It should be noted that this chapter might have taken a very different form if the editors had elected to commission it to an historian or a sociologist. In the first case the emphasis might have been upon the history of aging as a topic worthy of interest to psychologists and begun with an analysis of G. Stanley Halls seminal opus (1922) as well as focusing upon the meaning of historical events that may have impacted psychological inquiry (cf. Cole, 1993; Cole et al., 2008 ). In the second case, the theoretical focus might well have begun with the work of Mannheim (1952; Pilcher, 1994) on the problem of generations and then shifted to the more recent work of the Rileys (1994) on generations moving as convoys through time. Instead I decided that as a chapter fora handbook that emphasized psychological changeover adulthood as its prime dependent variables, it would be more appropriate to have a psychologist address the impact of historical changes in our society that have had important impact on many of the substantive variables typically studied by psychologists interested in the human aging process (cf. James, 2005; Schaie et al., 2005 ). Much of the interest by psychologists in historical influences on behavior has been stimulated by the literature on generational or cohort differences in both level and developmental trajectories of many psychological constructs. Particularly in the field of cognitive psychology it was found early on that the disassociation between the findings from studies of cross-sectional age differences and longitudinal age changes could best be understood by considering what was called the age-cohort-period model in sociology Ryder, 1965 ), and the age-cohort-time model in psychology ( Schaie, 1965 ).
R in a developmental or age-related study of inter-individual differences or intra-individual change is characterized by the form R f ( A , C , P where A is the chronological age, C is the birth cohort, and Pis the calendar time at which the observation is conducted. These components are confounded, much as temperature, pressure, and volume in the physical