Reaching back to the original theory of Spearman, Jensen presents a comprehensive synthesis of the past century's evidence and concludes that g and its nexus constitute no less than the central dogma of psychometrics, the science of mental ability. While his conclusions regarding behavioral and psychometric correlates of g are largely convincing, those regarding the presumed biological and evolutionary foundations of g are less strong. It is a formidable work that enriches our understanding of individual differences in intelligence and it is sure to stimulate more scientific research and debate.
2. Once Jensen essentially quieted the critics on the subject of bias, critics took another tack and dismissed the importance of the tests altogether, calling them trivial, "merely paper and pencil" (actually, the tests rely very little on paper or pencils), measuring only "academic intelligence" that is pertinent only in the limited venue of schools (e.g., Gould 1981/1996). Jensen's task now is no less than to mount a defense of the construct of mental ability (g) as it has been classically defined and measured. He does so by reconstructing g from the ground up, beginning with Spearman's 1904 formulation. He assembles an enormous number of studies and convincingly argues that g is a legitimate, robust scientific construct, and that it is a reflection of a basic biological dimension along which humans vary. It is, furthermore, a source of individual differences with profound implications for every aspect of life, including, but not limited to, school achievement.
3. In the present work (as in Bias in Mental Testing), Jensen seems to be speaking not only to scientists within his discipline, but also to scientists across disciplines and to nonscientists. To nonscientists, Jensen is saying, "My scholarship on this topic is within the domain of science. If you wish to understand what I have concluded, you must step inside the scientific realm and abide by the rules, which I have summarized in brief form. Rhetoric outside of these parameters is quite another matter." To scientists, he is saying, "Psychometrics is a legitimate scientific enterprise. It has rules of observation, quantification, measurement, and logic. What I propose is testable within the rules of science. Everything I conclude is subject to empirical disconfirmation, and those who wish to challenge me should bring data."
4. It is unlikely that nonscientists will readily relinquish their presumed authority on human intelligence. But Jensen argues for removing the term from the realm of ordinary discourse where it has equal status with other abstractions like love, justice, and the like. Rather, he argues that intelligence, like gravity or radiation, must be defined precisely for any reasonable dialogue about its properties to take place. Moreover, since it cannot be observed directly, we must take time to understand the highly standardized manner in which it may be indirectly observed and understood. To avoid confusion, Jensen suggests abandoning the term intelligence altogether, and starting anew with the g factor. He suggests always using the term in that form to remind us that g is a factor with a particular mathematical definition. I'm not altogether convinced that we need to abandon the term for scientific discourse, but I share his frustration with the gratuitous stretching of the term by contemporary writers such as Sternberg, Gardner, and Goleman. While many have eagerly embraced broadening of the construct to include a wider and wider range of human characteristics, Jensen successfully distinguishes the classical notion of intelligence from all of the misuses of the term popularized recently. He is kind to those who have offered what they believe to be serious challenges to a g based theory of intelligence, but ultimately concludes that while current speculations are interesting, they have diverged much too far to be subsumed under the term intelligence, they are largely lacking in scientific validity, and they do nothing to deny the reality of g. Given the misconceptions and disagreements surrounding the terms intelligence and IQ, it is perhaps justifiable that Jensen tries to distance himself from them early in the book.
5. Many contemporary psychologists view the field of differential psychology as moribund, or dormant at best. Differential psychology is regarded as a purely applied field, with the IQ test being a successful, if tired, tool for classifying individuals for educational and clinical purposes. And, most damningly, applied differential psychology has long been said to have no viable theories of intelligence. Jensen's claim is that we've had a theory all along, but we've just insufficiently acknowledged or appreciated it. In some sense, Jensen's work is a defense of early 20th century psychometrics. He concentrates primarily on how research of this century has essentially borne out the primary ideas of Spearman. Further, he elevates the discovery of g by Spearman in 1904 to the same status as Thorndike's law of effect. In other words, he regards (correctly in my view) the discovery of g as one of the very few singular, lasting, and influential accomplishments of psychology. It is a central dogma supported by a mountain of evidence and broad consensus among psychometricians.
6. Jensen's current work is an ambitious, comprehensive, and in my mind, successful articulation of the central dogma of the psychometrics of mental ability. As he did in Bias in Mental Testing, he goes to great length to present, often in primer format, the basic foundations for his arguments. Among these are the scientific foundations and methods of classic psychometrics, the rationale and methods of factor analysis, and principles of behavior genetics and evolution. While I have no doubt that critics will seize upon particular studies or their interpretation and conflate them into fatal flaws, my opinion is that Jensen's scholarship is accurate, his conclusions reasonable where much empirical data exist, and his speculations prescient. His arguments are strengthened by his positioning himself firmly within the scientific tradition of building hypotheses subject to disconfirmation. Within the rules of this endeavor, anyone wishing to bring down the overall construction (and many will try), will have a herculean task ahead of them. Critics of the conclusions reached by Herrnstein & Murray (1994) in The Bell Curve, had essentially one (albeit very sound) study to attack. In this book, Jensen has pulled together a nexus of hundreds of studies, and finding fault with one or another of them will not threaten the edifice.
7. While the edifice that Jensen has erected is an impressive one, a few weaknesses are present, some within Jensen's area of undisputed expertise, psychometrics, and others in his attempt to explore what he terms in the last chapter, the "vertical" directions of g's foundations. First the weaknesses within psychometrics. Jensen argues that experts cannot agree on a definition of intelligence. But to some degree (admittedly some less degree), the same can be said of definitions of g. Spearman was elusive when trying to pin down the essence of g, and so is Jensen. After presenting a definition deriving from factor analysis, many of the studies he relies upon for his case have used a multitude of different tests, some of which are unidimensional such as the Ravens, which Jensen regards as an "almost pure measure of g". But should not the definitive measure of g be the hierarchical 3rd order factor? And what of Spearman's own admonition (cited on p. 31), that with more tests, more g is captured? The problem is, of course, that a relatively small number of the total number of studies used by Jensen to marshal his evidence have been done with g determined according to Spearman's recommendation. Notwithstanding this point, my guess is that Jensen's argument would likely be strengthened, rather than diminished, had more of the research he cites used better measures of g. Another problem, though, is that Jensen regards psychometric measures of g to be reflections of an underlying set of biological variables that are now only dimly understood. This dim understanding is problematic as Jensen reaches "downward" through physiological mechanisms toward genetics.
8. In building up the case for g, Jensen gives only cursory treatment to mental ability factors other than g. While he builds a convincing case that the g factor is ubiquitous, and accounts for much variability in a wide range of human endeavors, what of the other factors in the hierarchy? Jensen states that Carroll's three stratum theory is not inconsistent with his views on g, but he pays scant attention to Carroll's other strata. Jensen's admiration for Spearman's theory is obvious, but why is there so little mention of Spearman's specific (s) factors? It seems that a goal of psychometrics is to account for more and more variance, and to do that, it must go beyond g. Little attention is given even to the best-known 2nd order factors, those of verbal and nonverbal abilities, which as Jensen documents, were even acknowledged by Spearman to be important. Since Wechsler's bifurcation in the 1930's, verbal and nonverbal IQ have become integral parts of intelligence tests, with both practical value in diagnostics and very good factor validity. But on this matter I cannot really fault Jensen, because his stated purpose in the book is to restore the centrality and dominant position of g, not to construct a comprehensive theory of intelligence.
9. When Jensen departs from pure psychometrics, his attempts to link g with with variables such as nerve conduction velocity, brain size, and myopia, seem very crude or at least preliminary. They seem something akin to showing a relationship between blood pressure and temperature to general health in an era where much more complex and sophisticated causal mechanisms are known. Jensen no doubt believes that future consilience between the neurosciences and psychometrics will only strengthen his case. It is probably best that Jensen is intentionally conservative and vague in his comments about what illumination neuroscience has provided thus far and is likely to provide in the future. He no doubt realizes that any pronouncements at this point are at best premature, and at worst, dead wrong. Here I am reminded of attempts to reconcile intelligence theory (and construct intelligence tests) with the discovery of hemispheric specialization. One gets the sense that Jensen yearns to build stronger bridges to the neurosciences than he does, but realizes we must wait for further research in other disciplines to be done.
10. In what will unquestionably be seen as his most controversial chapter, Chapter 12, Population Differences in g: Causal Hypotheses, Jensen turns to his hypotheses about race differences in g. Beginning with what he calls "Spearman's hypothesis", namely, that the degree of race differences on tests is proportional to the tests' g loadings, he casts a very broad net, extending from psychometrics to population genetics and evolutionary theory. Here he is certain to be criticized for not being an expert in those fields. Others may reach different conclusions about the data he reviews, and future research may prove him wrong on some counts, but he is to be admired for continuing to put the questions before us and place their investigation in the realm of science as opposed to rhetoric. One falsifiable hypothesis is worth more than thousands of pages of prose by self-proclaimed experts.
11. While much of the book is a reiteration (and a very good one) of the research of the 20th century related to g, in the last chapter, The g Nexus, Jensen offers numerous directions for research in the next century. He suggests that the prediction of behavior would be considerably strengthened by supplanting measures of g with measures of a variety of nonintellective (or personality) factors such as motivation, perseverance, and conscientiousness. He presents these in a section titled "The Limitations of g", and reminds us that while g is the best single predictor of many types of performance, future research should explore the interactions of other variables and factors with g. I have long been puzzled by the failure of psychology to seek and achieve a synthesis between so-called intellective and nonintellective components of human performance. The interaction between intelligence and personality is still for the most part uncharted territory. Consider the work of Eysenck, for example, who made major contributions to both intelligence and personality theory but offered very little integration to synthesize the two areas. The linking of g to other variables is one of Jensen's recommendations for enlarging the g nexus in a "horizontal" direction.
12. In the book's very last paragraph, Jensen proposes a possible heuristic for future research in the "vertical" direction, the neurophysiological basis of g. While I have commented above that Jensen's attempts to link g to biology are necessarily weak, given our present knowledge, he nevertheless sees further understanding of g to be in the realm of neuroscience, as apparently did Spearman. In fact, while Jensen's view seems to be that very little advancement (and considerable confusion) in intelligence theory has taken place since Spearman published The Abilities of Man in 1927, he does seem hopeful that the next century holds promise for greater understanding. Arthur Jensen has enriched our understanding of intelligence (no, I have not abandoned the word) for 30 years through his systematic and exhaustive scholarship. Count me among those who agree with his major conclusions about the past century's psychometrics. I would not bet against his intuitions concerning what the next century will bring.
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