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

356 Educability and Group Differences
the hypothesis that genetic factors are involved in the difference
between populations. High within-group heritability cannot prove
between-group heritability, but it does increase the a priori likelihood
of finding genetic components in the average difference
between groups.
Knowledge of the heritability of IQ within each of two populations
can also place certain constraints on the kinds of hypothesized
environmental influences that are invoked to explain population
differences. Analyses show, for example, that whatever
environmental factors account for the IQ differences between
identical twins reared apart have an exceedingly low probability
of explaining the one standard deviation IQ difference between
the Negro and white populations. If it were established that the
heritability of IQ is as high as 0*75 in both the Negro and white
populations, then it can be said that as yet no environmental
factor or combination of factors has been identified on which the
Negro and white populations differ sufficiently to account for the
15 points mean IQ difference solely in terms of non-genetic factors.
This makes it necessary, if one wishes to maintain a purely
environmentalist theory, to hypothesize the influence of subtle,
unmeasured, and perhaps unmeasurable environmental differences
which contribute to variance between races but not to variance
within races. If the subtle, hypothetical environmental factors
cannot be measured or cannot be shown to bear some relationship
to IQ differences within racial groups, then the theory that postulates
their existence as an explanation of racial IQ differences is
in principle untestable and therefore beyond the pale of science.
Such hypotheses will not do as a scientific explanation of population
differences. Theories that emphasize genotype x environment
interaction as an explanation of racial IQ differences imply a
genetic difference, since they are based on the hypothesis that
different racial genotypes respond differently to the same environmental
influences. There is no evidence as yet for such an interaction
between genetic and environmental effects in the determination
of IQ differences, either within or between racial groups.
COMBINING ENVIRONMENTAL EFFECTS
Many environmental differences have been shown to exist between
Negro and white populations, and some sizeable proportion of