286 THE MISMEASURE OF MAN
Charles Spearman and general intelligence
The two-factor theory
Correlation coefficients are now about as ubiquitous and unsur-
prising as cockroaches in New York City. Even the cheapest pocket
calculators produce correlation coefficients with the press of a but-
ton. However indispensable, they are taken for granted as automatic
accouterments of any statistical analysis that deals with more than
one measure. In such a context, we easily forget that they were once
hailed as a breakthrough in research, as a new and exciting tool for
discovering underlying structure in tables of raw measures. We can
sense this excitement in reading early papers of the great American
biologist and statistician Raymond Pearl (see Pearl, 1905 and 1906,
and Pearl and Fuller, 1905). Pearl completed his doctorate at the
turn of the century and then proceeded, like a happy boy with a
gleaming new toy, to correlate everything in sight, from the lengths
of earth worms vs. the number of their body segments (where he
found no correlation and assumed that increasing length reflects
larger, rather than more, segments), to size of the human head vs.
intelligence (where he found a very small correlation, but attributed
it to the indirect effect of better nutrition).
Charles Spearman, an eminent psychologist and fine statistician
as well* began to study correlations between mental tests during
these heady times. If two mental tests are given to a large number
of people, Spearman noted, the correlation coefficient between
them is nearly always positive. Spearman pondered this result and
wondered what higher generality it implied. The positive correla-
tions clearly indicated that each test did not measure an indepen-
dent attribute of mental functioning. Some simpler structure lay
behind the pervasive positive correlations; but what structure?
Spearman imagined two alternadves. First, the posidve correlations
might reduce to a small set of independent attributes—the "facul-
ties" of the phrenologists and other schools of early psychology.
Perhaps the mind had separate "compartments" for arithmetic,
verbal, and spatial aptitudes, for example. Spearman called such
* Spearman took a special interest in problems of correlation and invented a mea-
sure that probably ranks second in use to Pearson's r as a measure of association
between two variables—the so-called Spearman's rank-correlation coefficient.