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

Race Differences in Intelligence 171
general population. It is interesting, therefore, that the average
family income of the gifted is 1 -45 SDs above the national average.
Using only income as an index of environmental advantage, we
would estimate the IQ of the offspring of the gifted as the genetic
prediction (IQ = 127-3) plus the environmental advantage (1*45 x
3-35 = 4-9 IQ points) as 132-2, which does not differ significantly
400)-^
t CHILDREN PER
I TEN IQ POINT
_ INTERVAL
MENTALLY
DEFICIENT
NORMAL d is t r ib u t i o n
,5 2 5 W ITH
cr = 16.5
H = 132.7
0 ' 7 0 8 0 9 0 100 110 120 130 140 150 160 170 180 1 9 0 2 0 0
Figure 7.4 Distribution of Stanford-Binet IQs of the offspring of
Terman’s gifted subjects. A normal curve is superimposed on the
actual data indicated by crosses. (Terman and Oden’s [1959,
Table 61] data were plotted in this figure by W. Shockley, who
notes that ‘The offspring [of the Terman gifted group] have an
accurately normal distribution in the same IQ range in which the
parents do not fit the tail of a normal distribution’ (personal
communication). This point is interesting in that this result is
predictable from a polygenic model with a large [e.g. > 20] number
of loci and a considerable amount of heterozygosity in the parents.
Under these conditions, theoretically the offspring should show a
nearly normal distribution despite marked skewness of the distribution
of the parental values, and this in fact is what is found in
these data.)
from the obtained mean IQ of 132-7. Thus, our model fits the
Terman gifted data very well.
By reckoning from the same model, the average Negro income
gap of —0-80 ± 0-15 would account for about 0-80 x 3-35 = 2-7 IQ
points (or 18 percent) of the 15 IQ points difference between the
racial IQ means. It must be concluded that income differences can
account for only a small fraction (less than one-fifth) of the 15
points mean IQ gap between Negroes and whites.