declines in strength, memory , and hearing are equally well documented.
Most personality traits, on the other hand, do not show a marked pat -
tern of rising or falling as people age.
In all these examples, whether of change or stability , we are com -
paring the average levels of groups of individuals. There are good rea -
sons for concentrating on the average (or mean) level of a trait when we
are interested in the effects of age. We know that most traits show awide distribution, with some individuals high, some low , and many in -
termediate in the degree to which they manifest the trait. We can rarely
make assertions of the form “All 80-year olds are higher in Xthan all
70-year-olds.” If a psychologist is careless enough to say that old peoplehave poor memories, he or she is sure to hear dozens of stories aboutold people with good memories or young people with worse ones.When so pressed, the psychologist is likely to rephrase the statement tosay that, on average , old people have poorer memories. “Of course,” he
or she might continue, “some people have better memories, someworse; they may have been born that way , or they may have developedskills through practice or education. Medications or illnesses or psycho-logical states may interfere with memory performance. But I’m not in-terested in any of those things. I’m interested in the effects of age onmemory . So my strategy is to measure a large group of people, some old,some young. Some of the old ones will be naturally bright, some natu-rally not so bright; some will be well-educated, some poorly educated.When I look at the group average, all those differences will cancel eachother out. The only thing all these people have in common, and thusthe only thing that will be characteristic of the group average, is the factthat they are older. The same logic applies to the young group. When Icompare the average older subject with the average younger subject,any differences must be due to age.”
We have already challenged the assumption that the only respect in
which two such groups differed systematically was age, pointing outthat they might well also differ in average education, health status, orother features. But if we can assume that those other characteristicshave been controlled through a careful selection of subjects, the logic ofcross-sectional comparisons is sound. Individual differences in memoryare attributable either to age or to some other factors. The interest instudies of this sort is always in age as a source of individual differences;other differences are something of a nuisance, a source of possible con -
fusion. In statistical models, they contribute to what is called an errorThe Course of Personality Development 99