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

Race Differences in Intelligence 165
populations similar to the twin samples), with one SD on the scale
being equivalent to 4-74^0*3 IQ points. That is to say, two
genetically identical individuals who differ by 4-74 IQ points
(true-score values) can be said to differ by one SD on the scale of
the effects of environment on IQ.7
If, then, we hypothesize that the mean difference of 15 IQ points
between Negroes and whites is due entirely to non-genetic causes,
we must conclude that the two populations differ by 15/4-74 = 3-2
SDs on our environmental scale. With a difference this large, only
0-07 percent of the lower group exceeds the median of the higher
group.
But here we are considering the total non-genetic or environmental
effects in the twin samples. The total environmental
variance can be analyzed into two parts: (a) variance due to
environmental effects (i.e., differences) between families, and (b)
variance due to environmental effects within families, including
unequal prenatal effects on each member of a twin pair. (These
environmental differences operating within families and making
for environmental differences among children reared together are
sometimes referred to as micro-environmental effects.) The proportions
of variance attributable to the between and within components
are estimated from the difference between MZ twins reared together
in the same family (MZT) and MZ twins reared apart in different
families (MZA). The differences between the mean absolute
difference among MZA and MZT give an estimate of the withinfamilies
and between-families effects. The difference between MZ
twins reared apart is attributable to both the within-iwcmYits and
between-izrmXizs environmental effects; the difference between MZ
twins reared together is attributable only to the within-family
effects. Subtracting the difference for MZT from the difference
for MZA, therefore, gives us the difference attributable to betweenfamily
effects. When this is done on MZT and MZA data where
both types of twins are from comparable populations, the withinfamilies
environmental effect actually turns out to be slightly
larger than the between-families effect on IQ (Jensen, 1970a, p.
145). But to keep the argument simple, let us assume that the
between and within variances are approximately equal. This would
mean that half the within-MZA twin variance is due to environmental
differences between families. Since the variance of total
(i.e., between and within) environmental effects on IQ is (4-72)2