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

92 Educability and Group Differences
gain component of the achievement test score would be a smaller
proportion of total [consolidated] achievement for the brighter
sibs and thus would not so attenuate the correlation between them.
In other words, their phenotypic correlation would be closer to
their genetic correlation.) This result is in fact what has been
found. Burt (1943) divided sibling pairs into two groups: those
above the median in IQ (i.e., 100 IQ) and those below the median.
The correlation between siblings’ scholastic achievement test
scores was 0-61 for the above-average sibs and only 0-47 for the
below-average sibs.
Another inference from our model is that sibling correlations
(based on tests given at the same age for both sibs) in measures
of intelligence should be substantial and should increase with age,
while year-to-year measures of gain should show much lower or
even negligible correlations. The status measures, which increasingly
reflect C, therefore, would also increasingly reflect the genetic
factors which the sibs have in common, while the gains, which
reflect motivation and specific learning and largely fortuitous
environmental factors, should show little, if any, sib correlation.
This inference, too, has been substantiated in part in a longitudinal
study conducted at the Fels Research Institute (McCall, 1970).
The level (status) of intelligence at any given age was found to
show much higher heritability than the pattern of changes (gains)
in intelligence from one time to another (an average interval of
9 months). Although there is an increase in sib correlations with
age, it is not statistically significant. The model also predicts that
parent-child correlations should be higher when they are based
on measures of the parent as an adult than measures of the parent
taken at the same age as those on the child. McCall’s (1970) study,
which also included parent-child correlations of test scores
obtained when both parent and child were between 3 and 12 years
of age, showed significantly lowrer parent-child correlations than
have been found in studies of parent-child correlations in which
the parent was measured as an adult. (The one exception reported in
the literature is Burt’s [1966, Table 4] parent-child IQ correlation of
0-49 when the parents were adults and of 0-56 when the parents’
childhood IQs were used.) McCall (1970, p. 647) concludes:
.. . although the general level of IQ appears to show heritability,
the pattern of IQ change over age possesses far less heritability