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

134 Educability and Group Differences
popular notion that all the persons of each race are closely alike
mentally and very different from all the persons of any other
race is sheer nonsense. There is usually great variation within
the race and great overlapping between races. The case about
which most is known is intelligence in American Negroes,
including Negro-white hybrids, and whites mostly of English
and North European descent, (p. 321)
And Thorndike goes on to estimate the overlap3 here for persons
who have had equal numbers of years of schooling; his estimate is
that 10 percent of Negroes exceed the white median, which,
assuming normality of the two distributions, corresponds to a mean
difference of approximately 1-3 sigmas or 1-3 x 15 = 19-5 IQ
points. (This is a slightly larger difference than the 12 percent
median overlap estimated by Shuey on the basis of all existing
evidence up to 1965.)
Most modern geneticists would agree that Thorndike’s main
point is essentially correct. Any breeding groups are virtually
certain to have different gene pools and the only real questions
that remain concern the magnitude of the genetic difference (i.e.,
the heritability of the phenotypic group difference), its direction
(i.e., which group is higher on the characteristic in question), and
its significance in terms of the demands made by the environment.
Some differences, though real and statistically significant, do not
make any practical difference under existing conditions. The large
racial differences in ability to taste phenylthiocarbamide, for
example, are of no personal or social consequence. Differences in
mental abilities, on the other hand, can have important practical
consequences, depending upon their magnitude, both for individuals
and for society. What Thorndike recognized was the high
probability of genetic group differences, but he made no attempt
to estimate their magnitude, which is the more important question
in a practical sense. We are hardly any further ahead today.
The simple fact is that one cannot, in any strict, formal sense,
infer between-groups heritability from a knowledge of withingroups
heritability. This is true even if the heritability of the trait
is perfect (i.e., h2 = 1 *00) within each group and there is absolutely
no overlap of the phenotypic distributions of the two groups. As
a clear example we can point to various kinds of grasses. Grown
in complete darkness, their colors will vary from white to pale