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

Between-Groups Heritability 137
and the standard deviation (SD) of the environmental effects
would be y/o-S = 0*71 inches. This means, in effect, that two
genetically identical individuals (e.g., monozygotic twins) who
differ by 0-71 inches can be said to differ by 1 SD in the effects of
all environmental factors influencing stature. We can then express
the mean difference of, say, 10 inches between Watusi and Pygmies
in terms of the number of SDs by which they must differ in the
environmental factors affecting height within a population, if we
are to explain all of the differences in terms of these environmental
factors. This amounts to 10 inches/0-71 inches = 14-8 SDs difference
between the two groups in the effects of environment. Two
normal distributions whose means differ by as much as 14-8 SDs
are so extremely far apart that there would be absolutely no overlap
between the two groups in the environmental factors affecting
height. In other words, the probability is practically infinitesimal
that even the very largest environmental differences affecting
height within either population could begin to explain the 10
inches difference between the two populations. No naturally occurring
environmental effects within either population would alter
height more than about 6 SDs (which includes 99-8 percent of the
total range of a normal distribution) or 6x0*71 inches = 4*3
inches. Thus, the mean difference between groups is something
more than twice as large as the largest differences within groups
that could be attributable to naturally occurring environmental
effects within the groups. This is therefore so highly improbable,
that in order to go on entertaining a strictly environmental
hypothesis of the cause of the mean difference in statures one would
have to hypothesize that the environments of Watusi and Pygmies
differ in some very potent unknown factor (or factors), ‘X’, which
is present in one population and not in the other and which affects
all individuals in the one population and none in the other.
Furthermore, if factor ‘X* does not have an equal or constant effect
on all members of the population in which it is present, and if the
two populations are genetically identical for stature in accord with
the environmental hypothesis, then we should expect to find a
lower heritability for stature in the population affected by factor
‘X’, since it is a variable environmental effect which acts in the
one population and not in the other. If genetically identical, both
populations should be expected to respond similarly to the
environmental factors common to both. If the heritabilities of