In statistics, natural phenomena are modeled by what is called the Normal Distribution95
conceived by a mathematician named Carl Friedrich Gauss95. This is a continuous law of
probability which depends on its expectation95 (the mean) and its variance95 (the span of the
distribution values) which also amounts to the standard deviation95 squared. The Standard
Deviation (SD) value provides the extent of the range to consider. It’s a statistical quantity which
corresponds, to points on the curve, showing a tangible difference from the average population of
the set, in short, the variability in the population. The higher the SD, the more removed the set is
from the mean.
The scores are distributed along the “Gaussian Curve95” which concentrates most points in the
middle while diminishing rapidly at both ends. In that way, this curve resembles a bell.
This schematic96 represents the I.Q. scores’ dispersion in the general population. It essentially
follows a “Normal Distribution”, although some researchers have noticed more people on the far
right end97 than expected. The scores, grouped in ranges, are arranged horizontally while the
percentage of the population scoring at a particular level is viewed vertically (surface under the
curve).
On the 100-point mark, the distribution on the left mirrors that on the right side: for each
increment, just as many people are present above the mean (100) than under it. To emphasize
that effect, I have employed the same labels for the Gifted as for the Disabled. Psychologists are
in the habit of using slightly different designations for the intellectually challenged ranges (with
“exceptionally” being customarily replaced with “Severe”276 for example).
Throughout this book I use the Wechsler’s scale22 (the most widely used for I.Q. scores). On that
particular scale the SD is equal to 15 points. Other scales have different SDs, the standard
deviation of the Cattell scale is 24 for example.
This statistical number matters because a distinct emotional and behavioral pattern, as well as an
innate type of brain functioning and layout, is associated to each specific 15-point wide I.Q.
range.
So, unique personality traits are linked to each I.Q. range.