The Seattle Longitudinal Study (SLS; Hertzog, 2010; Schaie, 1996a, b, 2000, 2005a) began as Schaie’s doctoral dissertation at the University of Washington (Seattle, WA) in 1956. In an effort to resolve the discrepancies between cross-sectional and longitudinal findings in the study of adult intellectual development, Schaie designed a follow-up study, put into the field in 1963, that provided some answers, but also raised enough methodological and substantive questions to demand a continuing program of studies that is still in progress and has included eight major and several collateral data collections as well as three cognitive training studies. An important aspect of the study has been the investigation of cohort differences and their implications for the study of adult cognition. In the more recent phases of the study, a number of contextual, health, and personality variables have been identified that offer explanations for differential change and that provide a basis for possible interventions. Within the context of our monitoring of individual change, the authors therefore designed cognitive interventions that have been successful in reversing carefully documented declines in ability performance and in improving the cognitive functions of older persons who have remained stable. Many reasonable arguments have been made for the proposition that motivational and other personality variables might have greater potency in predicting adjustment and competence in midlife than does intelligence, but the empirical evidence for this proposition is less than convincing. Questions such as who should be retired for cause (read incompetence) in the absence of mandatory retirement at relatively early ages; whether there is sufficient remaining competence for independent living; or whether persons can continue to conserve and dispose of their property all involve the assessment of intellectual functioning (see Schaie, 1988a). We need to examine at what age developmental peaks do occur and assess generational differences as well as within-generation age changes. Most important, we must determine the reasons why some individuals show intellectual decline in early adulthood, while others maintain or increase their level of intellectual functioning well into advanced old age. Initial studies in 1949 (PMA 11–14; L. L. Thurstone & T. G. Thurstone, 1949) and 1953 (Schaie, Rosenthal, & Perlman, 1953), explored whether the factorial independence of the five abilities measured in the most advanced form of the PMA test would be retained in adulthood. In a 1963 follow-up study, in addition to tracking down and retesting as many of the individuals studied in 1956 as possible, we decided to draw a new random sample from the original population frame to provide the necessary controls for examining retest effects and began addressing the possibility that socio-cultural variation affects intellectual performance. The latter concern was stimulated by the thoughtful admonitions previously voiced by Raymond Kuhlen (1963). Our new sample therefore was extended over the original age range (22–70 years) plus an additional 7-year interval to match the age range now attained by the original sample. The second cross-sectional study essentially replicated the findings of the base study. The short-term longitudinal study, however, disclosed substantially different information about peak levels and rate of decline. Publication of findings was therefore delayed until a theoretical model could be built that accounted for the discrepancy between the longitudinal and cross-sectional data (Schaie, 1965, 1977, 2007). These analyses suggested that comparisons of age group means needed to be conducted for the repeatedly measured samples as well as for successive independent samples drawn from the same cohort. Results were reported that called attention to substantial cohort differences and that questioned the universality and significance of intellectual decrement with advancing age in community-dwelling persons (Nesselroade, Baltes, & Schaie, 1972; Schaie & Strother, 1968). The results from the third data collection seemed rather definitive in replicating the short-term longitudinal findings, but discrepancies between findings in the repeated-measurement and independent-sampling studies suggested the need for a replication of the 14-year longitudinal sequences, and it further seemed useful to follow the original sample for as long as 21 years, so a fourth data collection was conducted in 1977, again retesting the previous samples and adding a new random sample, this time from an expanded population frame (Schaie & Hertzog, 1983), adding collateral questions of the effects of monetary incentives on participant characteristics (Gribbin & Schaie, 1976); an examination of the aging of tests; and the beginning of causal analyses of health and environmental factors on change or maintenance of adult intellectual performance (Gribbin, Schaie, & Parham, 1980; Hertzog, Schaie, & Gribbin, 1978).
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