Early Development Of The Concept Of Cohort
Cohort approaches to the study of aging and social change have historically focused on the relationship between theoretical and empirical uses of the concept. During the 1970s, researchers continued to struggle with an operationalization of cohort study that did not invite ambiguities in the interpretation of research findings. For example, when comparing people of different ages at one point in time, cohort (based on year of birth) and age (the difference between current year and year of birth) cannot be distinguished. When extending the information across time— such as comparing people of the same or different ages at different points in time—cohort, age, and year (as the measure of historical time) are intertwined. Age equals current year (history) minus birth year (cohort). Irving Rosow argued in 1978 that focusing attention on methodological solutions to the confounding of age, period, and cohort resulted in the neglect of conceptual and empirical development of the concept of cohort. His question, "What actually is a cohort?" pointed to uncertainties in the definition of cohort and the difficulties inherent in measuring and modeling the complex processes and historical circumstances that cohort members collectively experience. Nevertheless, Rosow conceded that until the mechanisms that translated cohort characteristics into different attitudes and behaviors could be specified in models, cohort, measured simply as shared birth year, remained a useful proxy measure.
Norval Glenn, an early proponent of cohort analysis, conducted much research on social change in attitudes and behaviors. He also warned of the dangers of complete reliance on technical expertise at the cost of careful theoretical development in cohort analyses (see Glenn and Grimes, 1968; Glenn and Zody, 1970; Glenn, 1974)—sophisticated statistical techniques alone would not solve the linear restrictions of age, period, and cohort effects in quantitative analysis. Glenn maintained that while Ryder's classic essay developing the connection between cohort succession and social change was extremely influential, it contained few specifics on the techniques of research.
In addition to this emphasis on cohort analyses, a second branch of cohort literature attempted to ground cohort research more firmly in a consideration of social structural constraint and facilitation. Age stratification theory, an attempt to link a structural perspective to the study of individual behavior, emphasized the manner in which the age structure of societal roles organized members into hierarchies (see Riley, 1973; Riley, Johnson, and Foner, 1972; Riley, 1987). Matilda White Riley combined a cross-sectional approach to age group differences with a longitudinal perspective that considers what happens to particular cohorts as they move through time.
From age stratification theory grew the life-course perspective, which is now the dominant approach in social gerontology. The life-course perspective draws on diverse intellectual sources to study differences in aging across cohorts by capturing individual biography within the context of social structure and historical circumstance. Glen Elder was one of the first to conduct micro-level research using longitudinal data on children's lives to systematically study change in families and children over time. Since then, the use of the life-course perspective to connect trajectories of individual lives to larger societal changes has been fueled by the development of new analytic techniques. These techniques allow researchers to represent underlying dynamics of aging, to assess the multilevel contextual impact of environments on individual level outcomes, and to model the individual-level processes that correlate individual outcomes with individual characteristics over time (see Tuma and Hannan, 1987; Kreft and de Leeuw, 1998).