Educability-and-Group-Differences-1973-by-Arthur-Robert-Jensen

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Race Differences in Intelligence 175 A THRESHOLD HYPOTHESIS OF ENVIRONMENTAL EFFECTS Several years ago I proposed the hypothesis that the environment, with respect to mental development, displays ‘threshold’ effects (Jensen, 1968a, pp. 10-14). What this means is that environmental variations in one part of the total scale of environmental advantages have quantitatively or qualitatively different effects on the development of the phenotype than variations in another part of the scale. Such threshold effects for many characteristics are wrell known to geneticists (e.g., Falconer, 1960, Ch. 18) and there seemed reason to believe that intelligence might show similar threshold effects. In reviewing the literature reporting large shifts in IQ resulting from environmental changes, it wras clear that all instances of large gains in IQ were found in children whose environments had been changed from very poor to average or superior, while no IQ gains of comparable magnitude have been reported for children whose environments have changed from average to superior. This suggests a non-linear (or non-additive) effect of environment on mental development. Going from a typical slum environment to an average middle-class environment would presumably have a larger effect on IQ than going from a middle-class to an upper-class environment. We know that nutrition behaves more or less in this fashion with respect to stature. When the diet is deficient in certain vitamins, minerals and proteins, physical growth is stunted, but when adequate amounts of these foods are provided, growth is normal and further supplements to the diet will produce little effect. If we determined the heritability of stature in a population that included a sizeable percentage of persons whose nutrition had been inadequate for the full realization of their genetic potential for stature, we should find a much lower heritability than in a population in which everyone had adequate nutrition. In other wrords, the presence of threshold effects, or of a non-linear relationship between environment and phenotype, should result in lower heritability for populations located largely in the below-average part of the environmental scale as compared with populations located largely in the above-average part of the scale. This is illustrated in Figure 7.5. The phenotype/genotype ratio can be thought of as the degree to which the potential for development (genotype) is realized in actual development or performance (phenotype). It is assumed that phenotypic performance cannot
176 Educability and Group Differences exceed genotypic potential. The curves showing the hypothesized relationship between environment and the phenotype/genotype ratio are made to asymptote at some unspecified point below 1*00, since I do not wish to engage in a futile debate over whether persons ever realize their full intellectual potential.12 It should be understood that the asymptotic values of these curves for indi- E xtrem e F a ir Very R e strictio n Relevant Environm ent E nriched Figure 7.5 Hypothetical curves showing the relationship between the degree to which genetic potential is realized in the phenotype (i.e. actual performance) and the quality of the environment. Test A represents a relatively culture-free test, Test B a more culturally loaded test. viduals are assumed to be approximately normally distributed in the population. Test A in Figure 7.5 represents a relatively culture-free or culture-fair test; Test B is a more culturally loaded test. Figure 7.6 depicts an environmental hypothesis of Negro-white IQ difference. The hypothetical frequency distributions of the Negro and white populations are plotted on the same environmental scale as that shown in Figure 7.5. The bulk of the Negro
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Educability & Group Differences Art
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THE AUTHOR Since 1957 Dr. Arthur R.
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Ediicability and Group ^
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To the memory of Sir Cyril Burt e d
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List o f Tables 3.1 Correlations am
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17.3 Theoretical regression lines b
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2 Educability and Group Differences
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10 Educability and Group Difference
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Subpopulation differences in educab
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2 Current technical misconceptions
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j Intelligence and educability Educ
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Table 3.1 Correlations (decimals om
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Table 3.3 Correlations (decimals om
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100 Educability and Group Differenc
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244 Educabilty and Group Difference
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14 Teacher expectancy Credence in t
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ig Physical environment and mental
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20 Recapitulation The study of abil
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Appendix on Heritability Heritabili
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References a in s w o r t h , m . d
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398 Author Index Davis, A., 300 Dea
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400 Author Index Nichols, R. C., 10
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Author Index 401 Warren, B. L., 120
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cultural deprivation, 258 culture-b
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Making Xs Test of speed and persist
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Stanford-Binet Intelligence Scale,
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