alike in the traits
tested as other brothers and sisters. Though the difficulty of
discounting the effect of training in all these studies has been great,
yet in every case the investigators have taken pains to do so. The fact
that the investigations along such different lines all bear out the same
conclusion, namely, that intellectual differences are largely due to
differences in family inheritance, weighs heavily in favor of its being
a correct one.

The fifth factor that might account for individual differences is
environment. By environment we mean any influence brought to bear on the
individual. The same difficulty has been met in attempting to measure
the effect of environment that was met in trying to measure the effect
of inner nature--namely, that of testing one without interference from
the other. The attempts to measure accurately the effect of any one
element in the environment have not been successful. No adequate way of
avoiding the complications involved by different natures has been found.
One of the greatest errors in the method of working with this problem
has been found just here. It has been customary when the effect of a
certain element in the environment is to be ascertained to investigate
people who have been subject to that training or who are in the process
of training, thus ignoring the selective influence of the factor itself
in original nature. For instance, to study the value of high school
training we compare those in training with those who have never had any;
if the question is the value of manual training or Latin, again the
comparison is made between those who have had it and those who haven't.
To find out the influence of squalor and misery, people living in the
slums are compared with those from a better district. In each case the
fact is ignored that the original natures of the two groups examined are
different before the influence of the element in question was brought to
bear. Why do some children go to high school and others not? Why do some
choose classical courses and some manual training courses? Why are some
people found in the slums for generations? The answer in each case is
the same--the original natures are different. It isn't the slums make
the people nearly so often as it is the people make the slums. It isn't
training in Latin that makes the more capable man, but the more
intellectual students, because of tradition and possibly enjoyment of
language study, choose the L