Human evolution theory utilizing concepts of neoteny & female sexual selection
An etiology of neuropsychological disorders such as autism and dyslexia, and the origin of left handedness.

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S.F. Witelson

Neural sexual mosaicism: Sexual differentiation of the human temporo-parietal region for functional asymmetry: bibliographical excerpts


"For example, in the one neuropsychological study on lesbians reported to date, increased left handedness also was found, but the mechanism that is consistent with the literature is different than that for men: Increased levels of androgenic hormones appear associated with homosexuality and left-handedness in women (see McCormick et al., 1990, for discussion)." (Witelson SF (1991) Neural sexual mosaicism: Sexual differentiation of the human temporo-parietal region for functional asymmetry. Psychoneuroendocrinology 16: 147)

"We found that homosexual men showed an increased prevalence of left handedness (nonCRH) compared to the general population (McCormick et al., 1987; 1990; McCormick & Witelson, 1991), and that among CRH men, homosexual men showed less functional asymmetry on a verbal dichotic listening test (McCormick & Witelson, 1990). We also observed lower spatial ability in homosexual than in heterosexual men (McCormick & Witelson, 1991). Based on these results, we suggested an association between sexual orientation and cerebral lateralization and , by inference, a neurological component to the etiology of sexual orientation. With respect to more general issues, these results suggest that early neuroendocrine events which may influence sexual orientation also may be factors in determining aspects of functional asymmetry and cognition." (Witelson SF (1991) Neural sexual mosaicism: Sexual differentiation of the human temporo-parietal region for functional asymmetry. Psychoneuroendocrinology 16: 146)

"The corpus callosum has been shown repeatedly to vary greatly in size and shape (for reviews, see Clarke et al., 1989; Witelson, 1989). The difference may be as large as two-fold, for example, minimum/maximum scores= 614/906 for men and 450/824 for women (Witelson, 1989), and variation is comparable in each sex. Axon elimination is likely a factor determining a callosal morphology in both sexes. Genetic factors may influence the course of regressive events shaping callosal morphology as well as other anatomical regions, including right-left cortical asymmetries. The main point here is that an additional factor influencing axon elimination may be operative in men. Only in men is callosal size, and particularly isthmal area, related to hand preference, a measure of functional asymmetry. Similarly, only in men does posterior Sylvian fissure morphology vary with hand preference. Such sex differences suggest that some sex-related biological factor may influence the regressive mechanisms hypothesized to play a role in determining brain structure related to lateralization. Manipulation of early levels of sex hormones affects naturally occurring cell death (e.g.Arnold * Breedlove, 1985). In this context, prenatal or perinatal levels of androgenic hormones (I will use "testosterone" to refer to this more generic group) in males may regulate callosal axon elimination and development of associated structures related to functional asymmetry." (Witelson SF (1991) Neural sexual mosaicism: Sexual differentiation of the human temporo-parietal region for functional asymmetry. Psychoneuroendocrinology 16: 142)

"The morphology of the human corpus callosum appears to be different between the sexes. The question remains why the anterior (genu) and posterior (splenium) segments are or tend to be larger in men than women whereas the isthmal region of the body of the callosum is not; in fact, when hand preference is considered, the isthmal region shows a difference in the opposite direction for the majority of people. ... I have found the midsagittal area of the corpus callosum to be larger in mixed and left handers, referred to as non-consistent-right-handers (nonCRH), than among CRH subjects (Witelson, 1985). Hand preference is a rough index of the pattern of brain organization. Left handers (by various definitions) have a higher prevalence of atypical right-hemisphere representation of speech and language functions than do right handers and, in general, show a greater degree of bihemispheric representation of verbal and spatial skills (for review, see Bryden, 1988)." (Witelson SF (1991) Neural sexual mosaicism: Sexual differentiation of the human temporo-parietal region for functional asymmetry. Psychoneuroendocrinology 16: 139)

"I hypothesize that in men, lower levels of testosterone lead to less axon elimination, a larger callosal isthmus and associated temporo-parietal structures, greater left handedness (less consistent-right-hand preference), and greater bihemispheric representation of cognitive skills (less functional asymmetry), and that these same factors are not similarly operative in the development of the female brain." (Witelson SF (1991) Neural sexual mosaicism: Sexual differentiation of the human temporo-parietal region for functional asymmetry. Psychoneuroendocrinology 16: 143)

"Another possibility suggested here is that the effect of a second experimental variable, handling in infancy, which increases callosal size in male but not in female rats (Berrebi et al., 1988), may have masked the effects of reduced testosterone in males." (Witelson SF (1991) Neural sexual mosaicism: Sexual differentiation of the human temporo-parietal region for functional asymmetry. Psychoneuroendocrinology 16: 145)

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