Who was the investigator and what was the aim of the study? In the 1950s, little information was available concerning longitudinal changes in adults’ intellectual abilities. What there was showed a developmental pattern of relative stability or slight decline, quite different from the picture of substantial across-the-board decline obtained in cross-sectional studies. To provide a more thorough picture of intellectual change, K. Warner Schaie began the Seattle Longitudinal Study in 1956.

How did the investigator measure the topic of interest? Schaie used standardized tests of primary mental abilities to assess a wide range of abilities
such as logical reasoning and spatial ability.

Who were the participants in the study? Over the course of the study, more than 5,000 individuals have been tested at eight testing cycles (1956, 1963, 1970, 1977, 1984, 1991, 1998, and 2005). The participants were representative of the upper 75% of the socioeconomic spectrum and were recruited through a very large health maintenance organization in Seattle. Extensions of the study include longitudinal data on second – generation family members and on the grandchildren of some of the original participants.

What was the design of the study? To provide a thorough view of intellectual change

over time, Schaie invented a new type of research design—the sequential design. Participants were tested every 7 years. Like most longitudinal studies, Schaie’s sequential study encountered selectivity effects—that is, people who return over the years for retesting tend to do better initially than those who fail to return (in other words, those who don’t perform well initially tend to drop out of the study). However, an advantage of Schaie’s sequential design is that by bringing in new groups of participants, he was able to estimate the importance of selection effects, a major improvement over previous research.

Figure 1.9 Longitudinal changes in intellectual functions from age 25 to 88.

Source: From “Intellectual Development Across Adulthood" by K. Warner Schaie and Faika A. K. Zanjani, in Handbook of Adult Development and Learning, ed. by C. Hoare, p. 102. Copyright © 2006 by Oxford University Press.


25 32 39 46 53 60 67 74 81 88

Age (years)





Were there ethical concerns with the study? The most serious issue in any study in which participants are followed over time is confidentiality. Because people’s names must be retained for future contact, the researchers were very careful about keeping personal information secure.

What were the results? Among the many important findings from the study are differential changes in abilities over time and cohort effects. As you can see in Figure 1.9, scores on tests of primary mental abilities improve gradually until the late 30s or early 40s. Small declines begin in the 50s, increase as people age into their 60s, and become increasingly large in the 70s (Schaie & Zanjani, 2006).

Cohort differences were also found. Figure 1.10 shows that on some skills, such as inductive reasoning ability, but not others, more recently born younger and middle-aged cohorts
performed better than cohorts born earlier. An example of the latter is that older cohorts outperformed younger ones on number skills (Schaie & Zanjani,

2006) . These cohort effects probably reflect differences in educational experiences; younger groups’ education emphasized figuring things out on one’s own, whereas older groups’ education emphasized rote learning. Additionally, older groups did not have calculators or computers, so they had to do mathematical problems by hand.

Schaie uncovered many individual differences as well; some people showed developmental patterns closely approximating the overall trends, but others showed unusual patterns. For example, some individuals showed steady declines in most abilities beginning in their 40s and 50s, others showed declines in some abilities but not others, but some people showed little change
in most abilities over a 14-year period. Such individual variation in developmental patterns means that average trends, like those depicted in the figures, must be interpreted cautiously; they reflect group averages and do not represent the patterns shown by each person in the group.

Another key finding is that how intellectual abilities are organized in people does not change over time (Schaie et al.,

1998) . This finding is important because it means that the tests, which presuppose a particular organizational structure of intellectual abilities, can be used across different ages. Additionally, Schaie (1994) identified several variables that appear to reduce the risk of cognitive decline in old age:

• Absence of cardiovascular and other chronic diseases

• Living in favorable environmental conditions (such as good housing)

Figure 1.10 Cohort differences in intellectual functions from birth cohorts between 1889 and 1973.

Source: From “Intellectual Development Across Adulthood" by K. Warner Schaie and Faika A. K. Zanjani, in Handbook of Adult Development and Learning, ed. by C. Hoare, p. 106. Copyright © 2006 by Oxford University Press.




Verbal meaning Spatial orientation Inductive reasoning















Word fluency Intellectual ability






1889 1896 1903 1907 1910 1917 1924 1931 1938


1945 1952 1959 1966




Cohort (year of birth)


Studying Adult Development and Aging 29



• Remaining cognitively active through reading and lifelong learning

• Having a flexible personality style in middle age

• Being married to a person with high cognitive status

• Being satisfied with one’s life achievements in middle age

What did the investigator conclude? Three points are clear. First, intellectual development during adulthood is marked by a gradual leveling off of gains, followed by a period of relative stability, and then a time of gradual decline in most abilities. Second, these trends vary from one cohort to another. Third, individual patterns of change vary considerably from person to person.

Overall, Schaie’s findings indicate that intellectual development in adulthood is influenced by a wide variety of health, environmental, personality, and relationship factors. By attending to these influences throughout adulthood, we can at least stack the deck in favor of maintaining good intellectual functioning in late life.

What converging evidence would strengthen these conclusions? Although Schaie’s study is one of the most comprehensive ever conducted, it is limited. Studying people who live in different locations around the world would provide evidence as to whether the results are limited geographically. Additional cross-cultural evidence comparing people with different economic backgrounds and differing access to health care would also provide insight into the effects of these variables on intellectual development.