ly) between 93 and 108.

Three important conclusions are justified by the above facts:--

1. Since the frequency of the various grades of intelligence decreases
_gradually_ and at no point abruptly on each side of the median, it is
evident that there is no definite dividing line between normality and
feeble-mindedness, or between normality and genius. Psychologically, the
mentally defective child does not belong to a distinct type, nor does
the genius. There is no line of demarcation between either of these
extremes and the so-called "normal" child. The number of mentally
defective individuals in a population will depend upon the standard
arbitrarily set up as to what constitutes mental deficiency. Similarly
for genius. It is exactly as we should undertake to classify all people
into the three groups: abnormally tall, normally tall, and abnormally
short.[20]

[20] See Chapter VI for discussion of the significance of various I Q's.

2. The common opinion that extreme deviations below the median are more
frequent than extreme deviations above the median seems to have no
foundation in fact. Among unselected school children, at least, for
every child of any given degree of deficiency there is another child as
far above the average I Q as the former is below. We have shown
elsewhere the serious consequences of neglect of this fact.[21]

[21] See p. 12 _ff._

3. The traditional view that variability in mental traits becomes more
marked during adolescence is here contradicted, as far as intelligence
is concerned, for the distribution of I Q's is practically the same at
each age from 5 to 14. For example, 6-year-olds differ from one another
fully as much as do 14-year-olds.

THE VALIDITY OF THE INTELLIGENCE QUOTIENT. The facts presented above
argue strongly for the validity of the I Q as an expression of a child's
intelligence status. This follows necessarily from the similar nature of
the distributions at the various ages. The inference is that a child's
I Q, as measured by this scale, remains relatively constant. Re-tests of
the same children at intervals of two to five years support the
inference. Children of superior intelligence do not seem to deteriorate
as they get older, nor dull children to develop average intelligence.
Knowing a child's I Q, we can predict with a fair degree of accuracy the
course of his later development.

The mental age of a subject is meaningless if considered apart from
chronological age.