withstands all the shocks of life and of aging. Catastrophic events—
illnesses, wars, great losses—may alter personality , as may effectivetherapeutic intervention. But according to stability theory , the naturalcourse of personality in adulthood is unchanging.
INVESTIGATING THE COURSE OF PERSONALITY TRAITS
Cross-sectional studies of personality traits tell us nothing at all abouttheir natural histories. For that, we must trace the levels of traitsthrough the lives of aging individuals. If we are interested in formulat -
ing general principles and not simply writing individual biographies, we
must have data from reasonably large samples of individuals. And if wehave enough information to draw valid conclusions, we probably havetoo much to be able to understand it by simply inspecting it with theunaided mind. We have to resort to statistical summaries of the data,and of these the most important for the present purpose is the correla-tion coefficient.
The correlation coefficient, which expresses the direction and
strength of the association between two variables as a number between−1.0 and +1.0, is fundamental to trait psychology , being used to quan-
tify reliability and validity and forming the basic inputs for factor analy-sis. It also provides the basic metric of personality stability: A stabilitycoefficient is simply the correlation of a measure administered at onetime with the same measure administered at a later time. (Note that thisis also the operational definition of a retest reliability coefficient, al -
though this latter term is generally used when the retest interval is a
matter of days or weeks rather than years. We will return later to con -
ceptual differences between the two coefficients and some implications
for estimating true stability .)
Although they are familiar, correlation coefficients can be difficult
to interpret. What, for example, is a high orgood orimpressive orstrong
correlation? What is moderate? What is modest or weak? These disarm -
ingly simple questions are the source of profound and sometimes vitri -
olic controversy in psychology . Being human, researchers sometimes
tend to consider the correlations that support their point of view“strong,” and the correlations that support other positions “weak.”Even the most unbiased judgment, however, must somehow take intoaccount a range of considerations, including the expected magnitude of106 PERSONALITY IN ADULTHOOD