Age, cohort, and time development muddles: easy in practice, hard in theory.

The debate among developmental psychologists over how best to combine longitudinal and cross-sectional data sequences can be traced back at least four decades. During the 1970s, a variation of this theme received much attention: could the developmental influences of age, cohort, and time be unraveled by sufficiently ingenious application of combined data sequences? We believe discussion of this question has been needlessly parochial and confused. Substantive and methodological contributions from other disciplines have, until recently, been largely ignored by developmental psychologists. Moreover, the solutions debated by psychologists have generally been formulated in language that obscured, rather than explicated, the formal indeterminacy implicit in models of age, cohort, and time parameters. Methodologists have outlined formal solutions to problems of indeterminacy in the contexts of model identification and estimability theory. Sociologists have proposed solutions, based on explicitly theoretical assumptions, that permit model identification and unambiguous interpretation. We review these contributions, and suggest a hierarchy of solutions to the problem of age, cohort, and time indeterminacy.