But even if two groups did differ in means by as much as one
standard deviation, such a difference pales in comparison with the
within-group variability of five or six standard deviations. Thus a
person in the “inferior” group who scores one standard deviation
above its mean would still be at the mean in the “superior” group,
that is, better than 50 percent of the superior group; and a person in
the inferior group scoring three standard deviations above its mean
will be two standard deviations above the mean in the superior
group—better than 98 percent of the superior group.
The point is this: Group membership per se is a poor indicator
of a person’s standing, if only because the error term is so large—
equivalent to one standard deviation. Group membership cannot
substitute for individual assessment. Furthermore, characterizing all
members of a group by the group’s mean is indefensible, if not mali-
cious. Thus the psychology of individual differences shows why
each person has to be treated as an individual, not as a member of
a group. This conclusion alone should have dissipated the contro-
versies surrounding diversity.
Another important finding of the psychology of individual dif-
ferences is that many human traits do not correlate highly with one
another. (If they did, then only one trait would be needed to de-
scribe humans. This is essentially what is wrong with the “IQ” con-
cept.) When two traits do not correlate highly, a person’s standing
on one trait does not allow accurate prediction or estimation of that
person’s standing on the other trait because the standard error of
estimate will be large.
The number of human traits that do not correlate highly with
one another is large (Dawis, in press) and growing larger as research
on personality proceeds. Relying on the information provided by
one or even a few traits will produce a very restricted and biased
picture of a person. The psychology of individual differences shows
why a person must be assessed on many traits, not just on a single
trait, no matter how important it may be.
Thus the psychology of individual differences exposes the
fallacy of stereotypes, whether based on group membership or on
PERSON-ENVIRONMENT-CORRESPONDENCE THEORY 457