Age and Intelligence across the Adult Life Span

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Research on the relationships between aging and intelligence had its inception nearly 100 years ago in comparisons between adults and children (Kirkpatrick, 1903). The topic has captivated researchers in theoretical and clinical disciplines for over half a century (Jones & Conrad, 1933; Lorge, 1936; Miles & Miles, 1932; Willoughby, 1927). Whether intelligence declines with increasing age has long been the subject of research and debate by experts in the field (Baltes & Schaie, 1976; Botwinick, 1977; Horn & Donaldson, 1976), with both the research investigations and the controversies continuing to the present (Bengtson & Schaie, 1999; Birren, Schaie, Abeles, Gatz, & Salthouse, 1996; Craik & Salt-house, 2000; Kaufman, 2000a, 2001; Lawton & Salthouse, 1998; Lindenberger & Baltes, 1997; McArdle, Prescott, Hamagami, & Horn, 1998; Park & Schwarz, 2000). The nature of the complex relationship between aging and changes in intellectual functioning is of prime concern to clinicians who test clients across a wide age span, inasmuch as proper WAIS-III, KAIT, and WJ III interpretation demands understanding of normal, or expected, differential fluctuations in a person's ability spectrum from late adolescence and young adulthood to old age. Distinguishing between normal and pathological development is often the essence of competent diagnosis in clinical and neuropsychological assessment.

Probably the most comprehensive and cleverly conceived set of studies has been the life's work of K. Warner Schaie (e.g., 1958, 1983b, 1994) in collaboration with numerous colleagues (e.g., Hert-zog & Schaie, 1988; Schaie & Labouvie-Vief, 1974; Schaie & Strother, 1968; Schaie & Willis, 1993). His results have transformed the preconceptions of professionals throughout the world regarding the inevitability of declines in mental functioning along the path to old age. Although some of Schaie's findings are controversial, it is incontestable that his clever sequential combination of cross-sectional and longitudinal research designs has shown the importance of considering cohort (generational) effects when conducting research on aging. Further, Schaie's research program suggests that, when declines in intelligence do occur with age, they do so at far later ages than was formerly believed.

But Schaie consistently used the group-administered, speeded Primary Ability Tests (PMA; Thurstone & Thurstone, 1949), based on Thur-stone's theory of intelligence and normed only through age 18 years. As valuable as his findings are, they cannot replace research results based directly on the WAIS-III, its predecessors (Wechsler, 1939, 1955, 1981), and other intelligence tests for adults in helping clinicians understand the kinds of changes to anticipate during clinical, neuropsychological, or psychoeducational assessment. Evaluation of adolescents and adults depends on the WAIS-III or other individually administered clinical instruments as its primary or exclusive measure of intellectual functioning. Age changes on the PMA, Army Alpha (Yoakum & Yerkes, 1920), or other group instruments do not necessarily generalize to the profile changes to anticipate when testing the same person several times during his or her lifetime, or when comparing the subtest profiles of individuals or groups who differ in chronological age.

For these reasons, the studies conducted on the WAIS-III (Heaton, Manly, Taylor, & Tulsky, 2001; Kaufman, 2000a, 2001; Kaufman & Lichtenberger, 1999, pp. 187-200; Manly, Heaton, & Taylor, 2000), WAIS-R (Kaufman, 1990, pp. 212-222; Kaufman, Reynolds, & McLean, 1989), WAIS (Birren & Morrison, 1961; Botwin-ick, 1967, 1977), and Kaufman tests (Kaufman & Horn, 1996; Kaufman, Kaufman, Chen, & Kaufman, 1996; Wang & Kaufman, 1993) provide the most valuable research findings for the clinical interpretation of aging and intelligence. Especially valuable are data for the WAIS-III, based on both cross-sectional comparisons of education-adjusted IQs and longitudinal analyses of independent samples (Kaufman, 2001). Taken together, these recent analyses offer insight into aging and IQ for a contemporary sample that spans the broad 16-89-year age range. These WAIS-III analyses are discussed later in this chapter, and are integrated with the results of similar analyses conducted on the WAIS-R (Kaufman, 1990, pp. 212-222; Kaufman et al., 1989) and Kaufman tests (Kaufman & Horn, 1996; Kaufman et al., 1996; Wang & Kaufman, 1993), to gain insight into generational differences in the relationship of IQ to the aging process and into theoretical perspectives on these changes. The findings from Schaie's (1983b) landmark 21-year cohort-sequential Seattle longitudinal study, though based on the nonclinical PMA test, are also integrated into this discussion. However, to be consistent with the focus of this text, this chapter consistently emphasizes aging data obtained from studies of Wechsler's and Kaufman and Kaufman's individually administered intelligence tests for adults. In contrast to these tests, group-administered instruments like the PMA are subject to individual differences in test-taking behaviors, such as motivation level, attention span, and so forth; these variables are often important in testing elderly individuals. All data on tests like the WAIS-III and KAIT were obtained by well-trained psychologists who ensured the maintenance of rapport and motivation level throughout the testing session.

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