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

164 Educability and Group Differences
differences in IQ closely approximate a x distribution, we can
conclude that the effects of environment on IQ have a normal
distribution in this twin sample. Moreover, it is possible to determine
the standard deviation (SD) of the distribution of the effects
of environmental differences on IQ. The SD is 4-74 IQ points.6
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N -N e w m a n et al, 1937, N = 19
J J - J u e l- N ie ls e n , 1 9 6 5 ^ = 1 2
N S -S h ie ld s , 1962, N = 3 8
N B - B urt, 1966, N = 5 3
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S S N J
S S S J
- s s S N
S N s S J S
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B S B S S S
B S B S S B J
- B B B B s B N J J S
B B B B B B S J N N J N B S
B B B B B B S N S 8 S N B S N
B B B B B B B S S B B B B S N S S
- B B B B B B B B B B B B B B B B S N S N
0 2 4 6 8 10 12 14 16 18 2 0 2 2 2 4
Absolute D iffe re n c e in IQ
Figure 7.1 Distribution of absolute differences (\d\) in regressed
true-score IQ between co-twins reared apart. This distribution
closely approximates the chi distribution.
Since in a normal distribution six sigmas encompass virtually 100
percent of the population (actually all but 0-27 percent), and since
the SD of environmental effects on IQ in the total twin sample is
4-74, it can be said that the total range of environmental effects in
a population typified by this twin sample is 6 x 4-74 = 28-4 IQ
points. This value is referred to by geneticists as the reaction
range of IQ under natural conditions. This determination of the
reaction range is slightly greater than the values conjectured by
Gottesman (1968, p. 34) of 24 points, by Bloom (1964, p. 71) of
20 points, and by Cronbach (1969, p. 343) of ‘more than 25
points’.
Thus, we now have a scale of the effects of environment (in