Tan judges my target article as excellent, but expresses the view that in analyzing the long-term increases in brain size, myopia and intelligence, insufficient attention was paid to sex differences, handedness and sex hormones. I respond to some propositions advanced by Tan, and discuss these areas, including alternative interpretations for some of Tan's findings.
2. Conditions may differ in Tan's Turkish populations, where educational conditions for females may have recently undergone great improvement. Indeed, Miller & Corsellis (1977) found that, among 7,400 people age 20-50 autopsied in London, the trend toward increasing brain weight in the female began roughly thirty years later (1900) than in the male.
3. Also, the subjects of all of Tan's numerous studies were students at a Turkish medical faculty (and, in his MRI study, students at a high school for sports were also included). Thus, their brain morphologies are apt to differ from the general population (both on account of their very high IQs, and perhaps, their special talents). Although this would limit the generalizability of Tan's findings, his data would be very useful in a meta-analysis of the relationship between gifts of intellect/physicality and brain organization.
4. With respect to handedness, Tan asserts that if my theory is correct, rather than remaining nearly constant since early history, the proportion of left-handers should have risen in parallel with intelligence, since many studies report a relation between IQ and handedness. In response: first, I do not share Tan's belief that an intergenerational rise in the prevalence of left-handers has not occurred, since several parent-offspring studies led me to conclude that it had (e.g., Flemington, Dalton & Standage 1977; Porac, Coren & Duncan 1980; L. Tan 1983). In (Leslie) Tan's paper, 11.8% of the (917 Australian) offspring were left handed compared with 5.9% of their parents, and it was observed that large intergenerational differences were seen on tasks where there was unlikely to have been substantial social pressure for right-handedness in the earlier generation.
5. Second, although it's very well established that disproportionately more left-handers are found among people who've attained eminence in a variety of creative endeavours, this left-handed advantage diminishes, then reverses as one extends to lower IQs. I previously reported on the handedness of 2,720 adult members of high-IQ societies (using a modification of the Oldfield Handedness Test; see Storfer 1995), and classified 11.5% of the men and 10.5% of the women as left handed, and a further 12.5% of the men and 12.1% of the women as near- ambidextrous. These totals are larger than usually observed (Geschwind & Behan 1984), especially for females.
6. At least one large study (Hardyck, Petrinovich & Goldman 1976) found no difference in the average IQs of (5,600) left-and-right-handed schoolchildren, although the left-handers displayed considerably more IQ variability. I think this excess variability arises from there being 3 distinct brain organizations that can produce sinstrality: (1) hemispheric-reversal cases (15% or so of lefties) who, because of uterine problems, have IQs that average about a full standard deviation below the norm; (2) lefties with an otherwise 'normal' brain configuration (most lefties); and (3) people who are 'bilateralized' for speech (i.e., who have speech centers in both hemispheres), a phenomenon which, based on findings from open-brain operations (Ojemann 1983) or the observed effects of major stroke (c.f. Bradshaw & Nettleton 1983), occurs in about 15%-20% of left-handed people and 1%-2% of right-handers: implying that about half the bilateralized population is left handed (see Storfer, 1990; ch. 17). If the trend toward increasing IQ and brain size is making bilaterality more common, this would raise the percentage of left-handers. However, better uterine and neonatal care may be reducing hemispheric-reversal as a source of left handedness (and thereby raising IQ while reducing sinstrality).
7. Tan has intensively studied the relationship between testosterone levels and performance on spatial-reasoning problems, and reports that, among male (but not female) 'pre-med' students, a positive correlation exists. Thus, he asserts that, if intelligence and brain size were really increasing rapidly, we should have seen a large increase in the proportion of men and women having high testosterone levels (which we have not) and a shift to greater right-hemisphere activity. My analysis of data from Tan's 1990(c) study yields a different conclusion: 8 of the 19 right-handed male students in the moderate serum-testosterone- level range (550-950 ng/dl) had IQs above 130, compared with only 1 of the 6 students with higher levels, and none of the 6 with lower levels. Tan's data in a later paper (Tan & Tan 1998) display a steep progressive IQ fall among women students at levels above 500 ng/dl and a modest IQ decline among men at levels above 1,000 ng/dl.
8. I accept the idea that the higher levels of testosterone in the male brain are causally related the slight advantage men display on tasks of (three-dimensional) spatial reasoning, but I don't accept that this generalizes, either to the verbal or other non-spatial aspects of IQ, or their underlying biology. I base this on the following kinds of evidence: (1) other than in the 3-D spatial domain, males and females perform equally well on IQ-tests (Lynn 1994); (2) more difficult Ravens Matrices test problems evoke much additional left-brain, and little additional right-brain activity (Haier et al. 1983); (3) the primate studies cited in the Precis (paragraph 25) show a far greater intergenerational growth in the left hemisphere's speech strategy area than in the right; and (4) the development of the human intellect is thought to have been accompanied by a greater increase in neuronal activity in the left hemisphere than in the right (left hemisphere activation being associated with quiet concentration, conceptual- analytical processing and self-reflective thought, while the right is conceived as our externally-focused vigilant, perceptual- holistic processor). This is particularly true in language processing areas, where the two hemispheres differ greatly in neuronal architecture -- and where the left-hemisphere neurons fire far more often, both because they have many more dendrites, and because proportionately more dendrites receive excitatory transmissions (Scheibel et al. 1985).
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