Chapter 27 discusses the developmental aspects of intelligence in more detail. One of the leading theories had been that of Piaget, who proposed that the emergence of intelligence is accomplished in discrete stages related to age: sensorimotor, from 0 to 2 years; preconceptual thought, from 2 to 4 years; intuitive thought, from 4 to 7 years; concrete operations (conceptualization), from 7 to 11 years; and, finally, the period of “formal operations” (logical or abstract thought), from 11 years on. This scheme implies that the capacity for logical thought, developing as it does according to an orderly timetable, is coded in the genes. Surely, one can recognize these states of intellectual development in the child, but Piaget’s theory has been criticized as being anecdotal and lacking the quantitative validation that could be derived only from studies of a large normal population. Furthermore, it does not take into account an individual’s special abilities, which do not usually develop and reach their maximum at the same time as the more general intellectual capacities.
One would suppose that neurology, embodying an understanding of so many diseases affecting the cerebrum, might make it possible to verify one of these several theories of intelligence and to determine the anatomy of this cognitive entity. Presumably, Spearman’s g factor of intelligence would be maximally impaired, by diffuse lesions, in proportion to the mass of brain involved, an idea expressed by Lashley as the “mass-action principle.” Indeed, according to Chapman and Wolff, there is a correlation between the volume of brain tissue lost and a general deficit of cerebral function. Others disagree, claiming that no universal psychologic deficit can be linked to lesions affecting particular parts of the brain. Probably the truth lies between these two divergent points of view. According to Tomlinson and colleagues, who studied the effects of vascular lesions in the aging brain, lesions that involved more than 50 mL of tissue caused a moderate general reduction in performance, especially in speed and capacity to solve problems. Piercy, on the other hand, found correlations only between specific intellectual deficits and lesions of particular parts of the left and right hemispheres. It is important to acknowledge, for example, that lesions of the frontal lobes, and particularly the prefrontal regions, which disorder planning and “executive” functions, do not measurably affect overall IQ but do, of course, slow mental processing and degrade subtests specific to these skills. These problems are discussed in Chap. 21 on the localized functions of the cerebral cortex.
The authors conclude from experience and from evidence provided by neurologic studies that intelligence consists of a combination of multiple primary abilities, each of which may be inherited and each of which has a separate but poorly delineated anatomical representation. Yet we would disagree with both Thurstone and Gardner that these special abilities are of equivalent weight with regard to what is generally considered as “intelligence.” When viewed in the light of the classics of literature, history, and science, some of them, namely linguistic and mathematical, and perhaps spatial–dimensional abilities, are more closely integral to ideation and problem solving. Furthermore, they are affected most in the developmentally delayed and lost early in dementing diseases. To the extent that facility with general mental performance, that requiring the manipulation of abstract symbols and thoughts, marks an individual as “intelligent” and that these correlate with each other, we find Spearman’s g factor to be a credible but still not satisfying concept for intelligence.