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

106 Educability and Group Differences
Newman et al. correlated the differences in educational backgrounds
with differences of achievement for their pairs of MZ
twins reared apart; the resulting r is 0-91! The same correlation
for a group intelligence test (Otis IQ) is only 0-55, and for an
individual intelligence test (Stanford-Binet IQ) it is 0-79. The
achievement test has an h2 of 0*51. For the Otis IQ h2 was 0*73,
and for Stanford-Binet IQ h2 was 0-67. The total proportion of
non-genetic variance, i.e., 1 —h2, not including error variance
(estimated at 5 percent), is therefore 0*44 for achievement, 0*22
for Otis IQ, and 0*28 for Stanford-Binet IQ. The proportion of
total variance accounted for by differences between twins in
educational advantages, therefore, is given by the square of the
correlation between difference in environments and difference in
test scores, multiplied by the non-genetic variance (1 —h2), not
including error variance. Thus, in the study by Newman et al.,
twin differences in the index of educational environment account
for the following proportions of total variance: in scholastic
achievement = 0-36, in Otis IQ = 0-07, in Stanford-Binet IQ =
0-17.
Nichols (1965) also asked if the separate subtests (English,
maths, etc.) of the NMSQT had any heritability after the general
factor common to all the subtests was removed. He found that the
‘residual’ subtest scores had almost as high heritability as the
composite score. Nichols comments, ‘Thus, it appears that the
specific abilities measured by the NMSQT subtests have about
the same hereditary character as the more general ability which the
subtests measure in common.’ The general factor is probably
identical with theg of intelligence tests. A factor analysis of several
verbal and non-verbal intelligence tests and several achievement
tests showed that all the tests had very similar loadings on the
general factor (first principal component), and the proportion of
total variance accounted for by the general factor increased from
grades 4 to 8 (Jensen, 1971a, Table 5).
The heritability (h2) of scholastic achievement depends also
upon the degree of homogeneity or uniformity in the type and
quality of the instructional program of the schools from w'hich
the individuals in the heritability analysis have been sampled. The
school environment is more imposed upon the child than the extraschool
environment, which generally allows the child much more
freedom of choice of experiences according to his own proclivities.