| The G factor: the Science of Mental Ability |
[Sep. 8th, 2003|05:14 pm] |
Jensen, Arthur R. (1999)
http://psycprints.ecs.soton.ac.uk/archive/00000658/#html
2. The formal recognition of individual differences in mental ability as a subject for study in its own right arose as an outgrowth of the idea of evolution in the mid-nineteenth century. For the first time in history, animals' behavioral capacities and humans' mental abilities were recognized as a product of the evolutionary process, as were the physical systems of organisms. Darwin's theory of natural selection as the mechanism of evolution implied that organisms' behavioral capacities, along with their anatomy and physiology, evolved as adaptations to particular environments. In Darwin's theory, hereditary variation is a necessary condition for the working of natural selection. From this insight, Herbert Spencer, the early philosopher of evolution, interpreted individual differences in intelligence as intrinsic to the human condition. He further introduced the notion that human intelligence evolved as a unitary attribute.
3. Individual differences in mental qualities, however, did not become a subject for empirical study until the latter half of the nineteenth century, with the pioneer efforts of Sir Francis Galton, who is generally regarded as the father of differential psychology (the study of individual and group differences in human traits, which includes behavioral genetics). Galton introduced the idea of objective measurement of human capacities, devised tests to measure simple sensory and motor functions, and invented many of the statistical concepts and methods still used in the study of individual differences. He was the first to apply empirical methods to studying the inheritance of mental ability. Galton's conclusions, or beliefs, are consistent with his empirical findings but are not at all adequately supported by them. They may be briefly summarized as follows:
4. Human mental ability has both general and specific components: the general component is the larger source of individual differences; it is predominantly a product of biological evolution, and is more strongly hereditary than are specific abilities, or special talents. Mental ability, which ranges widely in every large population, is normally distributed, and various human races differ, on average, in mental ability. General ability is best measured by a variety of fairly simple tests of sensory discrimination and reaction time.
9. Spearman thought of g metaphorically as "mental energy" that could be applied to any and every kind of mental task, and likened group factors and specificity to specialized "engines" for the performance of certain types of tasks. According to Spearman, individual differences in potential performance on any mental task result from two sources: differences in the amount of mental "energy" that can be delivered to the specific "engine" that mediates performance of the task, and differences in the efficiency of energy utilization by the "engine." The efficiency of the various "engines" differs independently within the same person.
15. Among the various methods of factor analysis that do not mathematically preclude the appearance of g when it is actually latent in the correlation matrix, a hierarchical model is generally the most satisfactory, both theoretically and statistically. In a hierarchical analysis, a number of correlated group factors (first-order factors) are extracted first. The g factor then emerges as a second-order factor (or as a third-order factor in some very large and diverse batteries) from the correlations among the first-order factors (or among the second-order factors when g is at the third order).
23. It is wrong to regard g as a cognitive process, or as an operating principle of the mind, or as a design feature of the brain's neural circuitry. At the level of psychometrics, ideally, g may be thought of as a distillate of the common source of individual differences in all mental tests, completely stripped of their distinctive features of information content, skill, strategy, and the like. In this sense, g can be roughly likened to a computer's central processing unit. The knowledge and skills tapped by mental test performance merely provide a vehicle for the measurement of g. Therefore, we cannot begin to fathom the causal underpinning of g merely by examining the most highly g-loaded psychometric tests. At the level of causality, g is perhaps best regarded as a source of variance in performance associated with individual differences in the speed or efficiency of the neural processes that affect the kinds of behavior called mental abilities.
31. The fact that psychometric g has many physical correlates proves that g is not just a methodological artifact of the content and formal characteristics of mental tests or of the mathematical properties of factor analysis, but is a biological phenomenon. The correlations of g with physical variables can be functional (causal), or genetically pleiotropic (two or more different phenotypic effects attributable to the same gene), or genetically correlated through cross-assortative mating on both traits, or the nongenetic result of both being affected by some environmental factor (e.g., nutrition). The physical characteristics correlated with g that are empirically best established are stature, head size, brain size, frequency of alpha brain waves, latency and amplitude of evoked brain potentials, rate of brain glucose metabolism, and general health.
34. Individual differences in mental test scores have a substantial genetic component indexed by the coefficient of heritability (in the broad sense), that is, the proportion of the population variance in test scores attributable to all sources of genetic variability. The broad heritability of IQ is about .40 to .50 when measured in children, about .60 to .70 in adolescents and young adults, and approaches .80 in later maturity.
37. Traits that show genetic dominance provide evidence that they have been subjected to natural selection as a Darwinian fitness character over the course of evolution. IQ, and particularly its g component, manifest the theoretically predictable effects of genetic dominance: inbreeding depression in the offspring of consanguineous parents, and the opposite effect, hybrid vigor (or heterosis), that shows up in the offspring when each parent has a different racial ancestry. Tests' relative g loadings significantly predict the degree to which various tests manifest both inbreeding depression and heterosis. These data support the hypothesis that the g factor of psychometric tests has arisen through natural selection over the course of human evolution and therefore can be regarded as a fitness character in the Darwinian sense.
43. The physiological properties of the brain that might account for the speed-of-processing aspect of g are not yet known completely, but it seems safe to say that they would have to be properties that are common to all regions and modules of the brain that subserve cognitive functions in which there are reliable individual differences in the neurologically normal population. One obvious candidate is individual differences in nerve conduction velocity (NCV). Brain NCV increases along with measurements of mental growth from childhood to maturity and decreases along with mental decline in old age. NCV is also significantly correlated with IQ (Raven matrices) in college students.
44. At the level of complex psychometric tests the g factor is unitary. But it now appears most unlikely that g is unitary at the level of its causal underpinnings, as indicated by the timed measurements of performance on various ECTs and by neurophysiological measurements of variables such as NCV, rate of glucose metabolism (PET scan), and degree of myelination (MRI) of nerve fibers.
46. The validity of g is most conspicuous in scholastic performance, not because g-loaded tests measure specifically what is taught in school, but because g is intrinsic to learning novel material, grasping concepts, distinctions, and meanings. The pupil's most crucial tool for scholastic learning beyond the primary grades - reading comprehension - is probably the most highly g-loaded attainment in the course of elementary education.
50. The g factor is also reflected in many broad social outcomes. Many social behavior problems, including dropping out of school, chronic welfare status, illegitimacy, child neglect, poverty, accident proneness, delinquency, and crime, are negatively correlated with g or IQ independently of social class of origin. These social pathologies have an inverse monotonic relation to IQ level in the population, and show, on average, nearly five times the percentage of occurrence in the lowest quartile (IQ below 90) of the total distribution of IQ as in the highest quartile (IQ above 110).
68. In this section, sex differences are specifically examined in terms of their loadings on the g factor for a number of test batteries administered to representative population samples. When the sex differences (expressed as a point-biserial correlation between sex and scores on each of a number of subtests) were included in the correlation matrix along with the various subtests, and the correlation matrix was subjected to a common factor analysis, sex had negligible and inconsequential loading on the g factor, averaging about .01 over five test batteries. Applying the method of correlated vectors to these data shows that the magnitude of the sex difference on various subtests is unrelated to the tests' g loadings. Also, the male/ female variance ratio on diverse subtests (generally indicating greater male variability in scores) is unrelated to the subtests' g loadings. Although no evidence was found for sex differences in the mean level of g or in the variability of g, there is clear evidence of marked sex differences in certain group factors and in test specificity. Males, on average, excel on some factors, females on others. The largest and most consistent sex difference is found on a spatial visualization factor that has its major factor loadings on tests requiring the mental rotation or manipulation of figures in an imaginary three-dimensional space. The difference is in favor of males, and within each sex is related to testosterone level. But the best available evidence fails to show a sex difference in g. |
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