One would think that neurologic structure and function would correlate in some way with intelligence, but with the exception of the pathologically developmentally delayed (see Chaps. 27 and 37), such an association has been difficult to document. Brain weight and the complexity of the convolutional pattern are not correlated with intelligence—despite popular notions to the contrary, including a widely criticized analysis of the brain of Albert Einstein. (Witelson and colleagues proposed that an enlarged inferior parietal lobule, a crossmodal association area, accounted for Einstein’s visuospatial and mathematical genius, but this is certainly an oversimplification). Only laboratory measures of vigilance and facility of sensory registration (speed of motor responses/reaction time and rapid recognition of differences between lines, shapes, or pictures) have a consistent but still modest correlation with IQ. However, it is of interest that morphometric features of the regions of the cortex that are presumed to underlie IQ and verbal skills, such as the frontal and language areas, show a heritable component when measured on high-resolution MRI scans (see Thompson et al).
As to psychologic theories of intelligence, several have traditionally been influential at different historical periods. One is the two-factor theory of Spearman, who noted that all the separate tests of cognitive abilities correlated with each other, suggesting that a general factor (g factor) enters into all performance. Because none of the correlations between subtests approached unity, he postulated that each test measures not only this general ability (commonly identified with intelligence) but also a subsidiary factors specific to the individual tests, which he designated the s factors. A second theory, the multifactorial theory of Thurstone, proposed that intelligence consists of a number of entirely separable primary mental abilities, such as memory, verbal facility, numerical ability, visuospatial perception, and capacity for problem solving, all of them more or less equivalent. He proposed that these primary abilities, although correlated, are not subordinate to a more general ability. For Eysenck, intelligence exists in three forms: biologic (the genetic component), social (development of the genetic component in relation to personal relationships), and a number of specific abilities subject to measurement by psychometric tests.
Thurstone’s multifactorial theory of intelligence has been periodically reframed, for example by Gardner, who separated six categories of high-order cerebral ability but restated them in more modern terms: linguistic (encompassing all language functions); musical (including composition and performance); logical–mathematical (the ideas and works of mathematicians); spatial (including artistic talent and the creation of visual impressions); bodily–kinesthetic (including dance and athletic performance); and the personal (consciousness of self and others in social interactions). He referred to each of these as intelligences, defined as the ability to solve problems or resolve difficulties and to be creative within the particular field. Several lines of evidence are marshaled in support of this parceling of skills and abilities: (1) each may be developed to an exceptionally high level in certain individuals, constituting virtuosity or genius; (2) each can be destroyed or spared in isolation as a consequence of a lesion in a certain part of the nervous system; (3) in certain individuals, that is, in prodigies, special competence in one of these abilities is evident at an unusually early age; (4) in the autism spectrum, one or more of these abilities may be selectively spared or developed to an abnormally high degree (idiot savant). Each of these entities appears to have a genetic basis in so far as musical, artistic, mathematical, and athletic ability often runs in families, but their full development is influenced by environmental factors.
There are only limited data regarding the highest levels of intelligence, identified as genius. Terman and Ogden’s longitudinal study of 1,500 California schoolchildren who were initially tested in 1921 supported the idea that an extremely high IQ predicted future scholastic accomplishments (though not occupational or life success). On the other hand, most individuals recognized as geniuses have been especially skilled in one domain—such as painting, linguistics, music, chess, or mathematics—and such “domain genius” is not necessarily predicated on high IQ scores, although certain individuals display crossmodal superiorities—particularly in mathematics and music.