The assumption that aging occurs in context probably is an implicit or explicit core feature of all major models of aging, in biology, in the social sciences, as well as in psychological science. How such context is defined and what kind of impact is attributed to context strongly depends on the disciplinary perspective taken on aging. For example, Cowdry (1939), a pioneer in biogeron-tology, suggested distinguishing between endogenous and exogenous processes of aging. According to the endogenous view, aging is an involuntary process operating cumulatively over time and resulting in adverse modifications of cells. In the exogenous view, aging is seen as the consequence of infections, accidents, or environmental poisons. In most current theories in bio-gerontology, aging, or what is sometimes coined senescence in the biology of aging, is viewed primarily as an internal decline process related to the flow of chronological age and ending in the event of death (Cristofalo, Tresini, Francis, and Volker, 1999). Environmental conditions, however, shape the survival time of aging organisms, because it is generally acknowledged in models of longevity that genetic factors explain less than 30% of variability in survival time (Vaupel, Carey, and Christensen, 2003). The understanding of context in biological models of aging tends nevertheless to remain rather general, referring mostly to basic physical properties such as temperature, kind of nutrition, or environmental stress level, for instance, when young and old organisms are confronted with uncontrollable negative experiences in an age-comparative experimental design. It is worth mentioning that in the rapidly growing field of behavioral genetics in aging research, most recently a series of articles has underlined the still widely unmet need for a well-defined conception of the environment in order to better understand the operation of gene expression (or depression) in the flow of aging (Johnson and Crow, 2005).

At the other extreme, a social science concept of aging puts major emphasis on the operation of social, cultural, historical, and societal contexts, and how such influences are shaping aging processes. As was argued, different societal expectations and norms structure the sequence of the life course. Temporal inconsistencies between the individual and societal demands or norms may cause negative societal reactions and/or psychological stress on the individual level (Hagestad and Neugarten, 1985). Think of becoming pregnant at the age of 13 years, being unmarried still at the age of 50, or learning to drive a car at the age of 80 years. Additionally, historical context is changing dramatically over time, and this leads to changes of the flow of individual lives and personsociety interrelations. A classic example is Elder's (1974) research on Children of the Great Depression, which serves as an impressive illustration of the long-term influences of early childhood and adolescent experiences with the socioeconomic and family consequences of the Great

Handbook of Models for Human Aging

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Depression on the individuals' life course trajectories across adulthood and later life.

Psychological models of human aging have also put strong emphasis on the contextual component of life span and adult life development (Baltes, 1987; Carp, 1967; Dannefer, 1992; Kleemeier, 1959; Lawton, 1999; Rebok and Hoyer, 1977). Similarly to social science perspectives, the term context as used in psychological models of aging implies that the interaction between the aging individual and his or her environment is crucial for the understanding of the course and outcome of aging. According to psychological models of aging framed in a strong contextual view, a fundamental psychological challenge of aging individuals is to adjust or readjust their relation with the environment in which they live. Accordingly, a major research task of psychological gerontology is to describe and explain stability and change of person-environment dynamics as people age. As Dannefer (1992) has pointed out, the term context as also used in most psychological models of aging is predominantly composed of two large-scale entities, the social environment and the physical environment. The social environment refers to the totality of the diverse range of phenomena, events, and forces that exist outside the developing individual and are directly linked to other persons. This includes research issues such as social networks, social support, or the regulation of social relationships in later adulthood (Antonucci, 2001; Lang, 2001). The physical environment refers to the totality of the diverse range of phenomena, events, and forces that exist outside the developing individual and are directly linked to the material and spatial sphere. This covers research issues such as the impact of the physical-spatial home environment on aging, the role of neighborhoods and infrastructural characteristics, long-term institutions, as well as the challenge and outcome of residential decisions in later phases of the life span (Lawton, 1999; Wahl, 2001; Wahl and Gitlin, In press).

Moreover, emphasizing the context of aging in psychological aging research necessarily implies the need to consider natural settings, which are difficult to capture reliably and validly with experimental approaches in the laboratory. Furthermore, psychological models of aging strive to go beyond survival time as a major outcome of aging, as is often the case in biological models of aging. Instead, psychology focuses on outcomes such as subjective well-being, autonomy, or meaning of life, to name just a few, as products of the person-environment interactions across adulthood.

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