetus than in the negro fetus
Coincidentally, according to the second source I cite above (a review article from 2006):
Non-human apes and brachiating monkeys have shorter thumbs and longer fingers, and also lower 2D:4D.
Other differences reported in the 1921 study include smaller brain case and higher frequency of
evolution toe in black fetuses (
continue reading).
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