One of the most important research projects on adult intellectual development is the longitudinal study being conducted by K. Warner Schaie and his col­leagues in Seattle, Washington, which began in 1956 as Schaie’s dissertation (Schaie, 1996, 2005). This study has not only uncovered most of what we know about how primary mental abilities change across adulthood, but it also has been the basis for creating new research methodologies such as the sequential designs discussed in Chapter 1. Over the course of the study, more than 5,000 people have been tested over eight testing cycles (1956, 1963, 1970, 1977, 1984, 1991, 1998, 2005). All the participants were recruited through a very large health maintenance organization in Seattle that is representative of the upper 75% of the socioeconomic spectrum. Like most longitudinal studies, though, Schaie’s project has encountered selectivity effects; that is, people who return over the years for retesting tend to do better than those who fail to return. However, an

Cognitive abilities are assessed across the life span using standardized tests.

advantage of Schaie’s sequential design is that by bringing in new groups of participants, he has been able to estimate the importance of selection effects, a major improvement over previous research.

Schaie (1996, 2005) proposes a hierarchical relationship in intellectual abilities. Information­processing abilities such as perceptual speed and verbal memory are considered the most basic and are tied to neuropsychological functioning (Li et al., 2004; Salthouse, 1991). Mental abilities such as reasoning and numbering are products of acquired information. Finally, all mental abilities underlie all meaningful activities of a person’s daily life (Schaie & Willis, 1998; Willis, 1996a). Thus the developmental trends uncovered in the Seattle Longitudinal Study provide important insights into the course of intellectual changes that ultimately affect people’s work and daily living routines.

Schaie (2005) summarizes the findings as follows. Analysis of the data collected through the sixth time of measurement shows that people tend to improve on the primary abilities tested until their late 30s or early 40s. Scores then tend to stabilize until people reach their mid-50s or early 60s. But by their late 60s, people tend to show consistent declines in each testing. Although some people begin to show declines in their mid-50s, these decrements tend to be small until the mid-70s. Considering the mod­est improvements that most people make between young adulthood and middle age, scores are sig­nificantly lower than they were in young adulthood (roughly age 25) only by the mid-70s. These changes are depicted in Figure 7.2.

Do the general trends observed reflect global or specific changes in intelligence? That is, to what extent do people decline on all the primary abilities tested or only some of them? As you can see in Figure 7.3, even though by age 60 nearly everyone shows decline on one ability, very few people show decline on four or five abilities (Schaie, 1996). Even by age 88, only an extremely small number of people had declined significantly on all five abilities.

What happens when we consider both pragmatic types of abilities and mechanic types of abilities
related to Baltes’s two-component theory of intel­ligence? Abilities that are typical of mechanics such as reasoning, verbal memory, spatial orientation, and perceptual speed typically show a pattern of decline during adulthood, with some acceleration in very old age. These abilities show a steady pattern of decline. However, more pragmatic abilities, such as verbal meaning or ability and numerical ability, tend to remain stable or even increase up to the 60s and 70s. There are little or no age decrements before the age of 74. They start to show decline only in very old age. It appears that most of the loss occurs in highly chal­lenging, complex, and stressful situations that require activating cognitive reserves (Baltes et al., 2006).

These patterns may reflect a strategy of optimiza­tion of cognitive functioning in late life by selectively maintaining some abilities and not others (Baltes et al., 2006). Even so, the evidence is clear that signif­icant decrements in both types of intellectual abilities occur by the time people are in their 80s. In addi­tion, although the distinction between pragmatic and mechanic types of abilities still exists into very old age (over 80), they both show precipitous decline, just at different levels of functioning (Ghisletta & Lindenberger, 2003; Lindenberger & Baltes, 1997; Lindenberger & Reischies, 1999).