As every educated person recognizes, intelligence has something to do with normal cerebral function. It is also apparent that the level of intelligence differs from one person to another, and members of certain families are exceptionally bright and intellectually accomplished, whereas members of other families are just the opposite. If properly motivated, intelligent children excel in school and score high on intelligence tests, although this may be tautologic as the tests are designed specially to measure certain aspects of performance. Furthermore, the first intelligence tests, devised by Binet and Simon in 1905, were for the purpose of predicting scholastic success. The term intelligence quotient, or IQ, was introduced by the German psychologist Stern and used by Terman in 1916 for the development of intelligence testing. It denotes the figure that is obtained by dividing the subject’s mental age (as determined by the Binet-Simon scale) by his chronologic age (up to the 14th year) and multiplying the result by 100. The IQ correlates, but only broadly, with achievement in school and to a lesser extent with eventual success in professional work. An individual IQ increases with age up to the 14th to 16th years and then remains stable, at least until late adult life. At any age, a large sample of normal children attains test scores of a normal, or gaussian, distribution.
The original studies of pedigrees of highly intelligent and mentally less able families, which revealed a striking concordance between parent and child, lent support to the idea that intelligence is to a large extent inherited. However, it became evident that the tests used were also greatly influenced by the environment in which the child was reared. Moreover, tests were less reliable in identifying talented children who were not offered optimal opportunities. This led to the equally polarized but widespread belief that intelligence tests are only achievement tests and that environmental factors fostering high performance are the important factors determining intelligence.
Neither of these views is likely to be entirely correct. Past studies of monozygotic and dizygotic twins raised in the same or different families put the matter in a clearer light. Identical twins reared together or apart are more alike in intelligence than nonidentical twins brought up in the same home (see reviews of Willerman, of Shields, and of Slater and Cowie). A study of elderly twins by McClearn and colleagues is further instructive; even in twins who were older than 80 years of age, a substantial part (an estimated 62 percent) of cognitive performance could be accounted for by shared genetic traits. These findings suggest that life experience alters intelligence, but in a modest way. There is therefore a strong suggestion that genetic endowment is the more important factor—a view that was championed by Piercy and more recently by Herrnstein and Murray. However, there is equally valid evidence that early learning modifies the level of ability that is finally attained. In this way, intelligence may be looked upon not as the sum of genetic and environmental factors but as the product of the two. More importantly, it is generally appreciated that nonscholastic achievement or success is governed by factors other than intellectual ones, such as curiosity, a readiness to learn, interest, persistence, sociability, and ambition or motivation—factors that vary considerably from person to person and are not at all measured by tests of intelligence.
As to the genetic mechanisms involved in the inheritance of intelligence, a limited amount is known. There is an excess of males with what was previously called mental retardation, and now less pejoratively “developmental delay,” and there are several well-characterized syndromes in which the inheritance of mental retardation is X-linked as described in Chaps. 27 and 37. Also notable are the somewhat different patterns of performance between males and females on subtests of the various intelligence tests (males perform better in spatial ability and certain mathematical tasks). Males are more likely to be affected by advantageous or by aberrant genes on their single X chromosome, whereas females benefit from the mosaic provided by two X chromosomes. In some families, high intelligence segregates to certain individuals through an X-linked pattern. Further study will determine the validity of these views and of what will certainly prove to be a polygenic inheritance of intelligence and intellectual traits.