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.