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

184 Educability and Group Differences
the opposite direction. In short, subgroups of whites and Negroes
roughly equated for social class had about the same heritabilities,
and lower-class subpopulations of both racial groups had significantly
lower heritabilities for the aptitude tests. The lower heritability
in this group seems to be due principally to greater
within-pair variance for same-sex twins, which, of course, includes
all MZ twins. Scarr-Salapatek’s interpretation of this finding is
in terms of the threshold hypothesis, viz., that environmental
deprivations in the lowest SES group have increased differences
between co-twins and decreased variance among unrelated individuals,
with a consequent reduction of genetic variance, relative
to environmental variance, in test scores. When the lowest SES
groups were not included in the analysis, Scarr-Salapatek found
similar heritabilities in the Negro and white samples. But despite
higher heritabilities of both verbal and non-verbal aptitude in
middle-class Negroes than in middle-class whites, Scarr-Salapatek’s
data show that the Negro scores are distributed around means that
are nearly a standard deviation below whites of comparable and
lower SES. This is difficult to reconcile with the hypothesis that
Scarr-Salapatek seems to favor as an explanation of the mean
white-Negro ability difference. Scarr-Salapatek’s hypothesis should
predict that the heritability of the mental tests would be in general
lower in the Negro sample than in the white. Such a finding (which
in fact was not found) could be interpreted as consistent with an
explanation of the mean Negro-white IQ differences in terms of
environmental factors such as cultural deprivation. Scarr-Salapatek
writes: ‘The lower mean scores of disadvantaged children of both
races can be explained in large part by the lower genetic variance
in their scores’ (p. 1293). She adds: ‘If most black children have
limited experience with environmental features relevant to the
development of scholastic skills, then genetic variation will not
be as prominent a source of individual phenotypic variation; nor
will other between-family differences such as SES [socioeconomic]
level be as important as they are in a white population’ (p. 1294).
The data shown in Scarr-Salapatek’s Table 3 (p. 1288), however,
make this interpretation highly questionable. These data
allow comparison of the mean scores on the combined aptitude
tests for Negro children whose parents’ level of education and
income are both above the median (of the Negro and white samples
combined) with the mean scores of white children whose parents'