t the child has a "mental age" of 9 years, which in this case is
normal (our child being 9 years of age). If he goes only as far as
normal 8-year-old children ordinarily go, we say that his "mental age"
is 8 years. In like manner, a mentally defective child of 9 years may
have a "mental age" of only 4 years, or a young genius of 9 years may
have a mental age of 12 or 13 years.

SPECIAL CHARACTERISTICS OF THE BINET-SIMON METHOD. Psychologists had
experimented with intelligence tests for at least twenty years before
the Binet scale made its appearance. The question naturally suggests
itself why Binet should have been successful in a field where previous
efforts had been for the most part futile. The answer to this question
is found in three essential differences between Binet's method and those
formerly employed.

1. _The use of age standards._ Binet was the first to utilize the idea
of age standards, or norms, in the measurement of intelligence. It will
be understood, of course, that Binet did not set out to invent tests of
10-year intelligence, 6-year intelligence, etc. Instead, as already
explained, he began with a series of tests ranging from very easy to
very difficult, and by trying these tests on children of different ages
and noting the percentages of successes in the various years, he was
able to locate them (approximately) in the years where they belonged.

This plan has the great advantage of giving us standards which are
easily grasped. To say, for illustration, that a given subject has a
grade of intelligence equal to that of the average child of 8 years is a
statement whose general import does not need to be explained. Previous
investigators had worked with subjects the degree of whose intelligence
was unknown, and with tests the difficulty of which was equally unknown.
An immense amount of ingenuity was spent in devising tests which were
used in such a way as to preclude any very meaningful interpretation of
the responses.

The Binet method enables us to characterize the intelligence of a child
in a far more definite way than had hitherto been possible. Current
descriptive terms like "bright," "moderately bright," "dull," "very
dull," "feeble-minded," etc., have had no universally accepted meaning.
A child who is designated by one person as "moderately bright" may be
called "very bright" by another person. The degree of intelligence which
one calls "moderate dullness," another may call "extreme dullness,