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

178 Educability and Group Differences
evidence for it was the large IQ gains, amounting to an average of
some 30 points, shown by severely deprived orphanage children,
at about 18 months of age, when they were put into a more
stimulating environment and were eventually reared in good
adoptive homes (Skeels & Dye, 1939; Skeels, 1966). Bereiter argues
that this magnitude of IQ gains is within the reaction range of
IQ, assuming a heritability of 0-80 and equal additivity of environmental
effects across the whole range of environments. But a
30-point mean difference in group means suggests some individual
changes outside the normal reaction range for IQ. In fact, four of
the thirteen subjects in Skeels’ study showed IQ gains greater
than 30 points (Skeels, 1966, Table 1). So perhaps the threshold
hypothesis may be needed after all in order to account for these
data.
Probably the best evidence for the threshold hypothesis would
be the finding of significantly higher heritability in groups that
are above average in SES and environmental advantages than in
groups of low SES.13 No one has ever done this systematically.
The gifted children in Terman’s study came mostly from the
higher SES levels and unquestionably had considerably better
than average environmental advantages for intellectual development.
The mean IQ of their siblings was 123 and the correlation
between the IQs of the gifted and their siblings, estimated from
the sibling regression, is 0-44, which, when corrected for attenuation,
is close to the genetically predicted sibling correlations of
0*5 (with random mating) or 0-6 (with an assortative mating
coefficient of 0-5), and does not differ much from sibling correlations
reported in the general literature. The gifted group as adults
were, on the average, of higher SES than their own parents. Thus
the offspring of the gifted probably enjoyed even greater environmental
advantages. The narrow heritability of IQ in this group,
estimated from the midparent-midchild regression, is 0-85. This
is significantly higher than the best estimate of narrow' heritability
(0*71) given by Jinks and Fulker (1970, p. 342) on the basis of
Burt’s data, which includes a wride range of SES in the English
population. It is also higher than the midparent-midchild correlation
(0-69 + 0-03) found in a largely rural population sample in
Vermont in 1920, with environmental advantages presumably
much below those provided by the Terman gifted and their
spouses (Jones, 1928, p. 69). These heritability findings, then, are