In 1994, The Bell Curve by Richard Herrnstein and Charles Murray, with its apocalyptic vision of the social consequences for America of differences in intelligence, created such a media furore that some doubted whether any rational public debate on the subject could take place again. These books show that objectivity may prevail and prejudice be put aside - well, almost, because two chapters by Arthur Jensen will again inflame passions.
Jensen and Nicholas Mackintosh consider the scientific study of intelligence; Leila Zenderland, a cultural historian, examines intelligence tests as instruments of American social policy early in this century and the ensuing controversies. Her analysis is framed within a biography of Henry Herbert Goddard, who imported to America the innovative tests of Henri Binet and Theodore Simon.
Mackintosh was known best as a distinguished investigator of learning until his earlier magisterial review of Leon Kamin's The Science and Politics of IQ (1974). He makes insightful asides on the political conclusions of those for and against any genetic basis for intelligence. Jensen has been the egalitarians' bogey-man since his 1969 article in The Harvard Educational Review , in which he concluded that variations in IQ have a genetic basis between ethnic groups as well as for individuals and interventions to raise scholastic achievement had failed. Given the controversies he aroused, it is disturbing to find no direct reference to critics such as Kamin.
Turning to scientific controversies, Jensen forcibly argues the classical view, stemming from Herbert Spencer and developed by Francis Galton, that evolution must have led to genetic variations in "general intelligence". His hero is Charles Spearman who, in 1904, showed that intercorrelations among abilities might be explained by a single factor he labelled g. In response to criticisms of his 1969 article, Jensen began a new programme of research and proudly reports that the resulting books have been "citation classics". He claims not to have realised his critics' ignorance of the "sovereignty of g" for intellect, hence this book. An American reviewer has characterised it "an encyclopedia of mental ability" but, though wide ranging, it is very "g-ocentric", to borrow Robert Sternberg's phrase.
He has considered carefully his critics' arguments, although rarely acknowledging their source. The impression is gained of a liturgy for g as a highly heritable, nearly immutable trait, of great importance for society and individuals. Indeed, the final chapter maps out the future research needed on both.
Two conclusions are especially emphasised. First, that the correlation of g with brain size, physiological measurements of brain activity and the speed of performance on simple laboratory tasks shows it to reflect a fundamental neural property.
Contemporary neuroscience assumes that mental functions have biological bases and hence heritable variations. To be scientifically interesting, not correlations but particular hypothesised brain processes and genetic mechanisms are needed: it is not clear that Jensen has gone beyond what Eric Turkheimer has termed "weak biologism".
The second special issue is "racial" differences in intelligence. This takes up a third of the book. All investigators agree that the average IQ of American blacks is one standard deviation, 15 points, below the average for whites. Jensen develops his arguments that this has a substantial genetic basis. He attacks the case - the "mantra", he calls it - that heritability within groups (WGH) does not imply heritability between groups (BGH). Using formal mathematical relations between WGH and BGH (the parameters are not estimable), he defends his "default hypothesis": that assuming variation between two populations has the same basis as that within them gives the best account of kinship resemblances. This analysis deserves serious consideration.
Unfortunately, these chapters contain speculation about black/ white (B/W) genetic differences only tangentially relevant to IQ: for example, Luigi Luca Cavalli-Sforza's study of allele frequencies for 42 isolated populations to demonstrate that blacks show the greatest genetic distance from others. Charitably read, he is presenting the entire basis of his view of B/W differences. But one has to be concerned that this book appears in a series including one work titled Why Race Matters (1997). These chapters alone will make the book another "citation classic".
Jensen proclaims "the sovereignty of g", Mackintosh offers a sceptical but balanced analysis of studies of intelligence and its measurement. While Jensen is pursuing normal science in the Kuhnian sense, Mackintosh can be seen as attempting to engineer a "paradigm shift". His IQ and Human Intelligence is written as a text for advanced undergraduates, and it ranks among the most intellectually enjoyable examples of the genre. For a student, the drawback will be that so much informed scepticism is not matched by a fuller statement of his own theoretical position.
He agrees with Jensen on several issues. Both consider that starting with a definition of intelligence would be scientifically naive. Each concludes that IQ tests measure important cognitive capabilities and predict educational and vocational success better than social and economic status (SES). But both are wrong to plot success against SES and IQ expressed as standard deviations from their means, which artificially equates the units of underlying variables. Mackintosh accepts that kinship studies demonstrate substantial genetic influence, but brackets heritability between 0.30 and 0.75 against Jensen's estimate of 0.80 in maturity. Neither mentions recent work on differences in resemblance between monozygotic twins sharing a common placenta and the one-third who, like dizygotes, have separate prenatal environments.
Mackintosh also accepts that, at a psychometric level, there is support for a general factor as well as those common to certain test groups, but doubts whether this factor is the same in all test batteries. Mackintosh concludes from the correlations between g loadings of individual tests in different batteries that some are too low for g to be a true common factor. Jensen meanwhile claims communality on the basis of an average correlation of 0.85.
Mackintosh favours an interpretation of g in terms of overlapping processes rather than a basic property of the brain. He argues forcibly that searching for biological correlates, such as brain wave characteristics - a central theme for Jensen - is mistaken. He is equally sceptical about correlational studies of IQ and speed of performance on simple tasks. Jensen makes much of the relation between IQ and inspection time, or IT, the exposure duration needed for two stimuli to be discriminated.
Mackintosh claims IT correlates more with perceptual speed than g, and because reaction time and IT correlate weakly, speed of information processing cannot explain variations in IQ. He considers that g might reflect two uncorrelated processes: efficient sensory analysis and effective decision-making, although he seems uncertain here, for earlier he cites infants' speed of habituation to repeated stimuli as the best predictor of mature IQ and later speculates that habituation and IT may be related.
On B/W IQ differences, he agrees with Jensen that no factor "X" has been identified that varies between races and not within them. Yet Mackintosh concludes, from mixed-race offspring and adoption studies, that there is no indisputable evidence for a genetic basis for B/W IQ differences.
Mackintosh is concerned that if an IQ less than 70 defines mental retardation, 16 per cent of blacks would be so diagnosed. He cites adaptive characteristics, such as looking after oneself, not correlating with IQ as suggesting tests are biased against blacks. Jensen contends that blacks so diagnosed come from the lower tail of the IQ distribution, whereas for whites, syndromes of specific retardation are more frequently represented.
In his last three chapters, Mackintosh goes beyond Jensen in considering identifiable cognitive processes underlying intelligence and examines aspects of cognition which may be independent of IQ. Both explore the role of "working memory", postulated by Alan Baddeley and Graham Hitch as a possible underlying mechanism. Whereas Jensen analyses this in terms of the speed of neural processing, Mackintosh emphasises its planning and monitoring functions. He entertains the hypothesis of John Duncan and colleagues that the frontal lobes may be the locus of g but considers it unresolved whether there is a single control system or a cluster of autonomous component processes. Finally, he outlines the case for a "practical intelligence", reflecting a general purpose associative system dependent on conditioning processes, as elaborated by himself and others, including Kamin. He sees this as the basis of "implicit learning", in which performance on complex tasks improves with only fragmentary conscious involvement. So, broadly, intelligence is seen as dependent on a cognitive system generating and testing hypotheses together with this associative system.
Zenderland returns to the political passions aroused by intelligence tests. Analysing the rise and fall of Goddard's reputation, she concludes his work deserves more sympathy. Her approach is reminiscent of Adrian Wooldridge's Measurement of Mind (1994), which considered similar controversies in Britain. His theme was meritocratic selection; hers, approaches to mental handicap.
She is surprised Goddard should be America's first proselytiser for Binet's tests. Born in 1866 into an impoverished Quaker family, his early education was poor but, as a teacher, he developed a zeal for science as a servant of Christianity. Two men harnessed this: G. Stanley Hall and Edward Johnstone. Hall, an enthusiast for evolutionary approaches to the study of mind, sought to persuade teachers to investigate mental development. Johnstone was the principal of New Jersey's Vineland Institution for the "feeble-minded", who, in 1906, appointed Goddard to a new research post.
America faced problems resulting from the introduction of compulsory education, population drift into cities and massive immigration. These presented the new medical inspectors with two challenges: controlling epidemics and diagnosing mental handicap. To diagnose degrees of mental handicap at Vineland, Goddard turned first to laboratory tasks from experimental psychology, but these proved unsuccessful. He came upon Binet's tests when visiting Europe and was remarkably successful in persuading doctors of their value. He became America's expert on mental handicap, inventing the term "moron" for those closer to average ability but unable to take responsibility for their lives. Cities and states took his views seriously and, by the end of the first world war, the US was "feeble-minded" conscious. Zenderland stresses that Goddard's aim was to provide the handicapped with a safe environment and clemency in criminal trials.
When America entered the war, Goddard was invited to join Robert Yerkes's group developing tests for all recruits. By 1919, 1.7 million people had been tested. Later analysis painted a dismal picture of the nation's abilities: 30 per cent illiterate, and "morons" who could not be rejected since this would halve recruitment. This fed Goddard's growing pessimism about the mentality of Americans. In 1919, at the height of his career, he concluded in a prestigious lecture (prefiguring The Bell Curve ) that "four million superior people should devote themselves to the comfort and happiness of the other 96 million".
Invited to offer advice to doctors at Ellis Island responsible for excluding the mentally handicapped from immigration, he concluded 3 per cent of northern Europeans and 4 per cent of southern Europeans were handicapped. Zenderland emphasises that he was not in favour of national quotas and emphasised class over race.
By the mid-1920s, Goddard's work was being challenged and the US Army data on B/W differences fuelled new controversies about nature versus nurture and racism. Goddard's zeal was never matched by a grasp of statistical issues. Binet's standardisation sample was small and tests for adolescents unsatisfactory: the criterion for a moron - three or four years below chronological age - was dubious for adolescents. Goddard himself began to have doubts, accepting that the army results showed that most morons should not be classified as feeble-minded.
Goddard's reputation was enhanced by his The Kallikak Family: A Study of the Heredity of Feeble-mindedness (1912). Influenced by the geneticist Charles Davenport to study the inheritance of handicap, he identified "Deborah Kallikak" as a sixth-generation descendant of a child of a mentally handicapped woman by a man who also had children by his wife, presumably "normal". From a field study of the two matrilineal lines, he concluded that "feeble-mindedness" reflected a recessive gene. Originally hailed as a successful breakthrough, by the mid-1940s this amateurish study came in for justified criticism. By the age of 76, opinion of Goddard's career had plummeted, the final blow being Nicholas Pastore's PhD thesis arguing that scientists' views of the nature-nurture controversy reflected their politics and classifying him as a conservative.
Goddard responded that Pastore was interpreting 1910 papers in a 1940s context: discussing a recent controversy by reference to studies done before the problem existed. Zenderland's conclusions reflect Goddard's reply to Pastore. Her excellent epilogue draws attention to the admiration he engendered among colleagues, students and Vineland inmates. Combining christianity and science, he wanted to be his brother's keeper but, unfortunately for his reputation, how that is viewed changes continually.
All three books should be read by anyone with serious scientific or political interests in intelligence and its measurement. It is especially welcome that Jensen, and more so Mackintosh, offer, after nearly a century, a rapprochement between those studying individual differences and experimental psychologists concerned with general mental laws. It was unfortunate that intelligence tests came on the scientific scene when behaviouristic ideas with an extreme emphasis on learning began to dominate American academic psychology. The practical utility of tests, underlying the development of professional psychology, also alienated experimental psychologists whose approaches to measuring intelligence failed. But it seems likely that an even greater understanding of intelligence and its variation will come from the study of mental development, largely neglected by Jensen and Mackintosh.
Robert Audley is emeritus professor of psychology, University College London.
The g-Factor: The Science of Mental Ability
Author - Arthur R. Jensen
ISBN - 0 5 96103 6
Publisher - Praeger
Price - £31.05
Pages - 648