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

Race Differences in Intelligence 181
significance of the difference between the Negro and white Fs for
each test.18 If they do not differ significantly, there is little support
in these data for the hypothesis that the heritability for a given
test differs between the Negro and white populations. The largest
Negro-white difference in F on any of twenty tests in Vandenberg’s
study is on the spelling test (Negro F = 1-58, n.s., white F = 2-94,
p< 0-001). But the difference between even these most extreme
Fs is not significant at the 10 percent level, so of course none of
the other differences is significant. Vandenberg’s data, therefore,
hardly provide statistically reliable support for his conclusion
that ‘there is good evidence for the thesis that the ratio between
hereditary potential and realized ability was generally lower for
Negroes than for whites . . .’ (Vandenberg, 1970, p. 283). Clearly,
the trouble with this study is the small number (N = 14) of DZ
twin pairs in the Negro group. With so few cases, the sampling
error of the variance estimate is simply too large to permit any
statistically reliable inference.
There are only three other published heritability studies which
have included Negroes. One of these combines the Negro and white
MZ and DZ twin samples in arriving at heritability estimates for
a variety of tests of spatial ability (Osborne & Gregor, 1966). The
results, therefore, do not allow any comparison of Negro and
white heritabilities for these traits. The data, furthermore, could
not be analyzed separately for the two racial groups, since for
both groups combined there are only 33 pairs of MZ and 12 pairs
of DZ twins. These samples are much too small to detect significant
differences in heritabilities within the range we might expect. The
standard error of h2 with these size samples would be approximately
0-10, so group differences in h2 smaller than about 0*20 could not
be detected as statistically significant.19
Another paper based on the same twin samples as those of the
previous study estimated heritability for seven tests of simple
arithmetic (Osborne, Gregor, & Miele, 1967). The value of h2
for the composite score of all seven tests is 0-80 ± 0-10; the intraclass
correlations for MZ and DZ twins are 0-84 and 0-44,
respectively. But again, since there was no separation of Negro
and white twin samples, the study tells us nothing about racial
differences in h2. Moreover, the practice of estimating h2 in mixed
samples from two distinctive populations seems indefensible, since
the only population to which the result can be generalized is one