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

Between-Groups Heritability 133
selected) of the clearly specified population group from which
they were selected.
Population subgroups which have migrated are not necessarily
representative of their native parent populations. Studies of racial
or national groups in the United States, therefore, cannot automatically
be generalized abroad, and the reverse is also true. This
does not mean, however, that meaningful comparative studies of
various subpopulations within the United States (or elsewhere)
are not feasible.
INFERENCE FROM WITHIN-GROUPS TO BETWEEN-GROUPS
HERITABILITY
The first explicit recognition of this problem which I have come
across in the psychological literature is attributable to E. L.
Thorndike (1940, pp. 320-1). It is quite interesting to note how
close his estimate of heritability, based on the rather meagre
evidence of his day, comes to the estimates based on our present
more sophisticated methodology and more extensive data. He
ascribed the following percentages to the components of variance
in individual differences in intelligence:
Genes 80%
Training 17%
‘Accident’ 3 %
After discussing the predominantly genetic basis of individual
differences in intelligence, Thorndike goes on to say the following
about group differences:
Most of what has been said here about individual mental
differences is applicable to the mental differences of families
and races. Such exist as a consequence of differences between
the genes or training or both, of one family from another, one
race from another. A sample of man isolated from the rest in
breeding will only by rare accident have genes identical with
the rest of man. Whatever selective forces operate in the begetting
of that sample’s children will only rarely be just the same as
operate in the rest of man. But it is easy to overestimate these
family and racial differences, and in the interest of one or
another theory or prejudice this has often been done. The