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

Race Differences in Intelligence 179
consistent with the threshold hypothesis. But the total evidence
for the hypothesis must still be regarded as quite ambiguous. A
clear finding of an appreciable difference between h2 in the Negro
and white populations, however, would be consistent with the
hypothesis depicted in Figures 7*5 and 7*6. It could mean, in
effect, that the scale of environmental effects differs for the bulk
of the two populations and not simply that the two populations are
distributed about different means on the same additive (i.e., equal
interval) scale of environments. So now we must examine what
meager evidence exists on the estimation of h2 in Negro populations.
Estimations of h2 in Negro Populations
Recognizing that the threshold formulation implies a lower
heritability of IQ in the Negro population if in fact a substantial
proportion of them have been subjected to environmental conditions
in that part of the scale with the largest effects on cognitive
development, Vandenberg (1970) sought evidence to test this
hypothesis, viz., that there is a greater proportion of environmental
variance in IQs in a Negro (or any environmentally deprived)
population. Vandenberg estimated heritability by comparing MZ
and DZ twins. Let us look at the genetic model and its assumptions,
which form the basis for this method of estimating
heritability.
The total phenotypic variance,14 Vp, can be partitioned into
two main components: variance attributable to differences (both
genetic and environmental) between families (VB) and differences
(both genetic and environmental) within families (Vw). The
between-families variance (VB) can be partitioned into genetic
and environmental components: VBGand VBE, respectively.
The model assumes that the total variance is the same for MZ
and DZ twins, i.e., VMZ = VDZ. The total variance for DZ twins15
is composed of VDZ = \ V BG+$VWG + VBE + VWE. The total
variance for MZ twins is composed of: VMZ = VBG+ VBE+ V WE.
From the data we can obtain the between and within variances for
DZ and MZ twins. If we subtract the between-i&vmYits DZ variance
from the fotaumz-families MZ variance, we obtain one-half of the
between-izumlizs genetic variance:
^ bmz = I'bg + Vbe
Vbdz= W b g + V b e
Difference = \ V BG