Forum TRENDS in Cognitive Sciences Vol.5 No.3 March 2001 127 Historical Perspective Human intelligence differences: a recent history Ian J. Deary Differences among humans in their mental important5. As a result of The Bell Curve’s that it does not emerge (Guilford, abilities are prominent, important, and controversies, the APA put together a task Thurstone, Gardner, Cattell and Horn); that controversial. In part, the controversy force of 11 people to write a report on there is a hierarchy of mental abilities from arises from over-uses and abuses of mental ‘Intelligence: Knowns and Unknowns’. The the general factor through broad ability tests, from insalubrious events in the individuals concerned came from different factors to very specific, narrow abilities history of mental test research, and from research traditions within and outside (Burt, Vernon) or that there is merely a lack of knowledge about what is and is not intelligence and were known to hold very range of uncorrelated narrow abilities currently known concerning human different views on the topic. Yet, they (Guilford)10,11. The resolution of these intelligence. In this article, some of the managed to produce a wide-ranging review debates was available from the 1940s, but well-attested facts about human intelligence article that all contributors signed. It not widely recognized. By 1939, Eysenck differences are summarized. A striking remains a touchstone for disinterested and showed that even Thurstone’s own data limitation of this body of research is that, authoritative information about intelligence contained a general factor that refuted his whereas much is known about the taxonomy differences. The following sections provide own early ideas that there were only separate and predictive validity of human intelligence a summary of some of the ‘knowns’ about abilities12. And in 1940, Burt described a differences, there has been relatively little human intelligence differences that hierarchy of mental abilities that is not progress in understanding their nature, emerged during the 20th century. distinguishable from the ‘new’ consensus with the exception of behaviour genetic When ‘intelligence’ is referred to, what that emerged in the 1980s and 1990s13. studies. An article which will follow this one is meant is ‘psychometric intelligence’, the A hierarchical model of human explores attempts to understand human human differences measured by mental intelligence differences came to dominate in intelligence differences in cognitive terms. tests. Psychometric mental ability tests do the mid-to-late 1990s because of two lines of A previous article addressed recent not cover all the capabilities of humans6. converging research. First, the development research on the biological origins of human Nevertheless, in Carroll’s massive survey7 of structural equation modelling techniques intelligence differences1, and updates on and re-analyses of hundreds of data sets meant that hypotheses about the structure this topic are available elsewhere2,3. in intelligence research there are sections of psychometric abilities could be tested on abilities in the domains of language, competitively. Analyses of large, single Knowns and unknowns about human reasoning, memory and learning, visual datasets found that a hierarchical model intelligence differences perception, auditory perception, idea provided best fit to the data14,15 (see Box 1). The most recent furore about human production, cognitive speed, knowledge Second, Carroll published his survey of intelligence differences came in the wake and achievement, psychomotor abilities, over 400 datasets – many of them being of the surprise US best-seller, The Bell and miscellaneous other areas. Thus, the classic datasets in the 20th century’s Curve4. The book is a peculiar, incendiary there are speeded and non-speeded research on human intelligence differences sandwich, with a series of regression abilities, education-related and -unrelated – in which mental test batteries were given analyses of the National Longitudinal abilities, paper-and-pencil and other types to human subject samples from childhood Study of Youth in the USA enclosed of test, and tests covering a large range of to old age7. He subjected them to a uniform between essays relating the authors’ human mental functions. It is not the set of psychometric procedures. He worries about an intelligence-based social full gamut of human performance, as concluded from these re-analyses that apartheid and social policy considerations. emphasized by Sternberg6 and Gardner8, the best model of ability differences was a The arguments that followed its publication but it is an undeniably broad and important three-stratum hierarchy. resulted in two remarkable documents. range of human mental activities. Therefore, at present there is consensus The first was a full-page advertisement that there is a general cognitive factor that in the Wall Street Journal (December 13th, The taxonomy of psychometric accounts for about 50% or so of the variance 1994) entitled ‘Mainstream Science on intelligence differences in a broad assembly of mental tests given Intelligence’. Its 25 paragraphs described Spearman’s discovery, in 1904, of a general to a large sample of the population. The ‘conclusions regarded as mainstream factor common to many different mental general factors from different batteries of among researchers on intelligence’, and was abilities, has invariably been replicated in mental tests show very high correlations, signed by 52 researchers in the field. It is an datasets in which a large number of often well above 0.9 (Ref. 10). When that odd place for such a document, and readers humans undertook a variety of mental variance is taken into account, there is still might view the signatories as one-sided, tests9,10. To the onlooker during most of the variance attributable to separable ‘group’ largely committed to the psychometrics- 20th century, however, the taxonomy of factors of intelligence. The most commonly based intelligence research they were intelligence differences must have appeared emerging group factors are verbal, spatial, endorsing. That is why the second, the chaotic, with some past and present memory and processing speed, though report of the American Psychological researchers: insisting that the general different numbers and types of group Association’s (APA) Task Force, is so factor is important (Spearman, Jensen) or factors may be found depending on the http://tics.trends.com 1364-6613/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII:S1364-6613(00)01621-1 128 Forum TRENDS in Cognitive Sciences Vol.5 No.3 March 2001 Box 1. The hierarchical structure of mental ability differences For the present article a new analysis means that most of the variance apparently group factor has 88% (100 × 0.942) of its was performed on the American arriving at the tests from the group factors variance due to g, the factor shared with all standardisation sample of the Wechsler actually comes from g. Take the example of three other group factors. Therefore, about Adult Intelligence Scale-III (WAIS-III; Ref. a). matrix reasoning. Its parameter weight 52% of the variance (100 × 0.59 × 0.88) in The model was a hypothesis about the from the ‘perceptual organisation’ group matrix reasoning is due to g and only about structure of the variances and co-variances factor is 0.77, meaning that just over 59% 7% (12 × 0.59) due to the group factor. of the 13 tests of the WAIS-III. The (100 × 0.772) of variance in this test is due to hypothesis was tested using the EQS this group factor, which is shared with the References a Wechsler, D. (1997) Manual for the Wechsler Adult structural equation modelling program three other tests, and the rest is variance Intelligence Scale-III, Psychological Corporation (maximum likelihood method; Ref. b). The specific to the test and error variance. b Bentler, P.M. (1995) EQS Structural Equations model is a strict hierarchy: each test was However, the ‘perceptual organisation’ Program Manual, Multivariate Software, Inc. allowed to load on only one group factor, and all group factors were assumed to load on the general factor (g). No associations were allowed among tests or group factors, g other than through the latent variables at the level higher in the hierarchy. The results can be seen in Fig. 1. This 0.86 0.94 0.91 0.78 analysis was based upon the variances and co-variances among the 13 subtest scores available on 2450 subjects in the test Verbal Perceptual Working Processing manual. The model fits well by many comprehension organisation memory speed criteria. The 13 tests first agglomerate into four group factors, the names of which are derived from the WAIS-III manual. Note that each subtest has a high loading on only one 0.90 0.85 0.85 0.83 0.67 0.75 0.77 0.68 0.81 0.66 0.72 0.74 0.88 group factor. These parameter weights are estimated by the structural equation v s i c pc bd mr pa a ds ln cd ss modelling programme and are like partial beta weights in a regression model. Note TRENDS in Cognitive Sciences too that all four of the group factors have high associations with the general factor. Fig. 1. The blue squares represent the 13 WAIS-III subtests (for abbreviations see below). The ellipses and the circle containing g (general factor in intelligence) are latent variables whose names are adopted for convenience. Correlations among the group factors Fit indices are given for each variable and subtest. The average of the off-diagonal absolute standardized residuals are between 0.63 and 0.83 with a mean of was 0.027 (below 0.04 is good). The following fit indices have values of 0.9 or greater in well-fitting models: 0.76, refuting the idea that there might be Bentler-Bonett normed fit index = 0.965; Bentler-Bonett non-normed fit index = 0.959; and the comparative fit = independent ‘primary mental abilities’ at index 0.968.