ess which we
will call A, whereas those of a more advanced civilization employ other
mental processes, in addition to A, say B, C, D, or E, each individual
using them in different degrees for the performance of one and the same
test. Finally, there is in all likelihood a third kind of variability,
whose origin is ultimately environmental, which is manifested by
extremes of nervous instability. Probably the exceptionally defective
and the exceptional genius are more common among civilized than among
primitive peoples.

Similar features undoubtedly meet us in the study of sexual differences.
The average results of various tests of mental ability applied to men
and women are not, on the whole, very different for the two sexes, but
the men always show considerably greater individual variation than the
women. And here, at all events, the relation between the frequency of
mental deficiency and genius in the two sexes is unquestionable. Our
asylums contain a considerably greater number of males than of females,
as a compensation for which genius is decidedly less frequent in females
than in males.

7. Individual Differences[64]

The life of a man is a double series--a series of effects produced in
him by the rest of the world, and a series of effects produced in that
world by him. A man's make-up or nature equals his tendencies to be
influenced in certain ways by the world and to react in certain ways to
it.

If we could thus adequately describe each of a million human beings--if,
for each one, we could prophesy just what the response would be to every
possible situation of life--the million men would be found to differ
widely. Probably no two out of the million would be so alike in mental
nature as to be indistinguishable by one who knew their entire natures.
Each has an individuality which marks him off from other men. We may
study a human being in respect to his common humanity, or in respect to
his individuality. In other words, we may study the features of
intellect and character which are common to all men, to man as a
species; or we may study the differences in intellect and character
which distinguish individual men.

Individuals are commonly considered as differing in respect to such
traits either quantitatively or qualitatively, either in degree or in
kind. A quantitative difference exists when the individuals have
different amounts of the same trait. Thus, "John is more attentive to
his teacher than James i