Stanford-Binet IQ tests

In the 1980s it was discovered that the renormalizations of the Stanford—Binet IQ test, ongoing since its introduction in America in 1932, and the renormalizations of the other major IQ test, the Wechsler Adult Intelligence Scale (WAIS), ongoing since 1953, had been hiding the fact that the performance of the American population on IQ tests had been increasing about 3 points per decade or approximately 10 points per generation. 

Development (JQD) Scale at 3 years, and the Stanford—Binet IQ Test at 3 years. There were no statistical associations except for maternal PbB effects on the MDl, PDl, and KID variance. 

A later analysis (Emhart et al. 1987) related PbB levels obtained at delivery (maternal and cord blood) and at 6 months, 2 years, and 3 years of age to developmental tests (MDI, PDI, Kent Infant Development Scale [KID], and Stanford-Binet IQ) administered at 6 months, 1 year, 2 years, and 3 years of age, as appropriate. After controlling for covariates and confounding risk factors, the only significant associations of blood lead with concurrent or later development were an inverse association between maternal (but not cord) blood lead and MDI, PDI, and KID at 6 months, and a positive association between 6-month PbB and 6-month KID. The investigators concluded that, taken as a whole, the results of the 21 analyses of correlation between blood lead and developmental test scores were "reasonably consistent with what might be expected on the basis of sampling variability," that any association of blood lead level with measures of development was likely to be due to the dependence of both PbB and... 

Because the number of correlations among the n elements is n(n—1)/2, the sum of the item covariances increases more rapidly as a function of n than the sum of the item variances. In standard test batteries, such as the Wechsler, the Stanford—Binet and the British IQ scales, which have large numbers of items, the item covariances account for about 90% of the total variance. Hence most of a typical test s variance attributable to individual differences in performance results from the correlations, or common variance, among its various elements. 

In our thought experiment, we start out with 10 pairs of MZ twins. We might consider 1000 pairs—the result will not change substantially. These twins (all males) are bom into ordinary middle class families in a Midwestern suburb. Let s assume that the cotwins of each pair have identical IQs at the age of z. The twins are then separated at that age, one twin remaining at home and the other twin moved into a lower-class family in an urban ghetto. We allow 30 years to go by, during which time the twins are totally out of contact with each other. Then we give standard IQ tests (for example, the Stanford—Binet) to all the twins. 

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