THE MISMEASURE OF MANtesting lay with his own theory of a single g underlying all cognitive
activity. IQ tests worked because, unbeknownst to their makers,
they measured g with fair accuracy. Each individual test has a g-
loading and its own specific information (or 5), butg-loading varies
from nearly zero to nearly 100 percent. Ironically, the most accu-
rate measure of g will be the average score for a large collection of
individual tests of the most diverse kind. Each measures g to some
extent. The variety guarantees fhats-factors of the individual tests
will vary in all possible directions and cancel each other out. Only
g will be left as the factor common to all tests. IQ works because it
measures g.
An explanation is at once supplied for the success of their extraordi-
nary procedure of... pooling together tests of the most miscellaneous
description. For if every performance depends on two factors, the one
always varying randomly, while the other is constantly the same, it is clear
that in the average the random variations will tend to neutralize one
another, leaving the other, or constant factor, alone dominant (1914, p.
313; see also, 1923, p. 6, and 1927, p. 77).Binet's "hotchpot of multitudinous measurements" was a correct
theoretical decision, not only the intuitive guess of a skilled practi-
tioner: "In such wise this principle of making a hotchpot, which
might seem to be the most arbitrary and meaningless procedure
imaginable, had really a profound theoretical basis and a
supremely practical utility" (Spearman quoted in Tuddenham,
1962, p. 503).
Spearman's g, and its attendant claim that intelligence is a sin-
gle, measurable entity, provided the only promising theoretical jus-
tification that hereditarian theories of IQ have ever had. As mental
testing rose to prominence during the early twentieth century, it
developed two traditions of research that Cyril Burt correctly iden-
tified in 1914 (p. 36) as correlational methods (factor analysis) and
age-scale methods (IQ testing). Hearnshaw has recently made the
same point in his biography of Burt (1979, p. 47): "The novelty of
the 1900's was not in the concept of intelligence itself, but in its
operational definition in terms of correlational techniques, and in
the devising of practicable methods of measurement."
No one recognized better than Spearman the intimate connec-
tion between his model of factor analysis and hereditarian inter-
pretations of IQ testing. In his 1914 Eugenics Review article, he