A Little Randomness

January 3, 2009 | Leave a Comment

Category: Estrogen, Ontogeny

I’m posting some excerpts and abstracts that support or contest the conjectures from the last three posts hypothesizing that estrogen may influence our evolution.  I’m searching for studies that might explain, along with testosterone, specific ethnic physiological, psychological and neurological differences.  For those folks following this thread, jump on in.  Post in the comments section what you might have found that makes clearer (or less clear) what we are discussing.

Note we are looking for evidence of hormone levels in premenopausal women, those whose uterine environments influence the maturation rates of their children.

[citations removed]  “Schacter reported that women exposed in utero to the synthetic estrogen diethylstilbestrol had a handedness distribution on the Edinburgh Handedness Inventory (EHI) that was shifted away from strong right-handedness.  Nass et al. found that females with congenital adrenal hyperplasia (CAH), a disorder that results in increased androgen production during gestation, displayed a lesser degree of right-hand preference than unaffected sibling controls on the EHI.  However, males with CAH displayed a trend in the opposite direction.  More recently, Helleday et al. reported that females with CAH did not differ from controls in either degree of right-hand preference or in dichotic listening asymmetry.” (Moffat, S.D. & Hampson, E. (1996) Salivary testosterone levels in left-and right-handed adults.  Neuropsychologia 34 (3): pp. 225)

Responding to the above quote, might estrogen duplicate or amplify what we already know about testosterone?  Might estrogen levels influence rates of maturation manifesting in handedness appraisals?

“The differences in dizygotic twin frequency, and presumably ovulation rate, are in the same direction as the differences in testis size.  The frequencies of dizygotic twins are even higher (up to 49 per 1,000 births) among African blacks. …  Yoruba women, with the world’s highest frequency of dizygotic twins, have higher FSH and LH levels at the time of ovulation than do Japanese women, who have the lowest frequency of dizygotic twins.    This variation in female hormone levels may contribute to the distribution of the incidence of breast cancer, which is known to be related to oestrogen levels.  Even after all other risk factors for breast cancer have been taken into account, the incidence among Japanese women remains inexplicably low.  Perhaps this puzzle, the so-called ‘Japanese factor’ of (breast cancer, is related to the low double-ovulation frequencies and low hormone levels.”  Diamond, J. M. (1986) Variation in human testis size.  Nature (London) 320: 488-489)

This above study suggests that Japanese women have unusually low hormone levels, which would support our hypothesis.

“It is undisputed that in the fetal environment, testosterone can have profound effects on neonatal brain development.  In animal studies, when hormonal levels have been controlled experimentally, many developmental processes are affected by exposure to testosterone, and thus lead to anatomical differences between males and females (i.e., sexual dimorphism.).     Most of these are mediated by estrogen, from which testosterone is converted by normal enzymatic operations involving aromatase, a catalytic enzyme found within the brain and expressed quite early in development (McEwen, Lieberburg, Chaptal, & Krey, 1977).  Ironically, the role of the female hormone estrogen in this process occurs to a significant extent only in males and not females, since the estrogen secreted by the developing ovaries never reaches the brain.  High levels of alpha fetoprotein in the neonatal serum bind these estrogens and prevent their access to the central nervous system.  By contrast, estrogen has an effect on brain development in males, since conversion of testosterone to estrogen takes place within the developing brain itself.”  (Small S.L., Hoffman G.E. (1994) Neuroanatomical lateralization of language: sexual dimorphism and the ethology of neural computation.  Brain and Cognition 26: 307-8)

I’m not sure what that means, but it sounds interesting.


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