The biological correlates of g (brain size, evoked potentials, and sex hormones) analyzed by Jensen (1998, 1999) are critically considered in the light of the current literature.
2. Head Size and Brain Size (Chapter 6, pp. 146-149). Jensen devotes a relatively detailed section to the relation of IQ to head and brain size. The quintessence of this section is that brain size correlates significantly with IQ. Jensen emphasizes that there are at present eight MRI studies indicating a significant correlation between brain volume and IQ, which is close to +.40 after removing variance due to body size. Interestingly, we have also found an overall correlation of +.40 between MRI-measured total cerebral area (midsagittal section) and IQ, thereby further supporting the IQ-brain size hypothesis (Tan et al., 1999). On the other hand, we found in the same study that there was no significant correlation between IQ and MRI-measured cerebral area in right-handed men; although there was no sex difference in IQ, women had smaller cerebral areas than men, as frequently reported by others for IQ-brain size relations. This issue is nicely discussed in Jensen`s Book (p. 149). Nevertheless, Jensen does not mention a modification in IQ-brain size relation. Unfortunately, he largely neglects the studies inconsistent with the IQ-brain size hypothesis (see for instance Cain & Vanderwolf, 1990; Peters et al., 1998).
3. Average Evoked Potential (AEP) Correlates of G. Jensen's arguments about the importance of cortical evoked potentials for IQ have already been critically considered by Verleger (1999) in this journal, with the conclusion that "it appears that these results do not support the idea that ERPs provide a simple biological basis for the measurement of intelligence". Jensen (1998, p. 153) concludes that higher IQ subjects exert shorter latencies in AEPs (no reference), without providing evidence. This conclusion is not consistent with the current literature. For instance, Tan (1992) reported that N49 latency (somatosensory AEP) has a negative linear relation to IQ in females, but no correlation was found for males, indicating the importance of sex difference in neural speed hypothesis. Moreover, Tan et al. (1993) have reported that the N1 and P1 latencies of visual evoked potentials correlated inversely with IQ, but only for the right side of the brain. The H-reflex latency was inversely correlated with IQ, but this correlation was not always observed: it occurred in left-handers only for the left side and in right-handers only for the right side (Tan, 1991). These findings suggest that the simple hypothesis that speed is one of the biological correlates of g does not seem to be tenable. Yet Jensen believes that " higher IQ subjects show shorter latencies (faster neural reaction)".
4. Sex Differences in G (Chapter 13). The visuo-spatial abilities favoring males have the largest and most consistent sex differences in any psychometric abilities (Jensen`s book, p. 533). Jensen states that "Generally, females with markedly above-average testosterone levels (for females) and males with below-average levels of testosterone (for males) tend to have higher levels of spatial ability, relative to the average spatial ability for their own sex". Indeed, it seems to be well established that males outperform females in spatial abilities. If so, the male hormone testosterone (T) should be beneficial for this kind of cognitive function (cf. Mazur & Booth 1998). It has frequently been reported that T may be advantageous for spatial abilities in animals and humans (see Tan & Tan, 1998). There are also studies -- not mentioned by Jensen -- indicating that serum T level is correlated with spatial IQ in men (Tan, 1990; Tan & Akgun, 1992; Tan, 1998). Hence it is not justified to conclude that low T levels are associated with high spatial abilities in men.
5. Conclusions. My review has attempted to discuss the biological correlates of g summarized in Arthur Jensen`s book. Jensen emphasizes in his abstract that "the current frontier of g research is the investigation of the anatomical and physiological features of the brain that cause g", and "psychometric g also has more direct biological correlates than any other independent source of test variance, for example brain size, brain evoked potentials, nerve conduction velocity". Despite this main aim, only one chapter (Chapter 5, pp. 137-169) is devoted to the biological correlates of g. Here it seemed to me more as if the author were expressing his own subjective ideas rather than objective scientific observations and conclusions.
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Tan, U. (1992) Relation of nonverbal intelligence assessed by Cattell`s Culture Fair Intelligence Test to latencies of the somatosensory evoked potentials elicited by stimulation of the posterior tibial nerves in right-handed male and female subjects. International Journal of Neuroscience, 64: 107-112.
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Tan, U., Akgun, A., Komsuoglu, S., Telatar, M. (1993) Inverse relationship between nonverbal intelligence and the parameters of pattern reversal visual evoked potentials in left-handed male subjects: importance of right brain and testosterone. International Journal of Neuroscience, 71: 189-200.
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