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

346 Educability and Group Differences
their retarded fetal growth, short period of gestation and increased
risks during birth. There are very large variations in
intelligence in a general population of births in relation to
maternal age and birth order (Fig. 1); but these are due to
differences between rather than within families [emphasis added],
for there is little variation according to birth rank between sibs.
Thus, it is not yet established that the higher rate of reproductive
casualty in Negroes, as reflected in a higher incidence of fetal
loss, prematurity, and infant mortality, causes any substantial
proportion of the IQ deficit. But the question is much too important
to be dismissed or allowed to rest ambiguously on the
current inadequate state of the evidence. For we do have some
good statistics on certain population indices of reproductive
casualty, such as fetal loss, and if we make what seems to be a
reasonable assumption that fetal death represents merely a threshold
effect on a continuous variable of impaired development, we
have a firm basis for inferring some higher, though quantitatively
undetermined, incidence of physical (including neurological)
impairment in the Negro as compared with the white population.
In 1965, fetal deaths (for gestation periods of twenty weeks or more)
nationwide had almost twice as high a rate among Negroes as
among whites (25*8 v. 13-3 per 1,000 live births) (U.S. Department
of Health, Education, and Welfare, 1967, pp. 3-5). Assuming fetal
death to be a threshold effect on a normally distributed variable,
the Negro and white populations can be said to show a mean
difference of 046a on this variable. This is a large difference by
any standard. But even if this variable (organismic viability, freedom
from impairment, or whatever it is) were perfectly correlated
with intelligence, it could account for less than half of the Negrowhite
IQ difference.
But is the rate of fetal loss in a population entirely a function
of external environmental conditions It appears not to be. The
recent research on this matter may provide a clue to the hitherto
inexplicably higher rate of fetal loss and other less severe forms
of reproductive casualty among Negroes even as compared with
non-Negro groups of similar or greater environmental disadvantages.
Bresler (1970) has found that the probability of fetal loss is
directly related to the degree of genetic heterogeneity among the