Storfer's (2000) argument that there was a parallel increase in myopia, brain size, and intelligence due to urbanization (complex visual inputs) during the last century is analyzed in this commentary. Sex differences, sex hormones, and human handedness also need to be considered. In his excellent monograph, Storfer (1999) argues that human brains gradually increased in size during the past century along with a dramatic upsurge of myopia, which is highly correlated with human intelligence. Storfer also argues that these adaptive changes following urbanization may occur by adaptive epigenetic inheritance without violating the basic concepts of Darwin's theory.
2. If visual complexity causes an increase in the size of occipital neurons and an enlargement of the occipital cortex (p. 166), we should expect no sex difference in brain size-IQ relations, which is not always the case. Tan et al. (1999) reported an overall correlation of 0.40 between MRI-measured total cerebral area (highly correlated with brain size) and IQ for the total samples of males and females. Men's IQ was correlated with anterior cerebral area and women's IQ with posterior cerebral area. If a gradual increase in visual complexity were correlated with a gradual development of the occipital cortex, there should not be a gender difference and men's IQ should also be highly correlated with posterior cerebral areas. These results suggest that females might undergo occipital cortical development because of an increase in visual complexity arising from urbanization.
3. Storfer also fails to consider a unique human trait, handedness. The proportion of right- and left-handed people has remained nearly constant since early history. If Storfer's hypothesis were correct, then the proportion of right- and left-handedness should also change in parallel with intelligence, since many studies report a relation between IQ and handedness (see Annett, 1985; Tan, 1988a, b; Tan, 1990b; Tan, 1991).
4. Relating intelligence to cerebral lateralization and handedness: Degree of right-handedness is associated with some reduction in right brain function, i.e., a "stronger" right brain may be associated with weaker right-handedness and a "weaker" right brain with stronger right-handedness (see Annett, 1985). Tan (1990a, 1992a) has reported that left-handedness is determined by the left brain in both sexes, whereas right-handedness is determined by the right brain in men and by the left brain in women (Tan & Akgun, 1992b). Stronger right-handedness in women, resulting from "weaker" right brain function, would perhaps be disadvantageous for intelligence in women (see Annett, 1985). These studies suggest that intelligence may be related to the degree of cerebral lateralization in humans. If we have a higher IQ nowadays compared to 100 years ago, our handedness should have undergone a left shift towards right-brainedness compared to 100 years ago. No evidence supports this expectation, however. In contrast, right-handedness in the vast majority of the human population has remained the norm since prehistoric times (Coren & Porac, 1977; Spennemann, 1984).
5. Storfer does also not consider hormonal influences on brain size and intelligence. If brain size and intelligence showed a steady increase with time, as proposed by Storfer, we should also observe a steady change in hormonal levels in men and women. For instance, fluid intelligence was found to be positively correlated with serum testosterone levels in men (Tan, 1990c; Tan & Akgun, 1993; Tan & Tan, 1998). According to these results, high IQ is associated with high serum testosterone levels in men. If IQ increased steadily during the last century (Storfer's argument), blood testosterone should also have increased, until men became aggressive bulls, not the female-shifted men of our decade. Thank goodness, there are no signs of an increase in testosterone levels in men due to the urbanization that occurred within the last century.
6. Storfer (p. 170-171) states that females are more susceptible to myopia than males (higher incidence of myopia in females than males). This seems to contradict Storfer's hypothesis that myopia is associated with high IQ, since it is also known that there are fewer eminent women than men (Silverman, 1995a,b; Piirto & Fraas, 1995; Hanson, 1996). In conclusion, although I found the Storfer hypothesis very interesting, I think it could be elaborated by also taking sex differences, cerebral lateralization, sex hormones, and human handedness into account.
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