s below the median are vastly 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 roughly another
child as far above the average as the former is below." Lewis M. Terman,
_The Measurement of Intelligence_, pp. 66-67.]

It would be well to extend our view by measuring a whole population with
one of the standard tests. If the intelligence of a thousand children
picked at random from the population be measured, it will prove (as
outlined in Chapter III) that some of them are feeble-minded, some are
precocious or highly intelligent; and that there is every possible
degree of intelligence between the two extremes. If a great number of
children, all 10 years old, were tested for intelligence, it would
reveal a few absolute idiots whose intelligence was no more than that of
the ordinary infant, a few more who were as bright as the ordinary
kindergarten child, and so up to the great bulk of normal 10-year-olds,
and farther to a few prize eugenic specimens who had as much
intelligence as the average college freshman. In other words, this trait
of general intelligence would be found distributed through the
population in accordance with that same curve of chance, which was
discussed and illustrated when we were talking about the differences
between individuals.

Now what has become of the unit character, feeble-mindedness? How can
one speak of a unit character, when the "unit" has an infinite number of
values? Is a _continuous quantity_ a _unit_?

If intelligence is due to the inheritance of a vast, but indeterminate,
number of factors of various kinds, each of which is independent,
knowledge of heredity would lead one to expect that some children would
get more of these factors than others and that, broadly speaking, no two
would get the same number. All degrees of intelligence between the idiot
and the genius would thus exist; and yet we can not doubt that a few of
these factors are more important than the others, and the presence of
even one or two of them may markedly affect the level of intelligence.

It may make the matter clearer if we return for a moment to the
physical. Height, bodily stature, offers a very good analogy for the
case we have just been discussing, because it is obvious that it must
depend on a large number of different factors, a man's size being due to
the sum t