Human Intelligence Differences: a Recent History Ian J

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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

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. The present model is very economical. As is typically the case with large sample sizes the chi square value for the model is significant (= 663.4, d.f. = 61, p < 0.001). Abbreviations: v, vocabulary; s, similarities; i, information; this group factor level. The fact that the c, comprehension; pc, picture completion; bd, block design; mr, matrix reasoning; pa, picture arrangement; group factors are so closely related to g a, arithmetic; ds, digit span; ln, letter–number sequencing; cd, digit-symbol coding; ss, symbol search.

exact battery of tests that is employed. Stability and ageing of psychometric abilities are vocabulary, general Beyond the general factor and group intelligence differences knowledge and some number skills. ‘Fluid’ factors some test performance variance is Across several decades of adulthood mental abilities show decrements as people grow not accounted for, and is manifest in very ability differences show high stability older19–21. Fluid intelligence is often specific mental abilities. This so-called coefficients. Typically, more crystallized described by tasks that require abstract three-stratum model affords freedom from abilities (age-resistant) show higher reasoning, maybe under pressure of time, otiose arguments about being ‘for or against stability than more fluid (age-sensitive) with novel materials, in situations where the general factor.’ The three stratum account abilities; the former can be around 0.8 past knowledge and education can offer has been called a theory. It is not. And it is and the latter at or above 0.6 (Ref. 17). no assistance in coming to an answer. not a model of the modal human cognitive Our own follow-up study, 66 years later, Tasks including memory, processing architecture: rather, it is a taxonomy or of 101 participants of the Scottish Mental speed, and types of reasoning show ageing model of test variances and co-variances. Survey of 1932 found a stability coefficient decrements. Ageing largely affects the The taxonomy does not explain human of 0.63 (0.73 when corrected for attenuation general factor in batteries of tasks that intelligence differences, it describes them. of the ability range in the re-tested sample) decline with age22. Once that effect is It offers target pools of variance (general to for test scores on the Moray House Test taken into account, there is little residual specific) for explaining by investigations from age 11 to 77 years18. effect of age on more specific abilities. which inquire about the origins of variation So-called ‘crystallized’ abilities hold up Moreover, there are some strong data and in human mental abilities16. well with healthy ageing. Examples of such advocacy for the theory that cognitive

http://tics.trends.com Forum TRENDS in Cognitive Sciences Vol.5 No.3 March 2001 129 ageing is largely caused by slowing of and their causes – has long existed. It was experimental/cognitive psychologists and speed of information processing23. explicit in Huarte’s 16th century treatise differential psychologists in understanding on ‘mens’ wits’: he discussed the variety of human intelligence differences. Environmental and genetic influences on cognitive abilities, their predictive validity psychometric intelligence differences for professions, and discussed classical References 1 Deary, I.J. and Caryl, P.G. (1997) Neuroscience Reviews of family, adoption and twin (reared and then-contemporary evidence that and human intelligence differences. Trends together and apart) studies put beyond differences in the brain’s dryness and Neurosci. 20, 365–371 doubt that there is a substantial heritability temperature were the origins of mental 2 Vernon, P.A. et al. (2000) The neuropsychology to psychometric intelligence differences24,25. ability differences33. At the beginning of and psychophysiology of human intelligence. In Within the research area there is relatively the 20th century, Spearman was more Handbook of Intelligence (Sternberg, R.J., ed.), pp. 245–264, Cambridge University Press little concern with the exact estimate. concerned with the taxonomy and causes 3 Duncan, J. et al. (2000) A neural basis for general 9 Nevertheless, combining all studies puts of intelligence differences , whereas intelligence. Science 289, 457–460 heritability at about 50%, with strong Binet focussed on the predictive validity 4 Herrnstein, R.J. and Murray, C. (1994). The Bell suggestions that heritability might be of intelligence measures34. Curve, Free Press different at different ages, with especially Before psychology became an 5 Neisser, U. et al. (1996) Intelligence: knowns and unknowns. Am. Psychol. 51, 77–101 26 high levels in old age . Behaviour genetic experimental discipline the philosophers 6 Sternberg, R.J. (1999) Successful intelligence: studies also inform us about environmental Thomas Hobbes35 (17th century) and finding a balance. Trends Cognit. Sci. 3, 436–442 contributions, and it appears that, as one Christian Wolff36 (18th century) recognized 7 Carroll, J.B. (1993) Human Cognitive Abilities: grows from childhood to old age, the effect of individual differences in intelligence and A Survey of Factor Analytic Studies, Cambridge shared family environment declines almost discussed their varieties (taxonomy) and University Press 8 Gardner, H. (2000) Intelligence Reframed: Multiple to zero. One’s own specific environmental origins, essentially coming up with cognitive Intelligence for the 21st Century, Basic Books experience has a large effect at all ages. The accounts of mental ability differences. 9 Spearman, C. (1904) ‘General Intelligence’ search has begun, using molecular genetic This search for the psychological elements objectively determined and measured. Am. techniques, to identify individual genes of ability differences emerged again with J. Psychol. 15, 201–293 10 Jensen, A.R. (1998) The g Factor: The Science of which contribute variance to individual Galton’s37 suggesting and Spearman’s9 Mental Ability, Praeger 27–29 differences in psychometric intelligence . testing the idea that intelligence differences 11 Embretson, S.E. and Schmidt-McCollam, The common wisdom is that, if any such were based partly or wholly upon sensory K.M. (2000) Psychometric approaches to effects are found, there will be many genes discrimination differences, an idea that understanding and measuring intelligence. In each with a small contribution. has re-emerged recently38,39. Galton also Handbook of Intelligence (Sternberg, R.J., ed.), pp. 423–444, Cambridge University Press considered reaction-time differences to be 12 Eysenck, H.J. (1939) Primary mental abilities. Predictive validity of psychometric basic to higher cognitive differences. Br. J. Educ. Psychol. 9, 270–275 intelligence differences As psychology progressed through the 13 Burt, C. (1940) The Factors of the Mind, Psychometric intelligence tests have twentieth century the attempts to relate University of London Press applications in the field of education, psychometric intelligence differences to 14 Gustafsson, J.-E. (1999) Measuring and understanding G: experimental and correlational 5 occupation and medicine, among other sites . cognitive elements were desultory until approaches. In Learning and Individual In general, they are moderately strong the rise of cognitive psychology in the Differences: Process, Trait, and Content predictors of educational and occupational 1970s. There was then excitement and Determinants (Ackerman, P.L. et al., eds), outcomes. For example, in a review of optimism around the possibility that the pp. 275–289, American Psychological Association 15 Carretta, T.R. and Ree, M.J. (1995) Near identity thousands of reports over about 80 years, a new science of the mind would deliver a of cognitive structure in sex and ethnic groups. general mental ability test emerged as one catalogue of mental components that would Pers. Individ. Differ. 19, 149–155 of the strongest predictors of job success30. explain human intelligence differences 16 Deary, I.J. (2000) Looking Down on Human The correlations averaged about 0.5. (discussed in detail elsewhere16). Intelligence: From Psychometrics to the Brain, Clearly, there are other important things Oxford University Press ‘There seems to be widespread 17 Schwartzman, A.E. et al. (1987) Stability of contributing to job success. Other aspects concurrence among theoreticians and intelligence: a 40-year follow up. Can. J. Psychol. of the predictive validity of mental ability methodologists alike that new 41, 244–256 10 differences are discussed by Jensen and approaches to studying intelligence 18 Deary, I.J. et al. (2000) The stability of individual Gottfredson31 who emphasize the practical should somehow combine the differential differences in mental ability from childhood to old implications of general ability differences. and cognitive [information-processing] age: follow-up of the 1932 Scottish Mental Survey. Intelligence 28, 49–55 approaches that have been used in the 19 Schaie, K.W. (1996). Intellectual Development in Understanding intelligence differences past, and that the combination should Adulthood, Cambridge University Press Were it not for their predictive validity – somehow enable the investigator to 20 Pushkar Gold, D. et al. (1995) Structural equation their usefulness, that was evident since isolate components of intelligence model of intellectual change and continuity and that are elementary [at some level of their invention32 – it is doubtful whether predictors of intelligence in older men. Psychol. analysis].’ (Ref. 40, p. 196) Aging 10, 294–303 the taxonomy of mental ability differences 21 Baltes, P.B. et al. (1999) Lifespan psychology: would be so intensively studied. And it is The success of these ideas is theory and application to intellectual functioning. unlikely that so many researchers would covered in a review that follows this one. Annu. Rev. Psychol. 50, 471–507 be interested in the cognitive and biological It will describe the last few decades of 22 Salthouse, T.A. (1996) Constraints on theories of cognitive aging. Psychonomic Bull. Rev. 3, 287–299 origins of mental ability differences. This research on the cognitive bases of 23 Salthouse, T.A. (1996) The processing-speed tripartite examination of intelligence intelligence differences, and will call for theory of adult age differences in cognition. differences – their structure, their utility a stronger collaboration between Psychol. Rev. 103, 403–428

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24 Bouchard, T.J. (1998) Genetic and environmental 31 Gottfredson, L.S. (1997) Why g matters: the Empirica (Ecole, J., ed.), Georg Olms influences on adult intelligence and special complexity of everyday life. Intelligence 37 Galton, F. (1883) Inquiries into Human mental abilities. Hum. Biol. 70, 257–279 24, 79–132 Faculty, Dent 25 Plomin, R. and Petrill, S. (1997) Genetics and 32 Zenderland, L. (1998) Measuring Minds: 38 Deary, I.J. (1994) Sensory discrimination intelligence: what’s new? Intelligence 24, 53–77 Henry Herbert Goddard and the Origins of and intelligence: postmortem or resurrection? 26 McClearn, G.E. et al. (1997) Substantial genetic American Intelligence Testing, Cambridge Am. J. Psychol. 107, 95–115 influence on cognitive abilities in twins 80 or University Press 39 Deary, I.J. (2000) Simple information more years old. Science 276, 1560–1563 33 Huarte, J. de San Juan (1969, originally publ. processing and intelligence. In Handbook of 27 Chorney, M.J. et al. (1998) A quantitative trait 1575, transl. 1594) Examen de Ingenios, or A Intelligence (Sternberg, R.J., ed.), pp. 267–284, locus associated with cognitive ability in Triall of Wits (The Examination of Mens Wits), Cambridge University Press children. Psychol. Sci. 9, 1–8 Da Capo Press, Amsterdam 40 Sternberg, R.J. (1978) Intelligence research at 28 Plomin, R. and Crabbe, J. (2000) DNA. Psychol. 34 Binet, A. (1905) New methods for the diagnosis the interface between differential and cognitive Bull. 106, 808–826 of the intellectual level of subnormals. L’Annee psychology: prospects and proposals. Intelligence 29 Daniels, J. et al. (1998) Molecular genetic studies Psychologique 12, 191–244 (transl. 1916 by 2, 195–222 of cognitive ability. Hum. Biol. 70, 281–296 E.S. Kite in The Development of Intelligence in 30 Schmidt, F.L. and Hunter, J.E. (1998) The Children, Publications of the Training School Ian J. Deary validity and utility of selection methods in at Vineland, NJ, USA) personnel psychology: practical and theoretical 35 Hobbes, T. (1651, reprinted 1885) Leviathan, Dept of Psychology, University of Edinburgh, implications of 85 years of research findings. Routledge 7 George Square, Edinburgh, UK EH8 9JZ Psychol. Bull. 124, 262–274 36 Wolff, C. (1732, reprinted 1968) Psychologia e-mail: [email protected]

Book Review

The ensuing century has seen the of works published just over a decade Reading difficulty: an phenomenon of dyslexia emerge from the ago. The quickening pace of research update doctor’s surgery, coming to be recognized (‘explosion’ is Snowling’s term; ‘on a as a major educational problem as well as massive scale’ according to Miles and a fascinating object of study. The fact Miles) into dyslexia necessitated the Dyslexia: A Hundred that children often find it difficult to new editions; indeed because of this Years On (2nd edn) learn to read and spell was of course Snowling deemed it ‘necessary to write a by T. R. Miles and generally recognized as soon as universal completely new book’ (p. xiii) whereas Elaine Miles schooling was introduced in the latter the Miles duo have done ‘some extensive Open University part of the 19th century and, regrettably, rewriting’ (p. vi). Press, 1999. too often ascribed to lack of intellectual There are interesting commonalities £15.99 (vii + 208 pages) ability. The concept of dyslexia, on the as well as differences in the approaches ISBN 0 335 20034 6 other hand, assumes a specific disorder of the two books, although both make an of reading and spelling. It is worth honest attempt to come to grips with recalling that a different theory was what often seems ‘a bewildering conflict Dyslexia (2nd edn) also advanced early on that focused on of evidence’ or even ‘bewilderingly by Margaret J. Snowling the difficulties inherent in the contradictory’ research results, to quote Blackwell, 2000. nature of alphabetic writing and from Dyslexia: A Hundred Years On. Of £50.00 (hbk) / £14.99 individual orthographies. Thus we find, the two books, this is the more eclectic. (pbk), (xvi + 253 pages) in Iceland, the linguist Björn M. Ólsen It is historically organized, giving a ISBN 0 631 22144 1 (hbk)/ (who became the first rector of the succinct account of the history of ISBN 0 631 20574 8 (pbk) University of Iceland in 1911) research into dyslexia, tracing the publishing in 1889 the results of a beginnings back to attempts to localize It is now just over a century since detailed count of spelling errors by brain function in the 19th century, W. Pringle Morgan, a general practitioner students in the Reykjavik grammar before describing the early medical in Sussex, England, published what is school, and from the pattern of errors studies of Morgan, Hinshelwood, Orton commonly regarded as the first case arguing for specific changes to be made and others. From these beginnings, the history of developmental dyslexia. This in the Icelandic orthography. He found, book moves on to more recent work, was the case of Percy, a bright boy who however – as have almost all would-be dealing in different chapters with would have been ‘the smartest lad in the spelling reformers – that the force of current language-based models of school’ except for the fact that he was tradition is overpowering in matters dyslexia, especially with regard to the unable to read, even at the age of 14 of orthography. role of phonology, genetics and brain when Morgan saw him. Morgan thought The two books reviewed here give an research, as well as work on possible of his condition as ‘congenital word excellent view of the current state-of-the sensory deficits in dyslexia. The book blindness’, a term later replaced by art in dyslexia research, as is to be also has a practical touch; for example, ‘dyslexia’, or developmental dyslexia if expected from these authors, who are there are two useful chapters on there is a need to distinguish the recognized authorities in the field. In methodology and, at the end, chapters condition from acquired dyslexia. both cases the books are updated editions dealing with remediation and