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

Physical Environment and Mental Development 349
‘desirable’, hybrid vigor (e.g., greater size) is said to result. The
effect is quite independent of the effects of loci imbalance involved
in the Bresler study; it is a different phenomenon entirely. The
two effects can operate simultaneously, in ‘opposite’ directions;
the deleterious effects of genetic imbalance can override desirable
effects of heterosis. The opposite of heterosis genetically is inbreeding
depression, which shows up most clearly in consanguineous
matings, such as cousin marriage. Not all traits show
heterosis and inbreeding depression. Height and chest circumference
show it, as does IQ (Schull & Neel, 1965), but head shape
(cephalic index) and circumference apparently do not (Wolanski,
Jarosz & Pyzuk, 1970). Too close inbreeding causes depression
of some characteristics because of the increased likelihood of the
pairing of undesirable mutant alleles, while too much heterogeneity
of ancestral gene pools can have undesirable consequences due to
genetic imbalance caused by translocations and inversions of loci.
The role of these genetic mechanisms in the causation of reproductive
casualty and its differing rates in various subpopulations
calls for much further investigation, which hopefully will not
be hindered by ideologically motivated insistence that all such
effects must be attributable entirely to external environmental
factors.
NOTES
1. The locations of these studies: Cape Town, South Africa; Kampala,
Uganda; Guatemala; Mexico; Santiago, Chile; Sarajevo, Yugoslavia;
Hyderabad, India; Peru.
2. In autopsy studies of stillborn and newborn infants of poor, presumably
undernourished mothers in New York City, as compared
with infants of non-poor mothers, the magnitude of the effects of
‘poorness’ (presumably maternal undernutrition) on the growth of
various organs and body measurements was determined. Of the
eight measurements made on the babies, the brain was least affected,
suggesting that it is probably the nutritionally most highly buffered
organ in the fetus (Naeye, Diener, Dellinger & Blanc, 1969). The
index of relative effect of prenatal undernutrition for the eight
infant body measurements and the placenta were: thymus 38,
adrenals 25, spleen 23, heart 15, body length 15, liver 21, kidney 10,
brain 6, and placenta 4.