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

138 Educability and Group Differences
stature do not differ significantly in the two populations, there
would be two ways of getting around this fact while still maintaining
an environmental hypothesis. We can posit that factor ‘X’
has a constant effect on every member of the population in which
it occurs. Or we can posit two unknown factors, ‘X’ and ‘Y’, which
(i) have opposite effects on stature, (ii) exist exclusively in one
population or the other, and (Hi) have equally variable effects on
stature, thus increasing the non-genetic variance by equal amounts.
If these conditions seem untenable to us, we are apt to call factors
‘X’ and ‘Y’ genetic and reject the environmental hypothesis. But
this is admittedly a subjective judgment and neither a scientific,
statistical proof of a genetic hypothesis nor a disproof of the
environmental hypotheses. But we do know that genes can have
such large effects on stature (as can be proven by selective breeding
of plants and animals) and thus the genetic hypothesis seems more
reasonable than the hypothesis of unknown factors ‘X’ or ‘Y \
(c) The third reason that we intuitively accept a genetic hypothesis
in this case is that height is not the only physical difference
we see between Pygmies and Watusi. Suppose there are differences
in hair texture and distribution, in proportional differences
in body build, in facial features, and so on. In short, there would
be a whole consistent pattern of differences, not just differences
along a single dimension. Because of this consistent pattern of
physical differences, short Watusi still would not much resemble
tall Pygmies. The correlations among various body measurements,
after the general factor of stature is partialled out, would be different
for Pygmies and Watusi. This finding would accord with our
observation that Pygmies and Watusi look different even if one
ignores the overall difference in stature. All these differences could,
of course, be environmental, but it would be up to the environmentalist
to explain what kinds of environmental effects could
produce such consistently marked and different patterns of characteristics
in the two populations. The genetic hypothesis would be
more plausible because most of the elements entering into the
pattern differences are known to be highly heritable within each
population.
(d) The fourth reason that an environmental hypothesis strikes
us as implausible is that the extreme environmental conditions
which have the most extreme effects on stature within either
population also have other effects on the individual. These would