THE MISMEASURE OF MANSpearman on the inheritance of g
Two of Spearman's primary claims appear in most hereditarian
theories of mental testing: the identification of intelligence as a
unitary "thing," and the inference of a physical substrate for it. But
these claims do not complete the argument: a single, physical sub-
stance may achieve its variable strength through effects of environ-
ment and education, not from inborn differences. A more direct
argument for the heritability of g must be made, and Spearman
supplied it.
The identification of g and s with energy and engines again
provided Spearman with his framework. He argued that thes-fac-
tors record training in education, but that the strength of a per-
son's g reflects heredity alone. How can g be influenced by
education, Spearman argued (1927, p. 392), if g ceases to increase
by about age sixteen but education may continue indefinitely there-
after? How cang be altered by schooling if it measures what Spear-
man called eduction (or the ability to synthesize and draw
connections) and not retention (the ability to learn facts and remem-
ber them)—when schools are in the business of imparting infor-
mation? The engines can be stuffed full of information and shaped
by training, but the brain's general energy is a consequence of its
inborn structure:
The effect of training is confined to the specific factor and does not
touch the general one; physiologically speaking, certain neurons become
habituated to particular kinds of action, but the free energy of the brain
remains unaffected.... Though unquestionably the development of spe-
cific abilities is in large measure dependent upon environmental influ-
ences, that of general ability is almost wholly governed by heredity (1914,
pp. 233-234)-IQ, as a measure of g, records an innate general intelligence; the
marriage of the two great traditions in mental measurement (IQ
testing and factor analysis) was consummated with the issue of
heredity.
On the vexatious issue of group differences, Spearman's views
accorded with the usual beliefs of leading western European male
scientists at the time (see Fig. 6.9). Of blacks, he wrote (1927, p.
379), invoking^ to interpret the army mental tests: