The Behavioral Psychometric Approach

Extant psychometric data on aging and intelligence indicate three major phenomena. First, intellectual aging is multifaceted. Multifactorial models of intelligence (e.g., the Gf-Gc theory; Cattell, 1971; Horn, 1982; the dual-process model of life span intellectual development, Baltes, Staudinger, & Lindenberger, 1999) suggest that abilities in the fluid-mechanics (Gf) domain, which reflect an individual's capacity for problem solving, information organization, and concentration, are more biology based. In contrast, abilities in the crystallized-pragmatic (Gc) domain reflect the acquisition and use of culture-based information.

Verbal ability

Number ability Verbal memory

Reasoning

Spatial orientation Perceptual speed

Verbal ability

Number ability Verbal memory

Reasoning

Spatial orientation Perceptual speed

20 30 40 50 60 70 80 90 Age in Years

Figure 1. Cross-sectional age gradients in six primary mental abilities (N = 1,628). Abilities were assessed with three to four different tests and are scaled in a T-score metric.

Source: Data based on Schaie & Willis (1993); figure adapted from Lindenberger & Baltes (1994).

Babcock, Laguna, & Roesch, 1997; Balinsky, 1941; Baltes, Cornelius, Spiro, Nesselroade, & Willis, 1980; Lindenberger & Baltes, 1997; Lienert & Crott, 1964). Similar patterns have also been found in longitudinal studies, although the trends of dedifferentiation are not as strong as in the cross-sectional findings (McHugh & Owens, 1954; Schaie et al., 1998). Furthermore, ability dedifferentiation generalizes beyond the intellectual domain. A series of recent studies using simple measures of sensory acuity (Baltes & Lindenberger, 1997; Lindenberger & Baltes, 1994; Salthouse, Hancock, Meinz, & Hambrick, 1996), contrast sensitivity, and muscle strength (Anstey, Lord, & Williams, 1997) report an increase in the sensory-cognitive correlation with advancing age in age-heterogeneous samples. The strengthening of the sensory-cognitive link in old age has been interpreted as an indication of general neurological decline affecting both domains of functioning (e.g., Baltes & Lindenberger, 1997). The nature of the sensory-cognitive link is, however, still under debate. Afew recent studies took the experimental, instead of the correlational, methodology to study the sensory-cognitive link. Findings from these studies suggest that as people grow older they seem to allocate an increasing amount of resources to tasks that require maintaining balance in an upright posture or walking (e.g., Lindenberger, Marsiske, & Baltes, 2000; K. Z. H. Li, Lindenberger, Freund, & Baltes, 2001).

Figure 1 shows that cross-sectional age gradients of primary mental abilities (Thurstone & Thurstone, 1949) in the fluid-mechanics domain (i.e., verbal memory, reasoning, spatial orientation, and perceptual speed) decline linearly beginning in the 40s. However, abilities in the crystallized-pragmatic domain (i.e., verbal and numeric abilities) remain stable up to the 60s or 70s (Schaie & Willis, 1993).

Second, cross-sectional age differences are generally more pronounced than longitudinal age changes. Whereas modest cross-sectional negative age differences are found by the 40s for some abilities and by the 60s for most abilities, moderate longitudinal negative age changes in most abilities are usually not evident until the mid-70s or early 80s (Hayslip, 1994; Schaie, 1983, 1996). Discrepancies between cross-sectional and longitudinal age gradients are due to cohort effects (Schaie, 1965), practice effects, and selective attrition in longitudinal studies (Lindenberger & Baltes, 1994). After controlling for cohort and historical time effects, discrepancies between cross-sectional age differences and longitudinal age changes are reduced (Schaie, 1996). In addition, studies extending to very old age have provided opportunities for observing age differences and age changes in the 90s and beyond (e.g., the Berlin Aging Study, Baltes & Mayer, 1998; the Georgia Centenarian Study, Poon, Sweaney, Clayton, & Merriam, 1992; and the Kungsholmen Project, Small & Backman, 1997).

Third, aging contracts the factor space of intellectual abilities. Ample cross-sectional data show that correlations among subscales are generally larger in older samples, indicating an increasing degree of ability dedifferentiation (e.g.,

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