The origins of the Seattle Longitudinal Study can be traced back to work I did as an undergraduate at the University of California at Berkeley while doing directed studies under the supervision of Professor Read D. Tuddenham. He had introduced me, in an inspiring tests and measurements course, to the basic concepts of factor analysis and the writings of L. L. Thurstone (1938). I soon decided that, although the work of Wechsler (1939) on adult intelligence might be of great concern to clinical psychologists, the Wechsler Bellevue test and its derivatives, because of their factorial complexity, did not have the most desirable attributes for the exploration of developmental issues. I also learned that the more explicitly defined Primary Mental Abilities (PMA; Thurstone, 1938) had not been studied beyond adolescence and concluded that such an exploration might possibly be a fruitful topic for systematic research.
In an initial study I explored whether the factorial independence of the five abilities measured in the most advanced form of the PMA test (PMA 11-14; Thurstone & Thurstone, 1949) would be retained in adulthood. I then proceeded to ask whether adults would function at the same level as did adolescents. I also raised the question whether there might be ability-related differentials in adult PMA performance, and whether such differences in pattern would remain if the PMA test were administered under non-speeded conditions (Schaie, Rosenthal, & Perlman, 1953).
My appetite having been whetted by some provocative results from the early pilot study, I continued to explore a variety of corollaries of intelligence in adulthood during my graduate work at the University of Washington (Schaie, Baltes, & Strother, 1964; Schaie & Strother, 1968a, 1968d; Strother, Schaie, & Horst, 1957). As part of this work I also developed a new factored test of behavioral rigidity (TBR; Schaie, 1955, 1960, Schaie & Parham, 1975). These activities culminated in a doctoral dissertation designed to replicate the earlier work on differential ability patterns across a wider portion of the adult life span as well as to look at the effect of rigidity–flexibility on the maintenance or decline of intellectual functioning (Schaie, 1958a, 1958b, 1958c, 1959a, 1959b; 1962). This dissertation, of course, became the base for the subsequent longitudinal-sequential studies.
The search for an appropriate population frame for the base study was guided by the consideration that what was needed was a subject pool with reasonably-well known demographic characteristics, one that had been established for reasons other than research on cognitive behavior. That is, if possible the initial selection of volunteer participants for the study should be designed to minimize selection in terms of the potential participants’ interest in, concern with, or performance level on the dependent variables of interest. When plans for the study matured, my mentor, Professor Charles R. Strother, was by fortunate coincidence president of the lay board of the Group Health Cooperative of Puget Sound, one of the first broadly based health maintenance organizations (HMOs) in the United States. An arrangement was worked out with the administration of the health plan that permitted me to recruit potential research participants who had been selected by a random draw from the age/sex stratification of plan members aged 22 or older. The plan’s medical director made the appeal for participation as part of a membership satisfaction survey, the administration and analysis of which was my quid pro quo for gaining access to this population.
Results of the 1956 cross-sectional base study did not support a causal model involving differential patterns of intellectual performance across age for flexible and rigid individuals. The study did demonstrate significant relationships between flexibility–rigidity and intelligence at all ages. More important, however, it provided a sound demonstration of differential patterns of intellectual functioning across age and, by virtue of its design, serendipitously provided the basis for the following longitudinal-sequential studies.
Perhaps the most immediate stimulation leading to the conversion of a one-time cross-sectional study into a series of longitudinal studies was my reading of reports on longitudinal studies of individuals reaching middle adulthood, such as the articles by Bayley and Oden (1955); Jarvik, Kallman, and Falek (1962); and Owens (1953, 1959). Taken together, findings from these studies suggested to me that there was strong evidence that most intellectual abilities were maintained at least into midlife and that some abilities remained stable beyond that period. These findings clearly contrasted with the results of the earlier cross-sectional literature, including my own dissertation data. What seemed to be called for was the follow-up of a broad cross-sectional panel, such as the one I had been able to examine, by means of a short-term longitudinal inquiry. Intensive discussions of such a project with Charles Strother were followed by a grant application to the National Institutes of Health, which funded the study in time to collect the first set of follow-up data in the summer of 1963.
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 in order to provide the necessary controls for examining retest effects and to begin addressing the possibility that socio cultural change affects intellectual performance. The latter concern was stimulated by the thoughtful admonitions previously voiced by Raymond Kuhlen (1940, 1963). Our new sample extended over the original age range (22 to 70) plus an additional 7-year interval to match the age range now reached 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, 1967). 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, 1970; Schaie & Strother, 1968b, 1968c).
It soon became evident that the conclusions based on data covering a single 7-year interval required further replication, if only because two occasions of measurement permit the examination of cross-sectional sequences, but not of longitudinal sequences), the latter requiring a minimum of three measurement occasions. Only longitudinal sequences allow designs that permit contrasting age and cohort effects. Hence, plans were made for a third data collection, which was conducted in 1970. In that cycle as many persons as possible examined on the first two test occasions were retested, and a third random sample was drawn from the residual members of the base population (Schaie, 1979; Schaie, Labouvie, & Buech, 1973; Schaie & Labouvie-Vief, 1974; Schaie & Parham, 1977).
The results from the third data collection seemed rather definitive in replicating the short-term longitudinal findings, but a number of questions remained. 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 over as long as 21 years. A fourth data collection was therefore conducted in 1977, again retesting the previous samples and adding a new random sample, this time from an expanded population frame (Schaie & Hertzog, 1983, 1986). Continuous funding also made it possible to address a number of bothersome collateral questions. These included analyses of the consequences of shifting from a sampling-without- replacement to a sampling-with-replacement paradigm (Gribbin, Schaie, & Stone, 1976); an analysis of the effects of monetary incentives on participant characteristics (Gribbin & Schaie, 1976); an examination of the aging of tests (Gribbin & Schaie, 1977); and the beginning of causal analyses of health and environmental factors upon change or maintenance of adult intellectual performance (Gribbin, Schaie, & Parham, 1980; Hertzog, Schaie, & Gribbin, 1978).
My early introduction to the issues of cohort differences and secular trends led to serious questions as to what the meaning of these effects might be beyond their role as control variables or as bothersome design confounds. I therefore began to pay increased attention to the impact of social structures and micro-environments on cognitive change (see Schaie, 1974; Schaie & Gribbin, 1975; Schaie & O’Hanlon, 1990). This work was influenced early on by the writing of Matilda Riley (Riley, 1985; Riley, Johnson, & Foner, 1972) and later on by the work of Carmi Schooler (1972, 1987), as well as many other sociologists, anthropologists, and epidemiologists who have contributed to the Penn State social structure conference series (see Bengtson, Schaie, & Burton, 1995; Kertzer & Schaie, 1989; Rodin, Schooler, & Schaie, 1990, Schaie & Achenbaum, 1993; Schaie, Blazer, & House, 1992; Schaie & Hendricks, 2000; Schaie, Leventhal, & Willis, in press; Schaie & Schooler, 1989, 1998; Smyer, Schaie, & Kapp, 1996; Willis, Schaie, & Hayward, 1997).
Until the fourth (1977) cycle of the SLS we followed the then conventional wisdom of assessing each primary ability with the observable marker variable deemed to be the most reliable and valid measure of the latent construct to be estimated. With the widespread introduction of modern methods of confirmatory (restricted) factor analysis it became obvious that we needed to extend our concern with changes in level of intellectual functioning in adulthood to the assessment of structural relationships within the ability domain. This concern argued for collecting further data with a much expanded battery in which each ability would be multiply marked (Maitland, Intrieri, Schaie, & Willis, 2000; Schaie, Dutta, & Willis, 1991; Schaie, Maitland, Willis, & Intrieri, 1998; Schaie, Willis, Hertzog, & Schulenberg, 1987; Schaie, Willis, Jay, & Chipuer, 1989).
The fifth (1984) SLS cycle also marks the assumption of a major role in this project by Sherry L. Willis, who brought to this project her skills in designing and implementing cognitive training paradigms. Thus a major part of the fifth cycle was the implementation of a cognitive training study with our long-term participants aged 64 years or older, designed to assess whether cognitive training in the elderly serves to remediate cognitive decrement or increase levels of skill beyond those attained at earlier ages (Schaie & Willis, 1986b; Willis & Schaie, 1986b, 1986c, 1988, 1994b).
The database available through the fifth cycle also made it possible to update the normative data on age changes and cohort differences (Schaie, 1990a, 1990b, 1990c; Schaie & Willis, 1993) and to apply sequential analysis designs controlled for the effects of experimental mortality and practice (Cooney, Schaie, & Willis, 1988; Schaie, 1988d). Finally, this cycle saw the introduction of measures of practical intelligence (Willis & Schaie, 1986b), analyses of marital assortativity using data on married couples followed over as long as 21 years (Gruber & Schaie, 1987; Gruber-Baldini, Schaie, & Willis, 1995), and the application of event history methods to hazard analysis of cognitive change with age (Schaie, 1989a).
The sixth (1989-91) SLS cycle included a set of four related studies. First, with the collaboration of Robert Plomin, a noted developmental behavior geneticist, we took advantage of the longitudinal database to collect data to implement a study of cognitive family resemblance in adulthood. We did this by recruiting the participation of a large number of adult offspring and siblings of our longitudinal panel members (Schaie, Plomin, Willis, Gruber-Baldini, & Dutta, 1992; Schaie, Plomin, Willis, Gruber-Baldini, Dutta, & Bayen, 1993; Schaie & Willis, 1995; Schaie & Zuo, 2001). Second, we abstracted the health histories on our panel members and have conducted more detailed investigations of the relationship between health and maintenance of intellectual functioning (Gruber-Baldini, 1991a; Gruber-Baldini & Schaie, 1990; Gruber-Baldini, Willis, & Schaie, 1989). Third, we conducted a 7-year follow-up of the cognitive training study and have replicated the study with a more recent cohort of older persons (Willis & Schaie, 1992, 1994b). Fourth, we were able to conduct longitudinal analyses of cognitive ability structures and further update our normative data base with the collection of a sixth (1991) wave, using the standard approach of retesting and drawing a sixth new independent sample (Schaie, 1993, 1994a).
In between study waves, in 1993, we conducted a mail survey of health behaviors for those persons who had been in the 1989 family study and the 1991 longitudinal and sixth-wave studies.
In our seventh wave (1998), results from many aspects of which are reported in this revised volume, again consist of several related studies. First, we conducted a seven-year follow-up of the family study including the recruitment of relatives of panel members first entering the SLS in 1991. Second, we conducted follow-up studies of the first two training interventions to determine maintenance of training over seven and fourteen years; we also conducted training with a third sample of persons over 64 years of age. Third, we did another follow-up of our longitudinal panels and added a seventh (1998) wave. Fourth, we administered a neuropsychological battery and collected blood for ApoE genotyping from panel members who were aged 60 or older. We strengthened our personality assessment by means of a mail administration of the NEO Personality Inventory (Costa & McCrae, 1995) to all panel members who were successfully tested during the 1996-98 data collection. And finally we have begun recruiting members of a 3rd generation; i.e., persons who have at least one parent and one grand parent participating in the SLS. For most recent reports on the SLS study findings consult Schaie (2005) and Schaie and Willis (2010).
The Seattle Longitudinal Study had its administrative home at various universities with which the principal investigator was associated during his career that included the University of Nebraska, West Virginia University, University of Southern California and the Pennsylvania State University. In August 2008 it returned to its original home at the University of Washington. It is now administratively housed in the Department of Psychiatry and Behavioral Sciences of the UW School of Medicine.