The Seattle Longitudinal Study that involves extensive evaluation of intellectual abilities during adulthood was initiated by K. Warner Schaie (1994, 1996, 2005, 2010, 2011a, b, 2012). Participants have been assessed in seven-year intervals since 1956: 1963, 1970, 1977, 1984, 1991, 1998, 2005, and 2012. Five hundred individuals initially were tested in 1956. New waves of participants are added periodically. The main focus in the Seattle Longitudinal Study has been on individual change and stability in intelligence, and the study is considered to be one of the most thorough examinations of how people develop and change as they go through adulthood.
The main mental abilities tested are:
• Verbal comprehension (ability to understand ideas expressed in words)
• Verbal memory (ability to encode and recall meaningful language units, such as a list of words)
• Number (ability to perform simple mathematical computations such as addition, subtraction, and multiplication)
• Spatial orientation (ability to visualize and mentally rotate stimuli in two- and three-dimensional space)
• Inductive reasoning (ability to recognize and understand patterns and relationships in a problem and use this understanding to solve other instances of the problem)
• Perceptual speed (ability to quickly and accurately make simple discriminations in visual stimuli)
As shown in Figure 15.4, the highest level of functioning for four of the six intellectual abilities occurred in the middle adulthood years (Schaie, 2012). For both women and men, peak performance on verbal ability, verbal memory, inductive reasoning, and spatial orientation was attained in middle age. For only two of the six abilities—number and perceptual speed—were there declines in middle age. Perceptual speed showed the earliest decline, actually beginning in early adulthood. Interestingly, in terms of John Horn’s ideas that were discussed earlier, for the participants in the Seattle Longitudinal Study, middle age was a time of peak performance for some aspects of both crystallized intelligence (verbal ability) and fluid intelligence (spatial orientation and inductive reasoning).
When Schaie (1994) assessed intellectual abilities both cross-sectionally and longitudinally, he found decline more likely in the cross-sectional than in the longitudinal assessments. For example, as shown in Figure 15.5, when assessed cross-sectionally, inductive reasoning showed a consistent decline during middle adulthood. In contrast, when assessed longitudinally, inductive reasoning increased until toward the end of middle adulthood when it began to show a slight decline. In Schaie’s (2008, 2009, 2010, 2011a, b, 2012) view, it is in middle adulthood, not early adulthood, that people reach a peak in many intellectual skills.
In further analysis, Schaie (2007) examined generational differences in parents and their children over a seven-year time frame from 60 to 67 years of age. That is, parents were assessed when they were 60 to 67 years of age; and when their children reached 60 to 67 years of age, they also were assessed. Higher levels of cognitive functioning occurred for the second generation in inductive reasoning, verbal memory, and spatial orientation, whereas the first generation scored higher on numeric ability. Noteworthy was the finding that the parent generation showed cognitive decline from 60 to 67 years of age, but their offspring showed stability or modest increases in cognitive functioning across the same age range.
Such differences across generations involve cohort effects. In a recent analysis, Schaie (2011b) concluded that the advances in cognitive functioning in middle age that have occurred in recent decades are likely due to factors such as educational attainment, occupational structures (increases of workers in professional occupations and work complexity), health care and lifestyles, immigration, and social interventions in poverty. The impressive gains in cognitive functioning in recent cohorts have been documented more clearly for fluid intelligence than for crystallized intelligence (Schaie, 2011b).
The results from Schaie’s study that have been described so far focus on average cognitive stability or change for all participants across the middle adulthood years. Schaie and Sherry Willis (Schaie, 2005; Willis & Schaie, 2005) examined individual differences for the participants in the Seattle study and found substantial individual variations. They classified participants as “decliners,” “stable,” or “gainers” for three categories—number ability, delayed recall (a verbal memory task), and word fluency—from 46 to 60 years of age. The largest percentage of decline (31 percent) or gain (16 percent) occurred for delayed recall; the largest percentage with stable scores (79 percent) occurred for numerical ability. Word fluency declined for 20 percent of the individuals from 46 to 60 years of age.
Might the individual variations in cognitive trajectories in midlife be linked to cognitive impairment in late adulthood? In Willis and Schaie’s analysis, cognitively normal and impaired older adults did not differ on measures of vocabulary, spatial orientation, and numerical ability in middle adulthood. However, declines in memory (immediate recall and delayed recall), word fluency, and perceptual speed in middle adulthood were linked to neuropsychologists’ ratings of the individuals’ cognitive impairment in late adulthood.
Some researchers disagree with Schaie that middle adulthood is the time when the level of functioning in a number of cognitive domains is maintained or even increases (Finch, 2009). For example, Timothy Salthouse (2009, 2012) recently has argued that cross-sectional research on aging and cognitive functioning should not be dismissed and that this research indicates reasoning, memory, spatial visualization, and processing speed begin declining in early adulthood and show further decline in the fifties. Salthouse (2009, 2012) does agree that cognitive functioning involving accumulated knowledge, such as vocabulary and general information, does not show early age-related decline but rather continues to increase at least until 60 years of age.
Salthouse (2009, 2012) has emphasized that a lower level of cognitive functioning in early and middle adulthood is likely due to age-related neurobiological decline. Cross-sectional studies have shown that the following neurobiological factors decline during the twenties and thirties: regional brain volume, cortical thickness, synaptic density, some aspects of myelination, the functioning of some aspects of neurotransmitters such as dopamine and serotonin, blood flow in the cerebral cortex, and the accumulation of tangles in neurons (Del Tredici & Braak, 2008; Erixon-Lindroth & others, 2005; Finch, 2009; Hsu & others, 2008; Pieperhoff & others, 2008; Salat & others, 2004).
Schaie (2009, 2010, 2011a, b, 2012) continues to emphasize that longitudinal studies hold the key to determining age-related changes in cognitive functioning and that middle age is the time during which many cognitive skills actually peak. In the next decade, expanding research on age-related neurobiological changes and their possible links to cognitive skills should further refine our knowledge about age-related cognitive functioning in the adult years (Fletcher & Rapp, 2013; Merrill & others, 2012; Schlee & others, 2012).