term. In fact, if everyone started out with the exactly the same capacity
for memory , it would be much easier to see the effects of age.
But people do not start off with the same abilities, just as they do
not start off with the same levels of Anxiety or Assertiveness or Open -
ness to Ideas. In early adulthood we find wide variation in all the per -
sonality traits of interest to us. We find the same range of differences
among old people. We know from the studies in Chapters 4 and 5 thatpeople on the average do not change much during adulthood; but so farwe have said very little about what individuals do. Ms. Smith may bedocile and traditional as a newlywed, but 30 years later she may havebecome assertive and unconventional. Mr. Jones may be interested inautomotive mechanics and sports during his 20s, but he may be de -
voted to Bible reading by the time he is 60. Then again, Ms. Smith may
still be docile and traditional and Mr. Jones still interested in sportswhen both are past 70.
This is a question of stability or change not of a group of people but
of individuals. The study of changes in individuals is considerably morecomplicated and correspondingly more interesting than the study ofchanges in gr oups. If the group as a whole changes, we can be sure that
at least some of the individuals have changed, but the converse is notnecessarily true. The average level of the group may not be altered eventhough all the individuals change—if, for example, there are as manywho increase as decrease. Finding out that groups are stable, then, doesnot rule out the possibility that individuals change; and, in fact, a num-ber of fascinating possibilities for individual development are consistentwith the findings of group stability .
There is a major methodological difference between the study of
groups and the study of individuals. To make inferences about meanchanges with age, we need only compare groups of different ages (pro -
vided we can be relatively sure they do not differ in any other character -
istics). Any two groups of people will do, so the cross-sectional design
is a convenient, if not foolproof, method of obtaining a quick answer.We need not wait for the passage of time.
But a study of individual changes requires that we examine the
same person at two or more ages: Longitudinal studies with repeatedmeasurements of the same subjects are essential. As a result, the evi -
dence available on which to base conclusions is much slimmer and of
much more recent vintage. Only in the past two decades have morethan a handful of studies been published in which longitudinal data on100 PERSONALITY IN ADULTHOOD