Patterns of mental ability and socioeconomic status.

In a society that supports universal public education and allows a high degree of social and occupational mobility, a positive correlation is inevitable between socioeconomic status (SES) and intelligence. (The term intelligence is used here to mean those abilities primarily associated with scholastic aptitude.) Also, because there is a high correlation (of the order of 0.8-0.9) between phenotype and genotype for intelligence as measured by tests such as the Stanford-Binet,l" 2 it is inevitable that SES differences in intelligence are due largely to genetic factors. In reviewing the relevant evidence, the British geneticist C. 0. Carter remarked, "Sociologists who doubt this show more ingenuity than judgment."3 The conclusion that SES intellectual differences have a major genetic component and are not entirely attributable to environmental differences across SES is now practically beyond dispute among scientists who have studied the relevant evidence. The facts of the matter, which have been reviewed in detail elsewhere,48 come from a variety of sources. For example, identical twins separated in the first year of life and reared in widely differing social classes still show greater resemblance in intelligence than unrelated children reared together;9 the IQ's of children adopted in early infancy show a much lower correlation with the SES of the adopting parents than do the IQ's of children reared by their own parents;10 the IQ's of children reared in an orphanage from infancy and who have not known their parents show approximately the same correlation with their true fathers' occupational status as that found for children reared by their own parents (0.23 vs. 0.24);1" the correlation between the IQ's of children adopted in infancy and the education of their true mothers is close to that of children reared by their own mothers (0.44), while the correlation between children and their adopting parents is close to zero;'2 children of low and high SES show on the average an amount of regression from the parental IQ toward the mean of the general population that is precisely predicted by a polygenic model;6 when full siblings, who have on the average at least 50 per cent of their genetic inheritance in common, differ significantly in intelligence, those who are above the family average tend to move up the SES scale and those who are below the family average tend to move down.'3 It is noteworthy that intensive efforts by psychologists, educators, and sociologists to devise "culture-free" or "culture-fair" tests that would eliminate SES differences in measured intelligence have not succeeded.'4' 15 There are no standard intelligence tests known which eliminate SES differences. The present paper attempts to throw a new light on the nature of SES differences in intelligence. Problem and Method.-The research was initially stimulated by the puzzling observation made by many teachers and by the present writer that low-SES children with IQ's in the range 60 to 80 appear to be much brighter socially, on