Life-span developmental theory is usually considered to be a "meta-theory" in that it is a set of themes for approaching the study of development and aging. The life-span approach is not a set of empirically testable hypotheses; rather, it provides a broadened orientation to the study of aging. It should also be pointed out that life-span theory is not new, but has its origins in the work of several eighteenth- and nineteenth-century writers, especially Johann Nikolaus Tetens (1736–1807) and Adolphe Quetelet (1796–1874).
The main themes associated with the life-span approach are summarized Table 1. The idea that aging is multidimensional and multidirectional is one of the basic themes of a life-span orientation to the study of aging. Consistent with evidence suggesting increased inter-individual differences with aging in healthy adults, the experience of aging seems to produce cumulative differentiation within individuals along multiple dimensions. Although it might seem obvious, the idea that gain or growth as well as loss or decline can be observed throughout the life span has not usually been appreciated in research that is narrowly focused on the study of age-related deficits or decline. For example, the study of the characteristics of wisdom and mature thinking, the maintenance or continued evolution of cognitive expertise in the later years, and the emergence of emotional maturity are areas receiving increased research attention. Although there are relatively few research investigations aimed at describing the positive aspects and potentials of late-life functioning compared with the number of investigations in the literature aimed at description and explication of age-related deficit, research inspired by a life-span approach helps to provide a more balanced and accurate account of aging. Life-span theorists seem to enjoy the challenge of chipping away at the prevailing views of aging in science and in society as monotonic deterioration, decrement, and loss. In their efforts to dispel overly negative views of aging, life-span researchers are skeptical about overly narrow conceptions of aging. The results of research describing ordinary lives in real settings often provides a contrasting picture of aging compared with the results of research describing performance on tests and measures that are indigenous to youth and insensitive to the unique qualities of older adults. Another line of research evidence in support of a balanced view is derived from studies that distinguish between the characteristics of normal aging, illness-free aging, and successful aging. Research on successful aging has as one of its aims to identify the personal attributes and contextual characteristics of individuals who minimize or escape the negative consequences of aging and disease. From a gains/ losses perspective, there is a complex array of human capabilities; some show decline, some improve, and some remain the same across selected time periods.