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

126 Educability and Group Differences
differences in IQ, there are even visible genetic differences (e.g.,
skin color, hair texture, etc.) between the groups, the purely logical
status of which, in relation to IQ, is not different from the visible
environmental differences between the groups. In both cases, the
visible differences may or may not make a causal difference in
IQ. The visible environmental differences and the visible physical
genetic differences between two racial groups may have no causal
connection with IQ; both may be merely correlated with some other
factors which directly influence IQ. Since we know from studies
of the heritability of individual differences in IQ that genetic
factors have comparatively powerful effects and environmental
factors have comparatively weak effects, is there probabilistically
more reason to hypothesize environmental factors as of greater
importance than genetic factors in explaining group differences in
IQ The a priori preference for strictly or preponderantly environmental
explanations seems to stem more from ideological than
from any logical or scientific considerations. Thus, Jencks (1969,
p. 29) writes, ‘While a significant number of black children may
well suffer serious prenatal damage, Jensen’s evidence suggests
that we should probably look elsewhere to explain racial differences
in IQ scores. But it hardly follows that we must look to genes. We
might do equally well to look at patterns of child rearing.’ This
clearly expresses a preference which could determine one’s research
strategy, but the preference would seem to run counter to the
probabilities suggested by already established evidence. Genes
have already been established as having powerful effects on IQ;
individual genetic differences correlate about 0-85 to 0-90 (the
square root of the heritability) with IQ. Correlations between
child-rearing practices and IQ within racial groups are minute by
comparison, and the extent of their causal connection with IQ
differences between racial groups has not been determined. (If the
broad heritability, including GE covariance, of IQ is 0-75, for
example, the maximum correlation between IQ and all environmental
effects combined would be only yj\ —0-75 = 0-50 within
the population in question.) Pointing to some environmental
difference whose causal relationship to IQ is not established is
logically no more plausible as an environmental explanation of a
mean IQ difference than is pointing to some clearly genetic
difference, such as skin color, as a genetic explanation of the
difference.