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

306 Educability and Group Differences
from kinship correlations on various tests is consistent with this
formulation. For example, standard IQ tests show quite low
correlations (about 0*25), and consequently large IQ differences,
between genetically unrelated (and thus dissimilar) children
reared together, and show quite high correlations (about 0-80),
and consequently small IQ differences, between genetically identical
twins reared apart. On the other hand, certain highly cultureloaded
scholastic achievement tests show much less difference,
i.e., rather higher correlations (about 0-50) between unrelated
children reared together and lower correlations (about 0‘70)
between identical twins reared apart (Jensen, 1968a, Table 1 and
Figure 1).
Now, if we accept this premise that a test’s culture-loading is
inversely related to its heritability in a given population, let us
examine the consequences of comparing the regression of a
culture-loaded test upon a hypothetical culture-free test, and vice
versa, in each of two hypothetical populations, A and B. If
differences are found between groups A and B, one of three
hypotheses can be invoked to explain the difference: (1) the groups
are genetically equal but differ environmentally; (2) the groups are
environmentally equal but differ genetically; or (3) the groups
differ both genetically and environmentally. The consequences
of each hypothesis are shown in Figure 17.3. Our hypothetical
perfectly culture-free or environment-free (meaning h1 = 1) test
measures the genotype, G: the culture-loaded test measures the
phenotype, P. (The phenotypic value, P, is the sum of the genetic
and environmental values, i.e., P — G + E.) Assume that the
heritability of the phenotypic measure, P, is 0*80, so the correlation
between genotype and phenotype would be the square root of 0-80,
or 0-89. Also assume that the means of the two groups, A and B,
differ on the phenotypic measure by 1 SD.
Hypothesis 1, then, is the environmental hypothesis. It states
that the mean genotypes of the two groups are either equal (which
includes the hypothesis that the phenotypically lower group is
genetically equal to or higher than the phenotypically higher group,
i.e., Ga ^ Gb) or genotype B is above genotype A, and the average
environment of group A is more favorable than that of group B
(i.e., EA>EB). If this hypothesis is true, and if h2 is 0-80 in each
group, then the regression of P on G and of G on P for groups A
and B should appear as shown in Figure 17.3 in the two graphs