Much of the uncertainty about the relative influence of heredity and environment relates to our imprecise views of what constitutes intelligence. The authors tend to agree with those who view intelligence as a general mental capability, embracing a number of primary abilities: the capacity to comprehend complex ideas, learn from experience, think abstractly, reason, plan, draw analogies, and solve problems. Thus intelligence includes a multiplicity of abilities, which probably accounts for a lack of consensus about its mechanism. Everyday experience teaches us that it is not always abstract tasks that suffer most when intelligence is impaired. Indeed, even abstraction is not likely to be a unitary function. Theoreticians, like Carl Spearman, believed that intelligence comprises a general (g), or core, factor and a series of special (s) factors. In contrast, Thurstone conceived of intelligence as a mosaic of factors such as drive and curiosity, verbal and arithmetic ability, memory, capacity for abstract thinking, practical skills in manipulating objects, geographic or spatial sense, and athletic and musical ability—each of which appears to be largely genetically determined. These and other theories concerning intellectual development in the child, such as those of Eysenck and of Gardner, and particularly those of Piaget, were considered briefly in Chap. 20, which may be read as an extension of this section.
The beginnings of the development of intelligence are difficult to discern. It is readily assessed at 8 to 9 months of life, when the infant begins to crawl and explore. Now, learning proceeds rapidly as adults attach names to objects and help the baby manipulate them. Gradually the child acquires verbal facility (learning what words mean), memory, color and spatial perception, a concept of number, and the practical use of tools, each at a particular time according to a schedule set largely by the maturational state of the brain. Nevertheless, in these early achievements individuals differ considerably, reflecting to some extent the influence of their parents and others around them. Also, the young child exhibits elementary modes of thinking but is highly suggestible and often incapable of separating imagination from reality.
Neurologists who need a quick and practical method of ascertaining whether an infant or preschool child is measuring up to normal standards for a particular age will find Table 27-4 useful. The main items are drawn from Gesell and Amatruda and from the Denver Developmental Test. In addition, a variety of intelligence tests have been designed to measure the child’s special abilities and increasing success in learning in accordance with age (these are listed in Table 27-5). Starting at age 6 to 7 years, there is a steady improvement in intelligence scores that parallels chronologic age up to about age 13 years; thereafter the rate of advance diminishes. By age 16 to 17 years, performance reaches a plateau, but this is probably an artifact of the commonly used tests, which are designed to predict success in school. Only late in life do test scores begin to diminish, in a manner that is described in the next chapter on aging. Individuals with high or low IQs at 6 years of age tend to maintain their rank at 10, 15, and 20 years unless the early scores were impaired by anxiety, poor motivation, or a gross lack of opportunity to acquire the skills that are necessary to take such tests (language skill in particular). Even then, performance tasks, which largely eliminate verbal and mathematical skills, will disclose many individual differences.
The reliability of intelligence tests and their validity as predictive measures of scholastic, occupational, and economic success have been heatedly debated for many years. This aspect of the subject is discussed in the section on Intelligence in Chap. 21 and need not be repeated here. The most persuasive argument for these tests as an index of some type of native ability is that individuals drawn from a fairly homogeneous environment tend to maintain the same rating on the intelligence scale throughout their lives. While native endowment may set some limits on learning and achievement; opportunity, personality traits, and other factors clearly determine how nearly the individual’s full potential is realized.
The Development of Language