Abstract
It is reiterated. that the study of averages
makes the assumption that each age group represents
what the preceding age group will become anc , further
that a study of this nature reveals directly, nothing
regarding the individual difference in rate of growth i or individual variations of all kind that inay arise,
except in exceptional cases. The limitations thus
imposed upon the study of mental growth changes are , therefore obvious. But for the study of the main problem
here, the method of group testing is not entirely
without justification. Neither is it without value.
So much emphasis is laid on the imperfections of the
method that it is forgotten how few ,good tests are
available even for the individual testing of older
subjects. What it is desired to know here is the general
movement of the average curve from age to age.
It cannot be claimed that the choice of subjects here
employed facilitated the discovery of the upper limit
of the grovth of intelligence but at least with additional
true information, some idea of the age of mental
arrest for average subjects has been deduced.
The conclusions from the study of
GEORGE: WATSON'S BOYS' COLLEGE results are that a slowing down of average mental growth is apparent
after 14 years and that since selection operates
from 15 years onwards, there is no sign Of a counter
balance in increased average score from then onwards
(This result applies to both tests.) They assumption
then is, that after 15 years the slowing down will
continue and absolute zero average growth be apparent
by 16 years. The same conclusions can be derived
from the results of the first test applied. to EDINBURGH
LADIES' COLLEGE pupils, and under more favourable
circumstances would have been directly derivable
from the Second test.
The results obtained from both tests applied
to JAMES CLARK SCHOOL indicate a slightly lower
point of arrest but the conclusions are handicapped
by selective forces operating at both ends of the
school so that it is difficult to be definite in
fixating the age of mental arrest . Presumably shortly
after 15 years is the best point to suggest.
When due regard of the superior social
status of the GEORGE WATSON'S BOYS' COLLEGE and
EDINBURGH LADIES' COLLEGE subjects is taken and also
of the average social status of the JAMES CLARK
subject it is inevitable that adherence to 15 years
rather than 16 years should follow. The conclusion
that the upper limit of the growth of average intelligence
is ranched at or about 15 years receives particular
confirmation from the fact that DUNFERMLINE PHYSICAL TRAINING COLLEGE subjects in the first year
of testing did not reach and HERIOT WATT COLLEGE
subjects in the second year of testing did not exceed
the 16 year old level. Of all adult subjects they
most nearly approach average mentality. The adult
results in conclusion with. EDINBURGH PROVINCIAL
TRAINING COLLEGE indicate only a 17 year old level
for the subjects and they are undoubtedly of superior intelligence on the average.
Regarding the tendency of the general curve
to bend noticeably at 18 years and the tendency of
the adult average curve to approximate to the 18 year
old level only a few words need be said. The fact
that the subjects are generally of superior intelligence
beyond 18 years seem to indicate that the bend
of the curve is not lacking in significance and that
it is a suggestion of an average upper limit of the
growth of superior intelligence occurring at 18 year
Perhaps the desirability of finding such a result is
a. bias. The curve form may only be true to chance;
but it in thought not. The 13 year old school
children are rigorously selected.and also the post
graduate subjects. The addition of lower subjects
at the adult years is slight. Undoubtedly the
chief reason why the suggestion cannot be pushed
farther is, that an inadequate number of adult subjects have been suggested.
The outcome of the investigation is a
refutation of the finding of the American Army
testing:, that on the average mental growth ceases
after 13 -i years. The indications of these experiments
are that on the average, mental growth con-. tinues at a very slow rate from 12 to 15 years, and
that after 15 years no appreciable improvement can
be observed. The conclusion, are not contradictory
to the findings or opinions of the bast investigator$
in the realm of mental testing.
