liabilities in the social order.
On the other hand, there is a large number above the level of
average intelligence. The importance of this group for human
progress can hardly be overestimated. As we have seen
in other connections, progress is contingent upon variation
from the "normal" or the accustomed, and such variation
from the normal is initiated in the majority of cases by members
of this comparatively small super-normal group. If
civilization is to advance it must capitalize its intelligence;
that is, educate up to the highest point of native ability. But
in any case, its chief guarantee of progress lies in the
comparatively small group in whom native ability is exceptionally
high. For it is among this group that original thinking,
invention, and discovery almost exclusively occur.

CAUSES OF INDIVIDUAL DIFFERENCES. Among the chief causes
of individual differences may, in general, be set down the following:
(1) Sex, (2) Race, (3) Near Ancestry or Family,
(4) Environment. The particular fund of human nature
which an individual displays, that is, his specific native
endowments, as they appear in practice, will be a resultant of
these various causes. In the study of each of these characteristics,
we should be able ideally to eliminate all the others
and to consider them each in isolation.

THE INFLUENCE OF SEX. In the case of sex, for example, we
should not confuse individual differences due to the fact of sex
with individual differences due to divergent training given to
each of the sexes. In scientific experiments to determine sex
differences in mental traits, there have been careful attempts
to eliminate everything but the factor of sex itself. Thus in
Karl Pearson's studies of fifty twin brothers and sisters, the
factors of ancestry and difference of training and age were
practically eliminated.

In so far as allowance can be made for other contributing
factors, studies of individual differences due to sex have
revealed, roughly speaking, the following results. There have
been, in the field of sensory discrimination and accuracy of
motor response, slight--and negligible--differences of responses
made by male and female. The subjects stated were,
in most cases, selected so far as possible from the same social
strata, social and intellectual interest, and background.[1]

[Footnote 1: As, for example, the members of the graduating and junior
classes of the co-educational college at the University of Chicag