he injurious effects upon mental ability of malnutrition, decayed
teeth, obstructed breathing, reduced sleep, bad ventilation,
insufficient exercise, etc., we are met by endless assertion painfully
unsupported by demonstrated fact. We have, indeed, very little exact
knowledge regarding the mental effects of any of the factors just
mentioned. When standardized mental tests have come into more general
use, such influences will be easy to detect wherever they are really
present.

Again, the most important question of heredity is that regarding the
inheritance of intelligence; but this is a problem which cannot be
attacked at all without some accurate means of identifying the thing
which is the object of study. Without the use of scales for measuring
intelligence we can give no better answer as to the essential difference
between a genius and a fool than is to be found in legend and fiction.

Applying this to school children, it means that without such tests we
cannot know to what extent a child's mental performances are determined
by environment and to what extent by heredity. Is the place of the
so-called lower classes in the social and industrial scale the result of
their inferior native endowment, or is their apparent inferiority merely
a result of their inferior home and school training? Is genius more
common among children of the educated classes than among the children of
the ignorant and poor? Are the inferior races really inferior, or are
they merely unfortunate in their lack of opportunity to learn?

Only intelligence tests can answer these questions and grade the raw
material with which education works. Without them we can never
distinguish the results of our educational efforts with a given child
from the influence of the child's original endowment. Such tests would
have told us, for example, whether the much-discussed "wonder children,"
such as the Sidis and Wiener boys and the Stoner girl, owe their
precocious intellectual prowess to superior training (as their parents
believe) or to superior native ability. The supposed effects upon mental
development of new methods of mind training, which are exploited so
confidently from time to time (e.g., the Montessori method and the
various systems of sensory and motor training for the feeble-minded),
will have to be checked up by the same kind of scientific measurement.

In all these fields intelligence tests are certain to play an
ever-increasing role. With the exc