In the present chapter, we proceed from the premise that the study of ontogenesis requires a methodology that is inherently focused on the study of intra-individual change and inter-individual differences in intra-individual change (Bakes, P. B., Reese, & Nesselroade, 1977). We argue, however, that real-time longitudinal studies with single cohorts is not enough. Rather, as outlined already in the 1960s and 1970s in the field of life-span development and aging (Bakes, P. B., 1968; Labouvie, 1980; Nesselroade & Reese, 1973; Schaie, 1965,1979), the final power of longitudinal research rests in its creative use as a rather heterogeneous category of research strategies including microgenetic and simulation approaches (Bakes, P. B. & Goulet, 1971; Bakes, P. B. et al., 1977; Siegler & Crowley, 1991).
The call for a broad range of longitudinal methods is based on the assumption that behavioural development is the result of a complex, multilevel interaction of factors and mechanisms. It is unlikely that this nexus of biologically, socially and societally determined influences on development can be unravelled by longitudinal designs which are essentially descriptive or quasi-experimental in nature. For example, life-span theory suggests that development and aging are jointly determined by age-graded, history-graded, and non-normative systems of influence (Bakes, P. B., 1987; Bakes, Cornelius, & Nesselroade, 1979; Dannefer, 1987; Elder, 1986; Featherman, 1983; Kruse, 1992; Mayer, 1990). Some processes may exhibit a high correlation with age, whereas others are a reflection of historical change. In addition, some events or changes do not occur universally for all people.