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

182 Educability and Group Differences
comprised of the same proportions of the two racial groups. Also,
heritability estimates can be biased in one direction or the other
depending upon the relative proportions of the two racial groups
in the MZ and DZ samples. The between-izmxlxes, (or twin pairs)
variance will generally be increased relative to within-families
(pairs) variance when racial groups with widely differing means are
combined in heritability analyses.
In a third study, the twin samples were separated by race, and
heritabilities were computed separately for Negroes and whites
(Osborne & Gregor, 1968). The white sample was composed of
140 MZ and 101 DZ twin pairs; the Negro sample of 32 MZ and
11 DZ pairs. Nine tests of spatial ability were used. Heritabilities
for the white sample are in the range generally found for cognitive
tests, going from h2 = 0-38 to 0-82, with a mean h2 of 0-54. In
the Negro sample the values of h2 range from 0-02 to 1 *76, with
a mean of 0-94. Four of the nine estimates of h2 are larger than
TOO! This makes the estimates highly suspect, and no doubt the
trouble is in the large sampling error of the Negro estimates,
based as they are on Ns of 32 MZ and 11 DZ twin pairs. Despite
an average h2 value of 0*94, only two of the nine tests show heritability
values significantly greater than zero at the 5 percent level
of significance. (All the white h2 values, however, differ significantly
from zero at the 1 percent level.) There is no significant difference
between the white and Negro heritabilities, but this study could
not have statistically detected quite substantial group differences
in heritability even if such differences actually existed. The fact
that a statistically significant genetic component of variance shows
up on only two of nine tests for Negroes and on all of the tests for
w'hites certainly provides no support for the authors’ conclusion
that ‘environment does not play a more significant role in the
development of spatial ability of Negro children than of white
children’ (p. 736). But neither does this study provide any support
for the opposite conclusion. Because of the very few cases in the
Negro sample, the study throws no light whatever on Negro-white
differences in the heritability of mental abilities.
Even the largest study of IQ heritability in a Negro population
is statistically unsatisfactory in terms of sample size (Scarr-
Salapatek, 1971a). Since Scarr-Salapatek obtained all her data
from public school files, she was not able to determine the twins’
zygosity (i.e., whether they are monozygotic or dizygotic). She