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, for 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.[19] 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.

[19] See p. 52 _ff._ for method used to avoid accidental selection of
subjects for the Stanford investigation.

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.

[Illustration: FIG. 2. DISTRIBUTION OF I Q'S OF 905 UNSELECTED
CHILDREN. 5-14 YEARS OF AGE]

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 (approximate