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cal interventions are the concern of everyone and that it may be helpful to organize commissions of physicians, biologists, social scientists, theologians, lawyers, economists, and laymen to aid in the guiding of research and the application of knowledge. DwightJ. Ingle Professor Emeritus University of Chicago Educability and Group Differences. By Arthur R. Jensen. New York: Harper & Row, 1973. Pp. 407. $10.00. This book is nothing less than an attempt to prove that the minority population of socially defined Negroes in the United States is inferior to the white majority in respect to the genetic determinants of general intellectual ability. In spite of the title and perhaps half a dozen pages devoted to social class differences in general ability, the main burden of the book is a wide-ranging attack upon a purely environmental explanation of the typically poor average performance exhibited by groups of U.S. blacks relative to the white groups with which they have been compared in studies employing objective tests of general verbal ability. Expanding upon a section of his controversial paper in the 1969 Harvard Educational Review, the author draws on sources in the sociological, psychological , and medical literature to argue that this discrepancy in performance can be explained by the theory now widely accepted that individual differences in general ability are, in normal circumstances, partly determined by polygenic inheritance . The facts in this argument are not in dispute: the relatively depressed average scores of U.S. blacks in tests such as the Stanford-Binet, Wechsler Adult Intelligence scale, Lorge-Thorndike Non-Verbal scale, etc., amounting to about 1 standard deviation of the score distribution in the general population, are widely documented in the social science literature of the last 50 years [I]. Contemporaneously , a long series of family, twin, and foster-child studies [2] has established to the satisfaction of virtually all behavior geneticists that, in the populations studied, there exist a number, probably a large number, of genetic polymorphisms that, by some obscure mechanism, influence a person's ability to apply verbal memory and reasoning to the problems presented in this type of test. Jensen seeks to connect causally these two facts by inferring from indirect and fragmentary evidence that the black population is, in effect, characterized by a relatively lower frequency of those states of the polygene that dispose for successful solutions of these problems. He pushes on with this effort in full consciousness of the warnings of others [3] that formidable methodological difficulties stand in the way of this kind of population comparison. These difficulties arise from the necessity in genetic analysis of regarding psychological test scores as quantitative traits, the variation of which is due to multiple genetic and environmental effects acting conjointly. Because the states of the individual genes that influence general ability cannot be inferred from these scores, a simple comparison of population allele frequencies, which is unexceptional when applied to Mendelian traits, is quite precluded. The analysis can proceed only by the methods of biometrical genetics, introduced by R. A. Fisher and Sewall Wright, that are specifically directed toward trait variation 594 1 Book Reviews within population and are without relevance for comparison of mean levels of given, exclusive breeding populations. In the Fisher-Wright model for the components of trait variation due to heritability, allele interaction, environment, gene X environment interaction, measurement error, etc., only individual differences (i.e., the deviation of the trait value ofindividuals expressed as a deviation from the population mean) are explicitly treated. The data analysis based on the model deliberately excludes the population mean, the level of which is highly sensitive to the prevailing environment in which the trait—even one that is entirely heritable within the population—is expressed. Thoday [3] has given a number of examples of this phenomenon in plants and lower animals, but more apposite here is the remarkable effect of environment on the mean level of human stature. In the U.S. and European populations, stature has about the same heritability as general verbal ability, and yet the mean stature of these populations has increased nearly 1 standard deviation in two generations (comparable to the alleged difference in mean general ability levels of the U.S. black...


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