as to inborn traits, sex differences,
specialization of mental traits, speed of development, the respective
power of nature and of nurture. The various plans of promotion and
grouping of children found in different cities are in operation because
of certain beliefs concerning differences in general mental ability.
Coeducation is urged or deplored largely on the ground of belief in the
differing abilities of the sexes.

Exact knowledge of just what differences do exist between people and the
causes of these differences is important for two reasons. First, in
order that the most efficient measures may be taken for the education of
the individual, and second, in order that the race as a whole may be
made better. Education can only become efficient and economical when we
know which differences between people and which achievements of a given
person are due to training, and which are due more largely to original
equipment or maturity. It is a waste of time on the one hand for
education to concern itself with trying to make all children good
spellers--if spelling is a natural gift; and on the other hand, it is
lack of efficiency for schools to be largely neglecting the moral
development of the children, if morality is dependent primarily on
education. Exact knowledge, not opinions, along all these lines is
necessary if progress is to be made.

The principal causes for individual differences are sex, remote
ancestry, near ancestry, maturity, and training. The question to be
answered in the discussion of each of these causes is how important a
factor is it in the production of differences and just what differences
is it responsible for. That men differ from women has always been an
accepted fact, but exact knowledge of how much and how they differ has,
until recent years, been lacking. Recently quantitative measurement has
been made by a number of investigators. In making these investigations
two serious difficulties have to be met. First, that the tests measure
only the differences brought about by differences in sex, and not by any
other cause, such as family or training. This difficulty has been met by
taking people of all ages, from all sorts of families, with all kinds of
training, the constant factor being the difference in sex. The second
difficulty is that of finding groups in which the selection agencies
have been the same and equally operative. It would be obviously unfair
to compare college men and women, and expect