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

Between-Groups Heritability 139
make him differ from his fellows in more than just stature. Severe
malnutrition might make a Watusi abnormally small - perhaps as
small as the larger Pygmies. But malnutrition also makes Watusi
physically weak, while normal Pygmies, though small in stature,
are physically very strong. A Pygmy with pituitary gigantism
might be as tall as some normal Watusi, but he would be much
weaker physically. In other words, the extreme differences produced
by non-genetic factors within the populations involve a
constellation of other differences which do not resemble the
differences between the typical individuals of each population.
These four ‘arguments’ for the plausibility of genetic group
differences in stature, however, cannot prove a genetic hypothesis,
or, conversely, cannot disprove an environmental hypothesis,
because we can always posit factor ‘X’ as the unknown but crucial
environmental difference responsible for the difference between
the groups in stature. If there are specific hypotheses about causal
environmental differences, these hypotheses can be tested and
rejected without proving the genetic hypothesis. But at least we
could determine which environmental factors do not cause the
group difference in stature. If every known environmental difference,
singly or in combination, fails to account for the difference,
then the environmentalist must fall back on some unknown factor
‘X’. Unless he can formulate testable hypotheses concerning the
nature of ‘X’, we are left with an explanation which has little
utility in terms of prediction or control of the variable we wish to
explain. Even if a genetic hypothesis were wrong, the fact of high
within-group heritability leaves no doubt that the mean difference
between the populations could be decreased by genetic selection.
On the other hand, hypothesizing factor ‘X’ as the cause of the
difference provides no basis for control. But if such approaches
can never prove or disprove genetic or environmental hypotheses,
how can such hypotheses be put to scientifically definitive tests - as
definitive, that is, as anything can be in an empirical science
Discontinuous Traits
As Thoday (1969) has pointed out, there is usually little or no
problem in establishing genetic differences between populations in
discontinuous traits. Such traits are all-or-none and are due to the
presence or absence of a single gene. The genetic basis of the trait