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

Physical Environment and Mental Development 345
suggested by Amante et al. and by the many writings of Pasamanick
on this subject. The first counterfact is that independently
assessed complications of pregnancy are known to be reflected in
depressed performance on infant tests of psychomotor development
in the first year of life (Honzik, Hutchings & Burnip, 1965).
Yet on these very same tests, given at six months to one year of
age, large representative samples of Negro infants were found to
do as well as, or better than, comparable samples of white infants
(Bayley, 1965). Such findings could be compatible with a markedly
higher incidence of neurological damage in Negro infants only if
it is argued that the Negro infants are normally so very advanced
over white infants in psychomotor development that even with a
high incidence of brain damage the mean Negro performance is
still above the white mean. But this possibility should result in a
larger variance of Developmental Quotients for Negroes as compared
to whites, and Bayley’s data show no significant racial
difference in the variance of DQs.
The second item of evidence which is apparently inconsistent
with the hypothesis of high rates of brain damage as a principal
cause of lower Negro IQ is the heritability of IQ and the intrafamily
IQ variance (sibling differences) which are about the same
for Negro and white populations. If brain damage is an added
external source of environmental variance, it should significantly
lower the heritability of IQ and increase sibling differences. Negro
and white samples which do not differ significantly on these
variables still show an IQ difference of 1 SD or more (Scarr-
Salapatek, 1971a; and see Table 19.1, p. 339).8
These findings seem to accord with the conclusions drawn by
McKeown and Record (1971, p. 52) from their recent review of
the literature on prenatal environmental influences on mental
development:
Prenatal environmental influences appear to contribute little to
the variation in intelligence in a general population from which
those with recognized defects are excluded. There is little
relationship to abnormalities of pregnancy or labour. . . . But
the most convincing evidence that prenatal influences have little
effect on measured intelligence is the observation that twins
separated from their co-twin at or soon after birth have scores
which are little lower than those of single births, in spite of