Cognitive Abilities and Intelligence

Though there are more data about the inheritance of intelligence than about any other complex behavioral characteristic of humans, the word intelligence is viewed even by the proponents of IQ testing as misleading because it has too many different meanings. IQ researchers seem to prefer to use the expression "general cognitive ability," represented by the letter g (Jensen; Plomin, DeFries, et al., 2001). The notion of substantial genetic influences on individual variation in g or "intelligence" remains controversial even after almost a century of investigation.

Most investigators in behavioral genetics view the level of intellectual functioning (abstract reasoning, ability to perform complex cognitive tasks, score on tests of general intelligence, IQ) as a strongly heritable trait. In 1963, psychologists Nikki Erlenmeyer-Kimling and Lissy Jarvik summarized the literature dealing with correlations between the measured intelligence ofvarious relatives. After eliminating studies based on specialized samples or employing unusual tests or statistics, they reviewed eighty-one investigations. Included were data from eight countries on four continents spanning more than two generations and containing over 30,000 correlational pairings. The overview that emerged from that mass of data was unequivocal. Intelligence appeared to be a quantitative polygenic trait; that is, a trait influenced by many genes, as are such physical characteristics as height and weight.

The results did not suggest that environmental factors were unimportant, but that genetic variation was quite important. The less sensitive trait of height (or weight) can be used to illustrate this distinction. It is well known that an individual's height can be influenced by nutrition, and inadequate diets during development can result in reduced height. The average height of whole populations has changed along with changes in public health and nutrition. Yet at the same time, individual differences in height (or weight) among the members of a population are strongly influenced by heredity. In general, taller people tend to have taller children across the population as a whole, and the relative height of different people is strongly influenced by their genes. This also appears to be the case with intelligence. The Erlenmeyer-Kimling and Jarvik survey data suggest that about 70 percent of the variation among individuals in measured intelligence is due to genetic differences. The remaining 30 percent of the variation is due to unspecified (and still unknown) environmental effects.

Two decades later, in 1981, Thomas Bouchard and Matt McGue at the University of Minnesota also compiled a summary of the world literature on intelligence correlations between relatives. They summarized 111 studies, 59 of which had been reported during the seventeen years since the Erlenmeyer-Kimling and Jarvik review. Bouchard and McGue summarized 526 familial correlations from 113,942 pairings. The general picture remained the same, with roughly 70 percent of normal-range variation attributable to genetic differences and about 30 percent due to environmental effects.

However, researchers examining the behavioral genetics of cognitive ability estimate the heritability of g (or IQ) as substantially lower, about 30 to 35 percent. Statisticians Bernie Devlin, Michael Daniels, and Kathryn Roeder argue that the much of the difference between the high and low heritabilities can be accounted for by a substantial maternal environmental component. As in the height and weight example above, there is also a substantial general environmental component that increased IQ scores by about 30 points between 1950 and 2000. This is known as the Flynn effect (see Flynn).

Robert Plomin and colleagues have attempted to identify specific genes or gene regions, also known as quantitative trait loci (QTLs), that influence IQ. Though there has been one publication reporting an IQ-related gene (see Plomin, Hill, et al.), replication has not yet been forthcoming.

Much is known about the genetics of mental retardation and learning disabilities. The most common single causes of severe general learning disabilities are chromosomal anomalies (having too many or too few copies of one of the many genes that occur together on a chromosome). These genes may reside on additional chromosomes, for example trisomy 21 (an extra chromosome 21, or three instead of the normal two) is the cause of Down's syndrome, and the "fragile X" condition may by itself account for most, if not all, of the excess of males among people with severe learning disabilities (Plomin, DeFries, et al., 2001). A large number of rare single-gene mutations, many of them recessive, induce metabolic abnormalities that severely affect nervous system function and thus lead to mental retardation. Because the specific alleles involved are individually rare and recessive, such metabolic abnormalities can cause learning-disabled individuals to appear sporadically in otherwise unaffected families. The new field of molecular genetic technology holds a promise of future therapeutic regimens for many learning disabilities.

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