Throughout the history of the SLS, an effort now extending over more than 50 years, our focus has been on five major questions, which we have attempted to ask with greater clarity and increasingly more sophisticated methodology at each successive stage of the study. These are as follows.

Objective 1

Does intelligence change uniformly through adulthood or are there different life course ability patterns? Our studies have shown that there is no uniform pattern of age-related changes across all intellectual abilities, and that studies of an overall index of intellectual ability (IQ) therefore do not suffice to monitor age changes and age differences in intellectual functioning for either individuals or groups. Our data do lend some support to the notion that fluid abilities tend to decline earlier than crystallized abilities. There are, however, important Ability by Age and Ability by Cohort interactions that complicate matters. For example, gender difference trends suggest that women decline earlier on the active abilities and men on the passive abilities. Moreover, whereas fluid abilities begin to decline earlier, crystallized abilities appear to show steeper decrement once the late 70s are reached.

Although cohort-related differences in the rate and magnitude of age changes in intelligence remained fairly linear for cohorts that entered old age during the first three cycles of our study, they have since shown substantial shifts. For example, rates of decremental age change have abated, and at the same time there appear to be negative cohort trends as we begin to study members of the baby-boom generation. It is beginning to appear as if patterns of socialization unique to a given sex role in a specific historical period may be a major determinant of the pattern of change in abilities. More fine-grained analyses suggest, moreover, that there may be substantial gender differences as well as differential changes for those who decline and those who remain stable when age changes are decomposed into accuracy and speed.

With multiple markers of abilities we have conducted cross-sectional analyses of ability structure over a wide age range. Thus far it has been possible to demonstrate configural but not metric factor invariance across wide age/cohort ranges. Finally, we have examined the relationship of everyday tasks to the framework of practical intelligence and perceptions of competence in everyday situations facing older persons.

Objective 2

At what age is there a reliably detectable decrement in ability, and what is its magnitude? We have generally shown that reliably replicable average age decrements in most psychometric abilities do not occur prior to age 60, even though there is earlier slowing in response speed, and although some individuals may show earlier decline because of individual genetic factors, pathology, or socio-cultural disadvantages. However, reliable average decrement can be found for all psychometric abilities by age 74. Nevertheless, more detailed analyses of individual differences in intellectual change demonstrate that even at age 81 fewer than half of all observed individuals have shown reliable decremental change over the preceding 7 years. Average decrement below age 60 amounts to less than 0.2 of a standard deviation, while by age 81 average decrement rises to approximately 1 population standard deviation for most variables.

The data from the SLS attain increasing importance in providing a normative base to determine at what ages declines reach practically significant levels of importance for public policy issues related to mandatory retirement, age discrimination in employment, or the determination of the population proportions that can live independently in the community. These bases will shift over time, as we have demonstrated in the SLS that both level of performance and rate of decline show significant age by cohort interactions.

Objective 3

What are the patterns of generational differences, and what is their magnitude? Results from the SLS have conclusively demonstrated the prevalence of substantial generational (cohort) differences in psychometric abilities. These cohort trends differ in magnitude and direction by ability and can therefore not be determined from composite IQ indices. As a consequence of these findings, it was concluded that cross-sectional studies used to model age change will overestimate age changes prior to the 60s for those variables that show negative cohort gradients and underestimate age changes for those variables with positive cohort gradients. Our studies of generational shifts in abilities have in the past been conducted with random samples from arbitrarily defined birth cohorts. As a supplement and an even more powerful demonstration, we have recently conducted family studies that compare performance levels for individuals and their adult children. We have also recruited siblings of our participants to obtain data that allow extending the knowledge base in the developmental behavior genetics of cognition to the adult level, by providing data on parent -offspring and sibling correlations in adulthood. These data suggest that there has been a slowing in the rate of average decline over successive generations.

Objective 4

What accounts for individual differences in age-related change in adulthood? The most powerful and unique contribution of a longitudinal study of adult development arises from the fact that only longitudinal data permit the investigation of individual differences in antecedent variables that lead to early decrement for some persons and maintenance of high levels of functioning for others into very advanced age. A number of factors that account for these individual differences have been implicated, some of which have been shown to be amenable to experimental intervention. The variables that have been implicated in reducing risk of cognitive decline in old age have included (a) absence of cardiovascular and other chronic diseases; (b) a favorable environment mediated by high socioeconomic status; (c) involvement in a complex and intellectually stimulating environment; (d) flexible personality style at midlife; (e) high cognitive status of spouse; and (f) maintenance of high levels of perceptual processing speed.

Objective 5

Can intellectual decline with increasing age be reversed by educational intervention? Because longitudinal studies permit tracking stability or decline on an individual level, it has also been feasible to carry out interventions designed to remediate known intellectual decline as well as to reduce cohort differences in individuals who have remained stable in their own performance over time but who have become disadvantaged when compared with younger peers. Findings from the cognitive training studies conducted with our longitudinal subjects (under the primary direction of Sherry L. Willis) suggest that observed decline in many community- dwelling older people might well be a function of disuse and is clearly reversible for many. Indeed, cognitive training resulted in approximately two thirds of the experimental subjects showing significant improvement; and about 40% of those who had declined significantly over 14 years were returned to their predecline level. In addition we were able to show that we did not simply “train to the test,” but rather trained at the ability (latent construct) level, and that the training did not disturb the ability structure.

The dialectical process between data collection and model building that has been part of the SLS has made possible substantial methodological advances in the design and analysis of studies of human development and aging. In addition the study has provided baselines for clinical assessment and has made contributions relevant to education, basic instruction in psychological aging, and a variety of public policy issues.​​​​​