LIFE-SPAN DEVELOPMENT

Life-span developmental theory provides a framework for understanding human aging. The main purpose of theory in the study of aging is to provide a context for describing and explaining the regular transformations that occur with time to representative organisms living under representative conditions. Theories in general aid the process of articulating meaningful patterns from observations that would otherwise be disconnected pieces of a puzzle and less meaningful. In the study of aging, theories serve as frameworks for organizing research findings as well as general observations or intuitions about aging and the elderly. For example, the study of aging can be seen either as largely the inevitable accumulation of deficits and losses, or as a mixture of gains and losses that depends to some extent on the individual's actions.

Aging is universal in that similar changes in human behavior occur at about the same age for all persons. Some of these age-related changes are controlled by a "biological clock," and the term, primary aging, refers to the biologically based mechanisms that are responsible for breakdowns in particular organ systems. In contrast to the consequences of many disease processes that are relatively abrupt and that affect primarily one system or organ, primary aging eventually affects all systems within the organism, and aging occurs gradually and in a cumulative fashion. For example, it is known that all sensory systems become less acute and that the speed of processing information becomes gradually slower with advancing age. Further, there are predictable changes in physiological systems and in physical appearance with aging.

Despite the ubiquity and universality of aging processes, one of the most remarkable characteristics of human aging is the wide range of heterogeneity in the expression of the consequences of aging. Secondary aging factors such as stress, trauma, exposure to toxins, and disease exacerbate the rate of primary aging. However, it is also the case that some individuals successfully avoid disease, maintain high levels of physical and cognitive function, and continue to be actively engaged in life. Individual characteristics as well as the circumstances and the sociocultural contexts associated with particular periods of historical time (e.g., the Vietnam War era) are influential in producing distinctive patterns of aging.

No one is born old, and individuals become more unique as they grow older in part because of particular histories of life experience. During the life course, the repertoire of individuals is shaped by how the person invested time and energy. The life-span approach to the study of aging is one way of conceptualizing many of the factors that affect how individuals change as they grow older, and how different individuals show different patterns of change with aging.

Life-span theory

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.

Research methods and themes

The description or analysis of simultaneous gains and losses along multiple dimensions across time or age requires sophisticated research methods. Perhaps one of the most important methodological issues derivative from life-span theory has been the distinction between age, cohort, and time of measurement as sources of influence in developmental research. In the study of aging, it is known that observations of age differences in behavior are attributable to both cohort factors (influences associated with time of birth) and to age factors. Further, observations of longitudinal changes across multiple times of measurement are attributable to chronological age change and to the influences of changing socio-historical circumstances. The significance of age, cohort, and socio-historical factors as distinct sources of influence was not really appreciated until development was conceptualized in a broad life-span framework.

Another important methodological theme in life-span developmental theory has to do with the specification and meaning of the age variable. Although age-related change is usually described in terms of chronological age, years since birth provides a crude and unsatisfactory index of many aging phenomena. One theme of the life-span approach is to replace chronological age with index measures that more accurately capture the sources of time-related or age-related change. For example, the effects of aging on behavior are sometimes irreversible. The effects of aging on behavior can also be quantitative and continuous rather than qualitative, and reversible rather than irreversible. Further, markers of elapsed time are insensitive to the meaning of time as a relative and subjective dimension. Indeed, the experience of the passing of 365 days is not likely to be the same for different aged individuals. There are "social clocks" based on culture-related age prescriptions as well as "biological clocks" based on physiological time. Because age and time are not causes of change per se, an aim of aging research is to identify the mechanisms that are primarily responsible for age-related change. Careful description of the social and biological processes that produce aging would enable researchers to replace the index variable, time since birth, with the variables for which it is proxy. For example, there is some evidence to suggest that measures of brain reserve capacity can serve as more accurate measures of the effects of aging than chronological age.

Plasticity, reserve capacity, and resiliency

Most developmental research describes what is normal, not what is possible. Referring to Table 1, another main theme in life-span developmental theory is associated with plasticity, reserve capacity, and resiliency. Plasticity refers to the potential for intra-individual change. Intra-individual plasticity is evident when there is variability in levels of performance across different kinds of tasks or when there is variability in performing the same task measured at different times or under different conditions. Even the mature adult brain retains considerable functional plasticity, and there is continued cortical reorganization based on adult experience (e.g., Greenough and Black; Ramachandran). Life-span researchers are interested in understanding developmental differences in the gap between observable behavior and actual or potential competence. The potential for optimization is present throughout the life course, although it is likely that it becomes increasingly constrained near the end of the life span. In other words, there appears to be diminished reserve capacity near the end of the life span, such that the individual is vulnerable to a variety of circumstances associated with mortality.

Individuals also exhibit varying capacities to protect themselves from impairment and insult associated with aging and disease, and to adapt effectively to the demands of stressful situations. The term reserve capacity refers to the individual's resources for responding effectively to challenging conditions. The term resiliency is similar in its meaning, and refers to a capacity for successful adaptation and recovery in response to stressful life events. Although the concept of resilience has been used mainly in reference to protective resources in children, recently some life-span researchers have argued that resilience is a useful concept for describing individual adaptation throughout the life span.

The concepts of plasticity and reserve hold promise for providing a full understanding of the relationship between aging and behavior. Analogous to cardiovascular function, or muscular efficiency, healthy older adults usually function quite effectively in everyday nonstressful conditions, but their functioning is likely to be impaired under stressful conditions. That is, age-related deficits in behavioral function are most apparent when systems that are critical to maintaining performance are challenged or stressed. The study of reserve capacity is useful for describing the effects of aging on a wide range of functions under challenged conditions. Along these lines, work by Baltes and colleagues has addressed how selected aspects of development can be enhanced or compensated for in situations or domains where there is optimal support.

Biological and social processes

Another main theme of life span developmental theory is the idea that aging is continuously and simultaneously influenced by a wide range of dynamic biological and social processes. Although it seems trivial to point out that aging has multiple causes, most theories of aging emphasize the nonmalleable aspects of aging. From a life span perspective, it is recognized that some of the antecedents of aging are universal and species-determined, and that some are idiosyncratic or cohort-specific, culture-specific, or specific to a segment of historical time. Further, some of the biogenetic and sociocultural aspects of development and aging are gender-specific, and some of the biological and social aspects of development and aging are gender-invariant. Some of the environmental influences on aging are or seem entirely unique to individuals.

It seems that life span researchers also find it useful to look for conceptual linkages across disciplines. Occasionally, such linkages lead to major advances in how we think about aging processes. Significant advances in the study of aging often reflect multidisciplinary integrations of ideas. For example, recent work in the area of developmental behavioral genetics goes beyond the standard position on organism-environment interaction by calling attention to nonadditive synergistic effects. That is, there are unique combinations of nature and nurture that produce synergistic or optimal outcomes for development. As an example of a synergistic interaction between environmental conditions and heritability, it has been reported that there is a significant increase in the heights of second-generation Japanese persons raised in the United States compared with second-generation Japanese persons raised in Japan. Second-generation Japanese persons raised in the United States were over five inches taller than the American-reared sons of short Japanese fathers and the Japan-reared sons of tall Japanese fathers (see Bronfenbrenner and Ceci).

In conclusion, life span developmental theory provides a basis for describing both gains and losses associated with aging. From a life span developmental perspective, researchers tend to consider the potentials as well as limits of intra-individual change across the life span. Due to a combination of influences, many developmental outcomes are possible for each person, some outcomes are more likely than others, some outcomes can be made more likely, and some outcomes are not possible. From a life span orientation, the understanding of specific aspects of human aging are best understood in a larger context that encompasses the influences of biological and sociocultural factors across time and age.

William J. Hoyer

See also Age-Period-Cohort Model; Life Course; Life Span Theory of Control; Physiological Changes; Successful Aging.

BIBLIOGRAPHY

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