Intelligence testing during this century has had wide practical applications in educational, clinical, employment, and forensic contexts. Intelligence tests, such as the Wechsler Adult Intelligence Scale, have also been used extensively in research concerned with individual and group differences on a host of demographic and biological variables. The correlations of IQ scores with ethnicity, socioeconomic status, occupation, and certain other demographic variables are associated with level of education and cultural factors.

Declines in scores on intelligence tests after middle adulthood have been found in both cross-sectional and longitudinal studies, but the decline is steeper in the former than the latter type of study. The rate of age-related declines on intelligence tests varies significantly with the particular cognitive abilities measured by different subtests. The classic aging pattern is that of a greater age-related decline on performance tests than on verbal tests. Measures of fluid abilities also reveal greater age-related declines than measures of crystallized abilities. A rapid decline in scores on intelligence tests just prior to death—the so-called terminal drop—has also been found in a number of studies.

Piaget's four-stage theory of cognitive growth, culminating in the stage of formal operations at around 15 years of age, has had an important influence on developmental psychology. However, certain psychologists have maintained that cognitive abilities expand into dialectical thinking or postformal thought during adulthood.

Although some researchers view memory as consisting of a unitary process, most research on memory has been based on a three- or four-stage model: sensory memory (iconic or echoic); primary (short-term) memory, or STM; secondary (long-term) memory, or LTM; and tertiary (very long-term) memory, or VLTM. The rate of age-related declines in sensory, primary, and tertiary memory appears to be minimal, but the decline is quite pronounced on measures of secondary memory. The degree of age decrement in secondary memory, which has been documented by various methodologies, is affected by illness, persistence in scholarly activities, and psychological factors such as stress and motivation. The scores of many older adults on tests of memory and other cognitive abilities can be improved by special training, but it is not clear whether inherent abilities or merely test-taking skills are improved by such training, Educational attainment is inversely related to chronological age, a circumstance that may account, at least in part, for age declines on tests of cognitive abilities.

Like intelligence and memory, problem-solving ability and creativity vary with age. The efficiency of problem solving in old age is influenced by memory, motivation, cautiousness, rigidity, and other personality variables. The peak of creative productivity occurs at different ages for different fields of endeavor, ranging from early adulthood through late middle age. Research on creativity has been hampered by the difficulty of defining criteria of creative performance and of differentiating between originality, utility, and productivity.

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