Intelligence is a concept whose meaning has been fashioned by the discipline of psychology. Psychologists view intelligence as a set of mental abilities that are inferred from an individual’s performance on an intelligence test. In defining intelligence as one or more abilities, psychologists seek to demarcate it from the accumulation of specific knowledge to which only some individuals are exposed. In this view, intelligence is a broad cognitive capacity that, though often correlated with the acquisition of specific knowledge, is conceptually distinct from it.
In emphasizing intelligence as a quality of individual cognition, psychologists differentiate it from other individual qualities, such as personality, character, social skills, and physical abilities. Such qualities may be highly valued by a society, and those who possess them may have different life chances than those who do not; but because they are not cognitive skills, they are not acknowledged by most scholars as forms of intelligence. The prevailing view of intelligence as an attribute of the individual also does not admit the more social view of cognition that sees human cognition as distributed across a group of individuals in a particular social setting, who use tools and artifacts to represent knowledge (Hutchins 1995).
Intelligence can be characterized as a hierarchical set of cognitive abilities, with Carroll’s (1993) representation of three levels of mental abilities the most widely cited. At the lowest level are highly specific skills represented by performance on specific tests. One well known intelligence test, the WAIS III (1997), has 14 specific tests, ranging from digit span (in which the examinee repeats an orally presented sequence of numbers either forwards or backwards) to picture completion (in which an examinee must identify what is missing from a color picture of a common object or setting). These specific tests are distinct in the sense that performance on one does not perfectly predict performance on another. But individuals may perform better on some clusters of these specific tests than on others, and the correlations among scores on such tests can be accounted for by a smaller number of factors than the original number of tests. Carroll (1993), in an exhaustive study of the correlations among mental test scores, concluded that there are eight factors that can adequately represent the clusters: broad visual perception, broad auditory perception, broad retrieval ability, broad cognitive speediness, processing speed, general memory and learning, crystallized intelligence, and fluid intelligence.
As was true for performance on specific tests, performance on one of these factors does not perfectly predict performance on another, indicating that the factors are distinct. But individuals who score highly on one factor are more likely to score highly on others, with the average intercorrelation among factors about .70 (Deary 2001). Carroll (1993) concluded that most of the variation among individuals on these eight factors could be accounted for by one general factor, which has historically been referred to as g, or general intelligence. Because g is abstracted from performance on a great many tests, it is often described as a context free measure of general reasoning or problem solving ability, and as the ability to comprehend and respond to complexity in one’s environment.
Modern intelligence tests typically are constructed to have an overall mean of 100 and a standard deviation of about 15, and the distribution of scores within many populations assumes the shape of a bell curve, with about two thirds of the scores clustered between 85 and 115, and fewer than 5 percent of the scores either below 70 or above 130. In the US, individuals identified as African American have historically scored on average about 15 points below those identified as white, an average group difference that parallels the gap observed on a variety of tests of educational achievement. The average black-white difference in IQ test scores cannot be explained by a simple form of cultural bias, as the differences are observed on tasks that appear to require little knowledge of white middle class culture as well as those that are ”culturally loaded” (Jencks 1998). Nor can the black-white test score difference be easily attributed to the lower socioeconomic status of blacks, as substantial differences are observed even when blacks and whites are matched on measurable social and economic characteristics (Phillips et al. 1998). Unmeasured social differences between blacks and whites may be important, but they are not yet an adequate explanation for the black-white difference in IQ test scores.
There is little question that performance on intelligence tests is a function of both genes and environment. Heritability, an attribute of a population of individuals at a particular historical moment, refers to the extent to which variation in individual intelligence scores is due to differences among individuals in their genes and environments. A heritability estimate of zero indicates that all of the variation among individual test scores is due to differences in those individuals’ environments. A heritability estimate of one indicates that all of the variation among individual test scores is due to differences in those individuals’ genes. Although scientists are not yet able to identify the specific genes associated with particular cognitive abilities, the shared genetic heritage of monozygotic (”identical”) and dizygotic (”fraternal”) twins provides some purchase on the relative influence of genes and environment. Within middle class white populations in western societies, heritability estimates typically range from .50 to .70 (Cianciolo & Sternberg 2004).
Heritability estimates tell us little about the origins of a particular individual’s cognitive abilities. In part this is because heritability is defined in relation to a population, not to an individual. But it would be difficult to summarize the mix of genetic and environmental influences in a single number, as individuals experience remarkably different social environments, due to both their social locations in society (Gottfredson 2000) and the unique experiences of individuals within the same general social location or the same family (Gottfredson 1997; Deary 2001). Moreover, individuals have the capacity both to select themselves into new environments and to modify the environments in which they are situated, and these adaptations may be more easily negotiated by individuals with greater cognitive ability.
Some scholars have succumbed to the temptation to link the average black-white difference in IQ scores to the high heritability of intelligence within the middle class white population, suggesting that the origins of the black-white gap in intelligence scores are genetic (Herrnstein & Murray 1994). There is to date little empirical support for this view (Neisser et al. 1996). The existing evidence on black-white differences does not rely on a biological or genetic basis for racial classification, but rather a social basis, individuals’ self-reports. Hence, the observed score difference represents a mixture of social and biological influences (Gottfredson 1997). Moreover, it is widely recognized that the magnitude of the heritability of a trait within a population, such as US blacks, does not imply a similar magnitude of its heritability between populations.
Individuals who score higher on intelligence tests have differing profiles of adult social and economic success than do those who score lower. Intelligence test scores are correlated with educational attainment, job performance, and income, and with the likelihood of delinquent behavior and incarceration. A functional interpretation of these associations is that they reflect the complexity of the modern world, in which only those with advanced cognitive skills can successfully navigate their environments over sustained periods of the life course. It is a logical fallacy, however, to conclude that the advantages enjoyed by people with high IQ test scores are a necessary, and hence natural, feature of modern society. The impact of individual differences in IQ test scores on adult success is expressed within a system of structured social inequality, in which social origins either open or close the doors to opportunity (Fischer et al. 1996). Differences among individuals are real, but their impact can be magnified or muted by the political choices a society makes. Under the current institutional arrangements, individual differences in measured cognitive ability do matter, and perhaps increasingly so.
- Carroll, J. B. (1993) Human Cognitive Abilities: A Survey of Factor Analytic Studies. Cambridge University Press, Cambridge.
- Cianciolo, A. T. & Sternberg, R. J. (2004) Intelligence: A Brief History. Blackwell, Malden, MA.
- Deary, I. J. (2001) Intelligence: A Very Short Introduction. Oxford University Press, New York.
- Fischer, C. S., Hout, M., Sanchez Jankowski, M., Lucas, S. R., Swidler, A., & Voss, K. (1996) Inequality by Design: Cracking the Bell Curve Myth. Princeton University Press, Princeton.
- Gottfredson, L. S. (1997) Mainstream Science on Intelligence: An Editorial with 52 Signatories, History, and Bibliography. Intelligence 24: 13-23.
- Gottfredson, L. S. (2000) Intelligence. In: Borgotta, E. F. & Montgomery, R. J. V. (Eds.), Encyclopedia of Sociology, 2nd edn. Macmillan, New York, pp. 1359-86.
- Herrnstein, R. J. & Murray, C. (1994) The Bell Curve: Intelligence and Class Structure in American Life. Free Press, New York.
- Hutchins, E. (1995) Cognition in the Wild. MIT Press, Cambridge, MA.
- Jencks, C. (1998) Racial Bias in Testing. In: Jencks, C. & Phillips, M. (Eds.), The Black White Test Score Gap. Brookings Institution Press, Washington, DC, pp. 55-85.
- Neisser, U., Boodoo, G., Bouchard, T. J., Jr., Boykin, A. W., Brody, N., Ceci, S. J., Halpern, D. F., Loehlin, J. C., Perloff, R., Sternberg, R. J., & Urbina, S. (1996) Intelligence: Knowns and Unknowns. American Psychologist 51: 77-101.
- Phillips, M., Brooks-Gunn, J., Duncan, G. J., Klebanov, P., & Crane, J. (1998) Family Background, Parenting Practices, and the Black White Test Score Gap. In: Jencks, C. & Phillips, M. (Eds.), The Black White Test Score Gap. Brookings Institution Press, Washington, DC, pp. 103-45.