Modern Biological Views and Research

1. Speed of Neuronal Conduction

Recent theories have dealt with more specific aspects of brain or neural functioning. For example, one view suggested that individual differences in nerve conduction velocity are a basis for individual differences in intelligence. Conduction velocity has been measured either centrally (in the brain) or peripherally (e.g., in the arm).

Some investigators tested brain nerve conduction velocities via two medium-latency potentials, N70 and P100, which were evoked by pattern-reversal stimulation. Subjects saw a black-and-white checkerboard pattern in which the black squares would change to white and the white squares to black. Over many trials, responses to these changes were analyzed via electro des attached to the scalp in four places. Correlations of derived latency measures with IQ were small (generally in the 0.1 to 0.2 range of absolute value) but were significant in some cases, suggesting at least a modest relation between the two kinds of measures.

Other investigators reported on two studies investigating the relation between nerve conduction velocity in the arm and IQ. In both studies, nerve conduction velocity was measured in the median nerve of the arm by attaching electrodes to the arm. In the second study, conduction velocity from the wrist to the tip of the finger was also measured. Vernon and Mori found significant correlations with IQ in the 0.4 range as well as somewhat smaller correlations (approximately —0.2) with response time measures. They interpreted their results as supporting the hypothesis of a relation between speed of information transmision in the peripheral nerves and intelligence. However, these results must be interpreted cautiously because a later study did not successfully replicate these earlier results.

2. Glucose Metabolism

Some of the most interesting recent work using the biological approach has been done by Richard Haier and colleagues. For example, their research showed that cortical glucose metabolic rates as revealed by positron emission tomography scan analysis of subjects solving Raven matrix problems were lower for more intelligent than for less intelligent subjects, suggesting that the more intelligent subjects needed to expend less effort than the less intelligent ones to solve the reasoning problems. A later study showed a similar result for more versus less practiced performers playing the computer game of Tetris. In other words, smart people or intellectually expert people do not have to work as hard as less smart or intellectually expert people at a given problem.

What remains to be shown, however, is the causal direction of this finding. One could sensibly argue that the smart people expend less glucose (as a proxy for effort) because they are smart rather than that people are smart because they expend less glucose. Also, both high IQ and low glucose metabolism may be related to a third causal variable. In other words, we cannot always assume that the biological event is a cause (in the reductionistic sense). It may be, instead, an effect.

3. Brain Size

Another approach considers brain size. Investigators correlated brain size with Wechsler Adult Intelligence

Scale (WAIS-R) IQs, controlling for body size. They found that IQ correlated 0.65 in men and 0.35 in women, with a correlation of 0.51 for both sexes combined. A follow-up analysis of the same 40 subjects suggested that, in men, a relatively larger left hemisphere better predicted WAIS-R verbal ability than it predicted nonverbal ability, whereas in women a larger left hemisphere predicted nonverbal ability better than it predicted verbal ability. These brain size correlations are suggestive, but it is currently difficult to determine what they indicate.

4. Behavior Genetics

Another approach that is at least partially biologically based is that of behavior genetics. The literature is complex, but it appears that about half the total variance in IQ scores is accounted for by genetic factors. This figure may be an underestimate because the variance includes error variance and because most studies of heritability have been performed with children, but it is known that heritability of IQ is higher for adults than for children. Also, some studies, such as the Texas Adoption Project, suggest higher estimates: 0.78 in the Texas Adoption Project, 0.75 in the Minnesota Study of Twins Reared Apart, and 0.78 in the Swedish Adoption Study of Aging.

At the same time, some researchers argue that effects of heredity and environment cannot be clearly and validly separated. Perhaps, future research should focus on determining how heredity and environment work together to produce phenotypic intelligence, concentrating especially on within-family environmental variation, which appears to be more important than between-family variation. Moreover, peers seem to have a particularly large effect on the development of various personal attributes, probably including cognitive skills. Such research requires, at the very least, very carefully prepared tests of intelligen-ce—perhaps some of the newer tests described in the next section.

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