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

The Heritability of Scholastic Achievement 105
assessed; and the degree of their mastery by any individual will
be largely a function of the amount of time he spends in being
taught and in practising the skill. Thus children with quite
different IQs can be shaped up to perform more or less
equally in these elemental skills. If Johnny has trouble with
his spelling or arithmetic, his parents may give him extra tutoring
so that he can more nearly approximate the performance of
his brighter brother. This is not so easily accomplished for
more complex processes such as reading comprehension and
arithmetic applications, which more nearly resemble IQ in degree
of heritability.
Since school subjects increase in complexity with each grade
level, and the consolidated portion of achievement correlates more
and more with intelligence, we should expect that the heritability
of scholastic achievement should also increase with age. There is
evidence that this is the case. Husen (1963) found a decreasing
environmental component in achievement variance from fourth
to sixth grade. Increasing achievement differences among children
as they advance in age then increasingly reflect their genetic
intelligence differences. By the last year or two in high school, the
heritability of comprehensive measures of scholastic achievement
(rather than narrow tests which assess only the subject matter
taught in the immediately preceding school year) is quite comparable
to that of measures of general intelligence. This is shown in a
study by Nichols (1965) who obtained the scores of large samples
of MZ and DZ twins who, as high school juniors (eleventh grade),
had taken the National Merit Scholarship Qualifying Test
(NMSQT), which consists of subtests in English, mathematics,
social studies, natural science, and word usage. Nichols notes
that the intraclass correlations (on MZ and DZ twins) for the
NMSQT composite score are very similar to correlations for
measures of general intelligence obtained in other twin studies.
These findings, at first glance, might seem to be in conflict with
those of Newman et al. (1937), whose twins wrere adults but
showed quite different correlations for IQ than for achievement.
The reason is most likely that all the twins in Nichols’ sample
were very close to the same age and had the same number of years
of schooling. The twins of Newman et al. had quite different
amounts of schooling; for example, one twin went only through
the third grade while her separated co-twin was a college graduate.