Developmental psychology.pdf

388 Individual Differences
Figure 14.18
Age and Mental Ability: Cross-
Sectional Results. According to
these data, obtained from individuals
of different ages studied at the same
time, mental abilities begin to decline
early in the third decade of life
(Schaie & Strother, 1968).
55
I 50
cc
45
yr Numb }Г
Re isoning'
i
Composite IQ
\
Verbal m janing
—i —
40
23 33 43
Mean age
53 63
more pronounced for some functions than for others, but again the general pattern was
confirmed (Schaie, 1958; Figure 14.18).
Gradually, certain limitations in the cross-sectional method became apparent.
First, there is a loss of subjects at the older age levels. Second, because of steady cultural
improvements, younger subjects have received a better education than older ones.
The differences in performance perhaps were due to improvements in younger persons,
rather than a decline in older ones. Especially in the United States, the average number
of years of formal education has increased steadily, and in general the quality of formal
education has improved. Young adults might perform better than older people simply
because they have received a superior education. In a rapidly changing society, it is
difficult or impossible to obtain matched samples at the different age levels.
Longitudinal Method Another research approach is more time-consuming. It is
called the longitudinal method, for the same subjects are tested and retested at several
different age levels. The idea is to study them regularly as they grow older. This aim
makes these investigations relatively rare, for several years must pass before significant
effects can be observed.
Longitudinal investigations begun 25 to 50 years ago are yielding significant
results today, and they have indicated a pattern quite different from that found in the
cross-sectional approach. Follow-up studies of 768 Termites in adulthood have shown
that they, and their bright but generally less gifted spouses, experience continuous
growth in a number of areas of abstract thinking, even up to age 50 (Bayley & Oden,
1955).
Longitudinal studies in England with 141 college graduates yielded similar
results. This research involved a wide variety of tasks, including vocabulary, spatial
relations, numerical problems, analogies, and other forms of reasoning, and it was concluded
that gifted persons give no evidence of decline in test performance at least until
they are in their late forties (Nisbet, 1957).
But longitudinal studies are not without defect either, as a closer look shows.
Ideally, the only difference between a longitudinal subject at a younger and older age
occurs from the general impact ofsging per se, but subjects at the older ages are benefiting
from our expanding fund of knowledge and vastly improved methods of disseminating
it. Their increased intellectual capacity may be attributable to the greater
availability of knowledge within the culture. In short, cultures develop just as people
do, and cultural advances can influence the results of longitudinal studies, as well.
Dealing with Cultural Change One method of dealing with this problem involves
the use of control subjects. These people are the same age as were the longitudinal
subjects at the time of their first testing. The difference in performance.'if any, between
the longitudinal subjects at their first testing and the control subjects at this same age
represents the amount of cultural change. When this amount is subtracted from adult
performance at any point, it serves as a correction for cultural improvements.