For more than 40 years, researchers in gerontology have commented on the need for theories of aging. K. Warner Schaie suggested that theories were needed to integrate the large amount of research data that was accumulating across disciplines. Timothy Salthouse has commented that the field of cognitive aging has virtually no theories that can account for a wide range of phenomena. More recently, Vern Bengtson and colleagues have stated that many gerontology researchers have not focused on theory building, and therefore, the development of theories of aging has not progressed as rapidly as the accumulation of data. Despite this lag in theory building, some theories of aging have emerged that have influenced the conduction of many research studies in aging. The focus of the present chapter is to give an overview of these existing theories in the domains of psychology, sociology, and biology.
According to Bengtson, theory can be defined as "the construction of explicit explanations in accounting for empirical findings." In their view, theories of aging can help us understand and explain the normal aging process. More specifically, theories have several functions: (1) integration of knowledge, (2) explanation of knowledge, (3) predictions about what is not yet known or observed, and (4) interventions to improve human conditions. Theories allow integration of knowledge by providing a coherent framework for organizing a large amount of data obtained in empirical studies into a "brief statement that describes linkages among the crucial observations, variables, or theoretical constructs."
One of the most dominant agendas relevant to interventions is the goal of "successful aging," a theme that has guided much research in hopes of identifying the potential of aging and ways to improve the aging process. Indicators of successful aging have been defined in terms of length of life, biological health, mental health, cognitive efficacy, social competence and productivity, personal control, and life satisfaction. To understand how to age successfully and to develop theories accordingly, Paul and Margaret Baltes offered seven propositions about the nature of human aging:
1. There are major differences between normal, optimal, and sick (pathological) aging. This proposition suggests that declines produced by dementia and other age-related illnesses are not typical of the normal aging process.
2. There is much heterogeneity (variability) in aging. This proposition proposes that the aging process is not uniform; individuals age differently from each other.
3. There is much latent reserve. Reserve refers to the idea that we have a reserve capacity, or pool of resources that can be activated through learning, exercise, or practice. Older adults do not necessarily use their reserve capacities all the time, but when encouraged to do so, they can benefit to the same degree as young adults on a variety of tasks. For example, older adults benefit from practice with a task.
4. There is an aging loss near limits of reserve. Research has shown that performance in older adults declines when the limits of their reserve are reached. For example, older adults are unable to perform at the level of younger adults in tasks involving speed, even under the most optimal conditions.
5. Knowledge-based pragmatics and technology can offset age-related decline in cognitive mechanics. Even when the limits of reserve are reached, older adults can compensate for age-related declines by using their preexisting knowledge. For example, Salthouse showed that older typists were able to compensate for their slowing in tapping speed by reading farther ahead in the text to be typed, resulting in equivalent or sometimes better typing performance than younger typists.
6. With aging, the balance between gains and losses becomes less positive. A gain can be defined as an expected change with age that is desirable, such as becoming more intelligent, whereas a loss is an expected change that is undesirable, such as becoming less healthy. As we age, the ratio of gains to losses is thought to decrease.
7. The self remains resilient in old age. Older adults do not hold more negative views about themselves. In fact, older adults do not differ from young adults in reports of life satisfaction, personal control, or self-efficacy.
Several strategies and multidisciplinary theories for successful aging have been developed across fields of study in response to these propositions. One such theory, for example, emphasizes that increasing mental and physical functioning and decreasing the risk of disease and disability by eating healthy and exercising can reduce the occurrence of pathological conditions that disrupt the normal aging process. Additionally, it promotes active engagement with life.
Another theory, selective optimization with compensation, promotes successful aging through planning. Adults should: (1) select activities and abilities that are most important to their sense of well-being and concentrate their efforts on maintaining those abilities, (2) find strategies that will help them optimize performance on the chosen abilities, and (3) find ways to compensate for declines in other abilities.
Another successful aging theory maintains that aging is characterized by loss and decline in many areas. Successful aging is measured by how well older adults adapt to the unavoidable challenges they face as a result of age-related decline. The theory also establishes four tasks that older adults can perform to help them age successfully: (1) find a replacement for the ability that has been lost, (2) attempt to retrain faculties that are declining with age, (3) learn to make do with less, or (4) retain the remaining functioning.
In many respects, aging seems to be an individual process, demonstrating considerable variability between individuals. For example, two 82-year-olds may be very different in terms of health, their ability to live independently, and their cognitive abilities. Keeping the aforementioned multidisciplinary propositions and theories in mind, we turn to theories that can explain more discipline-specific aspects of the aging process: how and why we age the way we do.
Much research has gone into the problems associated with old age, including dementias such as Alzheimer's disease and Parkinson's disease. Large bodies of literature in psychology, medicine, sociology, and other fields focus on aspects of aging for specific groups of individuals (e.g., clinical patients) whose aging experience can be quite different from the norm. Until fairly recently, few researchers concentrated on studying normal, healthy older adults to get a sense of which aspects of our mental states change over time, which remain stable, and how those changes or stabilities manifest themselves in older adults' daily lives.
Cognitive theories of aging attempt to explain the nature of age-related differences in cognition, the factors responsible for these differences, and the mechanisms underlying why age-related differences occur. Although cognitive theories have developed primarily in the past 20 years, several dominant frameworks have emerged: (1) reduced processing resources, (2) general slowing, (3) inhibition deficits, and (4) transmission deficits.
One theory of cognitive aging proposes reduced processing resources in old age, where a processing resource is defined as some internal input necessary for processing that is available in limited quantities at any given point during processing. A variety of resources required for cognitive processing have been named, including working memory, attention, speed of processing, and inhibition, where the latter two have evolved into their own theories. In a reduced processing resources framework, people have a limited pool of processing resources from which they can draw when performing cognitive tasks, and the amount of available resources decreases as we get older. Therefore, age-related declines in cognitive performance are expected whenever a cognitive task requires more of these resources, e.g., more complex tasks. Consistent with this view, older adults exhibit greater decrements in performance on a variety of complex tasks, such as comprehension of syntactically complex sentences, mental arithmetic, and verbal reasoning.
A second class of theories, theories of general slowing, suggests that a major factor contributing to age-related differences in cognitive functioning is a reduction in older adults' processing speed, or the speed of executing cognitive operations. This age-related slowing occurs in the central nervous system and therefore affects all tasks regardless of complexity. Some researchers have proposed that the rate of slowing in older adults is predictable. For example, some researchers have suggested that young and older adults' processing speeds are linearly related. Others have argued that cognitive processes slow down at a constant rate; more specifically, older adults' processing speed is one and a half times slower than young adults. Although most researchers accept the idea that speed of processing slows with age, there is considerable debate over whether this slowing is the cause of all age differences.
A third cognitive theory of aging proposes that older adults have a deficit in their inhibitory processes: Aging impairs the ability to inhibit or suppress irrelevant information that becomes activated in the course of cognitive processing. Inhibitory processes serve two functions: to prevent irrelevant information from entering working memory (where people hold and manipulate information that is currently being focused on) and to delete information that is no longer relevant to the task at hand. When inhibitory processes are impaired, people will get more interference from irrelevant information because of their inability to suppress it. Inefficient inhibitory mechanisms have been used to explain why older adults are more likely to entertain thoughts that are irrelevant to cognitive processing, such as personally relevant thoughts or daydreams, or why they produce speech that is off-topic when describing events related to their lives.
The fourth cognitive theory of aging uses a connectionist framework where words are represented as nodes that are connected on many levels, including phonology (or sounds), orthography (or spelling), and semantics (or meanings). The Transmission Deficit hypothesis proposes older adults' connections weaken over time by virtue of aging. As a result, any task involving weak connections will be susceptible to age declines. Therefore, older adults are particularly susceptible to cognitive declines when new connection formation is required (as it is for new learning) or when preexisting connections have weakened, as will happen over time when words are not used frequently or recently. Research on older adults' memory, both for tasks involving new learning and for retrieval of infrequently used, existing knowledge, has shown declines, consistent with this theory.
Most theories of human development take one of two views of development: the life stage perspective or the life-span perspective.
Life stage perspectives view development as a series of stages through which all people pass in their lives. Progression through the stages occurs in a fixed order, and movement from one stage to another depends on performance in the earlier stage. Generally, later stages are seen as more advanced than earlier stages.
Erik Erikson's theory of psychosocial development, in which development takes place over eight stages, is the most widely known stage theory dealing with older adulthood. Each stage is characterized by two conflicting ways of dealing with life events typically encountered at that stage, called a psychosocial crisis. For example, the period of late adulthood, roughly after age 65, is marked by the struggle between integrity, or looking back on one's life positively, and despair, or feeling negatively about the life one has led. The adaptive resolution to the crisis is the acceptance that death is relatively near and the ability to review one's life with satisfaction. Successful resolution of this crisis brings with it the development of wisdom. However, this development is a lifelong process, as each psychosocial crisis is never fully resolved.
Daniel Levinson describes life as a series of four 25-year eras, or major life stages. As in Erikson's theory, each era is marked by a general developmental goal, such as becoming independent. However, Levinson's stages are further divided into developmental periods, which are alternately stable and transitional as life goals and circumstances change.
While Erikson's and Levinson's theories characterize the life trajectory as that of a series of stages (e.g., birth, middle age, death) that all humans go through, other theories are more consistent with the life-span developmental perspective. The life-span perspective emphasizes development as the result of a lifelong interaction between a person and his or her environment. In this view, development is composed of growth and decline throughout life, in every area of functioning, caused by many different factors. The rate of growth and decline is quite changeable, and individuals can experience several periods of gains and losses in a single domain, such as cognitive functioning, over a lifetime. Development is affected by the individual's historical, cultural, and social environments as well as by the physical environment, and as such is best understood by drawing on several different fields of knowledge.
The stress process framework proposes a mechanism by which people deal with the life changes associated with old age. It is based on the interaction of stressors, such as undesirable events and/or chronic problems; the moderators, or resources, marshaled to deal with the problems; and the outcomes or net effectiveness of the moderators in dealing with the stressor.
Examples of stressors include the loss or change of a societal role, such as retiring from employment or becoming a widow. Stresses often proliferate, and a primary stressor such as losing a spouse can prompt the occurrence of a secondary stressor, such as financial difficulty. Chronic strains such as illness also function as stressors. Moderators can take the form of internal resources that are rooted in one's personality, such as a preferred coping style or feelings of self-efficacy. Other people, in the form of social support networks, also serve to moderate the effects of stress on older adults. Successful moderators can also bolster people's ability to deal with future stressors, while moderators that are ineffective in dealing with problems can be adapted or discarded.
Similarly, continuity theory posits that people do not deal with stressors in order to resolve a specific developmental challenge, as Erikson argues. Instead, they develop, maintain, and change coping strategies over a lifetime, monitoring their effectiveness and changing strategies to fit each individual situation. Both the stress process framework and continuity theory emphasize the role that individual characteristics play in dealing with stressors, as well as the reciprocal effects stressors and moderators have on each other.
In contrast to developmental theories, which focus on age-related change within individuals, sociological theories attempt to explain the relationship of older adults as a group to the rest of society. Most sociological theories tend to emphasize the life course perspective, examining how social norms, including cultural, historical, and familial contexts, influence older adults' social roles and role transitions. The timing of a particular event (e.g., childbearing) in an individual's life and the historical context in which the event occurred are particularly important to the life course perspective.
A social role comprises not only one's personal self-concept but also the ideas held by the rest of society about how a person of a certain status should act. A problem facing most older adults is that of changing roles: from employed to retired, from spouse to widow, from being generally healthy to facing more frequent health problems. According to some researchers, societies have cultural age deadlines or general ages at which people are expected to have completed life milestones, such as marrying, having children, and retiring. Problems arise when these life milestones are completed "off time," or in a drastically different manner from the rest of society.
The role-theory framework focused on the negative changes that came with losing social roles after retirement age and gaining social roles that were less desirable, such as widowed or dependent on others. Studies carried out under this framework found that older adults who were unemployed, widowed, over 70 years old, or who simply viewed themselves as being elderly or old, were less accepting of old age than their working, married, under 70 years old counterparts who viewed themselves as middle aged. Additionally, when studying personality traits and adjustment to social roles, older adults whose personality incorporated both masculine and feminine traits were more accepting of their age than adults whose traits were strongly masculine or strongly feminine.
Activity theory takes the view that older adults' social needs do not diminish with age, even as the actual amount of social interaction declines. Adults who continue previous as well as new social interactions as they transition into old age will have higher life satisfaction than will adults who are more segregated from the rest of society. Although some studies have supported activity theory, other studies have found that older adults voluntarily decrease the amount of their social involvement for a variety of reasons. Additionally, activity theory does not make predictions about the activity level for adults whose environment changes.
In contrast, disengagement theory posits that older adults naturally withdraw from society as they age, becoming more self-involved and less involved with others. Concurrently, society withdraws from the individual, who is seen to have less to offer with increasing age. Later researchers have argued for differential disengagement, whereby older adults become less involved with some activities, but remain active in other ways. While disengagement theory and activity theory are responsible for generating a considerable amount of research on adjustment in late life, some researchers feel the theories fail to incorporate individual differences and preferences sufficiently to adequately explain social interactions in old age.
Continuity theory is a relatively recent sociological theory. As discussed in the previous section, it posits that older adults seek to maintain a stable environment. This environment can be maintained either externally, by remaining in the same physical and social contexts as in previous years, or internally, by maintaining beliefs, attitudes, and personality traits as in younger years. A mismatch between the desired and actual levels of continuity can be problematic; too little continuity (i.e., too many new experiences) can induce anxiety, while too much continuity can induce boredom with current circumstances.
Another recent theory is the socioenvironmental theory, which emphasizes the effects of one's surroundings on one's social interactions. Older adults are more likely to interact in situations where there are many older adults close by, as opposed to when there are fewer older adults or they are far away. This theory accounts for the popularity of large retirement communities and apartment buildings with large numbers of older adults and has been supported by research findings on friendship patterns in older adults. Social exchange theory is based on the idea that social interactions are conducted with the understanding that both partners will benefit equally from the contact. Interactions may also be conducted without expecting reciprocity, simply to help another person. In this framework, interactions with older adults would tend to fall under the second category, as society views older adults as having very little to give and much to receive, in terms of assistance, time, and money. Studies conducted with older adults on the amount of aid given to and received from others have found mixed results.
One of the early theories about biological aging was the "wear and tear" theory, or the idea that the body was analogous to a machine. Aging occurred simply because the machine was gradually wearing out. More recently, biological theories of aging have more precisely focused on specific factors that contribute to age-related declines, and these theories can be categorized into two groups: theories that argue that the aging process is largely genetically preprogrammed (although it is an oversimplification, one may think of these as "nature" theories), and theories that hold that the aging process is due to events that occur as a part of everyday life (these can be described as "nurture" theories). One current view, however, takes the approach that aging is due to an interaction of genetic and environmental factors.
While recent physiological aging theories have postulated that our aging process was encoded in our DNA and thus predetermined even before we were born, it is currently thought by some researchers that only 10 to 30% of longevity differences are inherited. Nevertheless, many researchers concentrate their efforts on people who have remained healthy into very old age, studying them and their relatives in an effort to determine what genetic characteristics these older adults share that may contribute to their longevity.
Other researchers believe that aging is due to problems at the cellular level. The error catastrophe hypothesis claims that errors or mutations sometimes occur during the process of transcribing DNA into RNA and RNA into proteins. As the cell divides and multiplies, these errors are passed on, and eventually impair the cell's functioning or lead to cellular death.
Free radical theories are the aging theories with which the general public is most familiar. Free radicals are byproducts of cell metabolism, and they damage other parts of the cell, thus reducing cellular functioning. Free radicals are unpaired electrons, commonly produced during radiation (e.g., sun exposure) or oxygenation. Much research has gone into the use of topical or ingested antioxidants to pair with the free radical's electron, thus rendering them harmless.
Physiological changes in the body's regulatory and immune systems also contribute to physical aging. The immune system becomes less effective with age, as do the body's other systems and organs. However, declines in immune system functioning lead to a decreased ability to detect and fight diseases, making older adults more vulnerable to illnesses such as influenza. Studies have shown that long-lived individuals have well-preserved immune systems.
In addition, the endocrine system has been shown to become less efficient with age at preserving the body's natural balance in areas such as blood sugar levels or hormones. When environmental changes such as lower blood sugar levels occur, the damage is exacerbated by the body's inability to repair the damage and restore balance as quickly as it did when it was younger.
While these are the dominant theories, many other theories exist that are in varying stages of development. In addition, theories are constantly being tested and modified in conjunction as new data from aging research emerge. It is only in the last half-century that researchers have begun to focus their attention on older adults as a population worthy of special consideration. While earlier theories of aging were few in number, narrowly focused, and generally negative, recent theories have emerged that establish aging as a multidimensional process. These theories view aging as characterized by positive as well as negative qualities and are more interactive in nature. They emphasize the interaction of biological, physical, and social factors in each individual's age trajectory, and attempt to explain how older adults can minimize the negative and maximize the positive aspects of aging, in order to more fully enjoy the increased life span that comes with living in the 21st century.
The American Geriatrics Society, http://www.americangeriatrics.org
The American Society on Aging, http://www.asaging.org
Baltes, P. B., & Baltes, M. M. (1990). Psychological perspectives on successful aging: The model of selective optimization with compensation. In P. B. Baltes & M. M. Baltes (Eds.), Successful aging: Perspectives from the behavioral sciences (pp. 1-34). New York: Cambridge University Press.
Bengtson, V. L., Rice, C. J., & Johnson, M. L. (1999). Are theories of aging important? Models and explanations in gerontology at the turn of the century. In V. L. Bengtson & K. W. Schaie (Eds.), Handbook of theories of aging (pp. 3-20). New York: Springer.
The Gerontological Society of America, http://www.geron.org
Lockshin, R. A., & Zakeri, Z. F. (1990). MINIREVIEW: Programmed cell death: New thoughts and relevance to aging. Journal of Gerontology: Biological Science, 45, B135-B140.
Salthouse, T. A. (1991). Theoretical perspectives on cognitive aging. Hillside, NJ: Erlbaum.
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