to grow in mental health with the years and to overcome the handicaps
of a less-than-optimal upbringing. Then we would say that society as awhole is getting healthier and healthier as each new generation starts offbetter and as each individual, every day in every way , gets better andbetter. Note that under this optimistic set of assumptions, there wouldbe little evidence of age differences when cross-sectional comparisonswere made: The older individuals would have risen from a level of poormental health to good mental health, but they would not now differfrom young adults whose good mental health is the result of enlight -
ened child-rearing practices. Thus two major effects on well-being—
one linked to socialization, one to aging—could jointly give the appear -
ance of “no difference.”
Lest the reader be unduly encouraged by this hypothesis, note that
the opposite set of premises could yield the same conclusion. Perhapswhat we have seen in the past decades is the decline of civilization, abreakdown of discipline that leaves each successive generation withfewer and weaker inner resources. If at the same time aging has thesame deleterious effects on mental health as on physical health, then allof us are getting worse with each passing day . And yet, again, a compari-son of young with old people would show equal levels of mentalhealth—or, in this case, mental illness.
Alternative interpretations like these are the despair of social scien-
tists and have prompted many to abandon altogether the cross-sectionalstudy . These researchers prefer to study aging as an ongoing process inindividuals. They conduct longitudinal research.
LONGITUDINAL DESIGNS: TRACKING CHANGES
OVER TIME
Most of the problems associated with cross-sectional studies can be
solved by using the other major method of aging research, longitudinaldesigns. In these, a group of individuals is selected and measured at onetime, then followed and measured repeatedly over a period of years,perhaps for decades. In contrast to the age differences estimated bycross-sectional studies, longitudinal studies estimate age changes. Thatis, they can see directly how the individuals under study have changedand perhaps at what point the changes occur. (As we will see, this doesnot necessarily mean that the changes are caused by aging.)Growth or Decline in Personality 67