The Distribution Of Intelligence
The question as to the manner in which intelligence is distributed is one of
great practical as well as theoretical importance. One of the most vital questions
which can be asked by any nation of any age is the following: "How high is the
average level of intelligence among our people, and how frequent are the
various grades of ability above and below the average?" With the
development of standardized tests we are approaching, fo
the first time
in history, a possible answer to this question.
Most of the earlier Binet studies, however, have thrown little light on
the distribution of intelligence because of their failure to avoid the
influence of accidental selection in choosing subjects for testing. The
method of securing subjects for the Stanford revision makes our results
on this point especially interesting. It is believed that the
subjects used for this investigation were as nearly representative of
average American-born children as it is possible to secure.
The intelligence quotients for these 1000 unselected children were
calculated, and their distribution was plotted for the ages separately.
The distribution was found fairly symmetrical at each age from 5 to 14.
At 15 the range is on either side of 90 as a median, and at 16 on either
side of 80 as a median. That the 15- and 16-year-olds test low is due to
the fact that these children are left-over retardates and are below
average in intelligence.
The I Q's were then grouped in ranges of ten. In the middle group were
thrown those from 96 to 105; the ascending groups including in order the
I Q's from 106 to 115, 116 to 125, etc.; correspondingly with the
descending groups. Figure 2 shows the distribution found by this
grouping for the 905 children of ages 5 to 14 combined. The subjects
above 14 are not included in this curve because they are left-overs and
not representative of their ages.
The distribution for the ages combined is seen to be remarkably
symmetrical. The symmetry for the separate ages was hardly less marked,
considering that only 80 to 120 children were tested at each age. In
fact, the range, including the middle 50 per cent of I Q's, was found
practically constant from 5 to 14 years. The tendency is for the middle
50 per cent to fall (approximately) 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
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.
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.