Differential Aging Research Paper

According to the principle of differential aging, the rate of functional losses and gains over time shows both inter-individual variability and intra-individual variability. Inter-individual variability is shown by the fact that there are differences across individuals in the rate at which changes occur over time. Intra-individual variability is demonstrated by the finding of differences in the rates of change over time within the same individual. The principle of differential aging stands in sharp contrast to the previously held views in developmental psychology that aging is a uniform process of decline, also known as dedifferentiation. Although it is true that death is the inevitable outcome of the aging process, this fact does not rule out the possibility that growth can occur in psychological functions even at the end of life. Plasticity is another component of differential aging, meaning that the potential exists throughout later adulthood for growth and adaptation.

1. Evidence For Differential Aging

Support for the concept of differential aging has emerged from a variety of sources within the field of gerontology. One of the strongest arguments used to support differential aging is a meta-analysis of a wide range of empirical studies based on data available through the late 1980s. This analysis yielded evidence of greater variability among older, compared to younger, samples of adults (Nelson and Dannefer 1992). However, without providing explicit tests of the concept of differential aging, many other investigations have led to similar conclusions. The concept of differential aging is becoming a well-established fact within the field of psychology and aging (Whitbourne 2001). It is no longer unexpected to show heterogeneity across older samples and within areas of functioning in the same individual.

Explanations of differential aging relate to models in lifespan developmental psychology that emphasize the importance of person–context transactions as influences on development (Lerner 1995). Such trans-actions affect the rate of aging within and across individuals as they are exposed to and select environments that differentially affect their physical, psycho-

logical, and social characteristics. Furthermore, rates of aging across and within individuals may reflect exposure to opportunities for prevention and compensation as represented, albeit crudely, by the variable of socioeconomic status. The higher mortality rates of individuals from lower socioeconomic backgrounds may be viewed as a manifestation of these differential opportunities (Lantz et al. 1998). To the extent that higher mortality rates reflect a more rapid acceleration of deleterious changes either directly or indirectly linked to aging, variations among socioeconomic status may be seen as a critical contextual variable within the framework of differential aging.

A concept highly related to differential aging is that of selective optimization with compensation (SOC; Baltes and Baltes 1990). Selection refers to the process of actively or passively reducing the number of goals and domains in which to become involved. The purpose of selection is to conserve energy and motivation for more important goals or to select new goals that will serve particular developmental tasks, such as coming to terms with mortality. Compensation in this model is defined as searching for and using alternate means of reaching a goal once it is no longer possible to use previous methods to achieve that goal. For example, an individual may wish to remain physically active but when limitations occur in mobility, the method of involvement may change accordingly. Optimization is the process of refining the methods and resources needed to reach a goal and thereby achieve success in particular domains. Applications of the SOC model to the processes through which individuals attempt to control and regulate developmental change over the lifespan are providing further elaboration of this model (Heckhausen 1997).

1.1 Application To Physical Aging Processes

The physical aging process is a prime example of the concept of differential aging. Aging of the body occurs across all organ systems, but at different rates within and across individuals. These concepts are expressed in the multiple threshold model (Whitbourne 1996), which postulates a reciprocal relationship between the aging of the systems within the body and the individual’s identity or self-conception throughout the years of adulthood. It is known that changes occur within bodily systems at varying rates over time, reflecting both genetic predisposition and the impact of the environment. A threshold is the point at which the individual becomes aware of having experienced an age-related change in an area of functioning. After the threshold is crossed, changes in identity occur as the individual becomes closer to defining the self as ‘old.’

Predictions of the multiple threshold model were tested in an investigation of a large sample of adults from 40 to 95 years of age (Whitbourne and Collins 1998). Respondents were asked to indicate the extent to which they had experienced a set of physical and cognitive changes related to the aging process as well as the way that these changes had affected their identities. There were clear individual differences in the areas of functioning identified as significant, as well as variations in the responses that individuals had to these age-related changes. Changes with regard to aging were clearly distinguished within the minds of respondents in this study, who reacted differently to changes in cognitive processes compared to changes in mobility and physical strength.

Another facet of the multiple threshold model is that individuals vary in the preventive and compensa- tory mechanisms available to them across a wide area of functions. For example, in the case of aerobic capacity, it is well known that regular activity can help to offset the aging process, even if this activity is begun quite late in life. By contrast, individuals can also choose to alter the rate of the aging process by the adoption of harmful health habits, such as cigarette smoking or a sedentary life style. Their decision to engage in these activities may reflect social factors as well as psychological processes. As noted, education and occupational background are further contributing factors, as individuals from higher socioeconomic levels are more likely to be aware of the strategies they need to employ to moderate the rate of their own aging (Lantz et al. 1998).

The extent to which individuals possess resources based on their sensory, cognitive, personality, and social skills provide another factor to be considered in differential aging (Baltes and Lang 1997). People who are low in these personal and social resources have more difficulty negotiating the tasks of daily life in later adulthood. In part, differences between resource- rich and resource-poor older adults occur because these individuals have unequal opportunities to take advantage of compensatory measures that would help offset the negative effects of aging.

One caveat to the notion of increasing variability with age in physical functioning comes from the BASE (Mayer et al. 1998). Among the oldest-old in the sample, i.e., those over 85, there was a decline across a variety of abilities and a tendency for greater similarity across domains of functioning. This finding suggests that individuals in the latest period of life, until recently not widely available for study, may be undergoing changes that are discontinuous with the earlier years of the lifespan. Changes in biological functioning may become more widespread after this point, affecting a broad range of abilities that depend on the integrity of the body’s systems.

1.2 Application To Intellectual Development

Differential aging also appears in research on intellectual development in later adulthood. Researchers in this field recognized by the 1960s that there was a considerable discrepancy between age patterns based on verbal vs. non-verbal tests of intelligence as well as differences between cross-sectional and longitudinal studies. Findings from the Seattle Longitudinal Study (SLS) revealed an overall picture of intelligence in later adulthood as one of stability until the 50s or 60s followed by decline through the oldest age tested. However, there are important qualifications on this conclusion (Schaie 1996). First, although some individuals may show declines in intelligence by the mid- 50s, significant losses do not occur overall until the decade of the 70s. A second point is deterioration of functioning does not occur across all abilities, even into the decade of the 80s. Third, most people are able to retain competent performance of familiar skills, particularly those that are of importance to them. Finally, there was evidence for plasticity as indicated by the success of training studies intended to improve the scores of older individuals on tests that had otherwise shown age-related declines.

A fundamental distinction has emerged in studies of intellectual functioning and age indicating that there are different trajectories over the lifespan for the two theorized components of intelligence: ‘fluid’ abilities, or the mechanics of intelligence, and ‘crystallized’ abilities, or the pragmatics of intelligence (Baltes et al. 1999). With increasing age in adulthood, losses occur in the functions that underlie the mechanics of intelligence, such as attention, working memory, and psychomotor speed. The pragmatic abilities involving mastery of a body of declarative and procedural knowledge, a well as expertise in specific domains, are maintained into later life. These abilities can allow older individuals to perform at relatively high levels despite the aging of functions that contribute to losses in the mechanics of intelligence. The area of wisdom, or optimal expression of knowledge in the fundamental pragmatics of life, provides a prime example of the growth of abilities in later life even in the face of sensory, perceptual, and cognitive losses (Baltes and Staudinger 1993).

1.3 Application To Personality And Social Functioning

The concept of differential aging, with the implication of variations in gains and losses across and within individuals, is implicit in the theory of socioemotional selectivity, the proposition that throughout adulthood, individuals reduce the range of their relationships to maximize the social and emotional gains and minimize risks (Carstensen 1992). According to this theory, aging is associated with a desire to become more focused on maximizing the emotional rewards of relationships and less involved in seeking information or knowledge through social interactions. The desire to maximize emotional rewards leads adults increasingly to prefer spending time in their relationships with people who are familiar to them rather than seeking out new friends and acquaintances. Although this theory postulates a universal process, that of seeking out the familiar rather than the new in relationships, it also has relevance to the differential aging principle. According to the theory, it is not aging but the recognition of limited time that stimulates the shift in motivation for relationships. Younger individuals will show similar tendencies to prefer spending time with close friends and family rather than strangers when they perceive that an ending is drawing near.

That older adults are able to maximize their feelings of well-being by active steps they take to regulate their relationships is also reflected in research on subjective well-being in later adulthood. In one large-scale survey of a national sample of adults (Mroczek and Kolarz 1998) the majority of older adults were found to rate their happiness levels as ‘high’ or ‘pretty high.’ However, there were variations in the relationship between subjective well-being and age as a function of gender, personality, and marital status. Furthermore, the results differed according to which component of subjective well-being was investigated. Positive affect was higher in the older age groups, particularly so for women. For men, personality interacted with age in ratings of positive affect. Higher positive affect scores were found only in the older group of introverted men. Men who were extroverts had high positive affect scores throughout the adult age range tested. Only the married men showed a relationship between age and ratings of negative affect. The older groups of married men had lower scores than the younger married men in the sample. Although these data only reflect one study out of many conducted within the field (Diener 1998), they are of particular relevance to the principle of differential aging because they point to the roles of personality and contextual factors as influences on levels of well-being in middle and later adulthood.

2. Suggestions For Future Research

Despite the existence of the principle of differential aging, many researchers within psychological gerontology and lifespan psychology continue to focus their efforts on the search for consistent patterns of age differences that would characterize older adults as a homogenous group. Theories such as the general slowing hypothesis (Salthouse 1996) propose that the aging process involves a unilateral loss of speed which in turn accounts for reductions in processing abilities. Trait theories of personality propose, in contrast, that there is overall stability of personality throughout the decades of middle and later adulthood (McCrae and Costa 1990). However, even within these frameworks, there are attempts to identify patterns of differences associated with biological or experiential factors. For example, experienced typists may preserve their abilities to perform well-learned tasks by compensating for loss of speed with changes in strategy (Salthouse and Saults 1987). Similarly, in the area of personality, although the search for universal trends continues (McCrae et al. 1999), researchers also attempt to identify patterns of life events that alter levels of personality traits or their expression in behavior (Helson et al. 1995).

3. Summary

The examples provided here of research supporting the concept of differential aging suggest the need for further empirical investigation that employs a multidimensional and multifaceted approach to the study of lifespan processes as these unfold in later life. It is no longer sufficient to document the existence of age differences or changes over time employing standard methods using correlation or analysis of variance. With the increasing availability of structural modeling techniques, lifespan researchers have the opportunity to translate the principle of differential aging into operational terms. Research based on this principle will not only reflect more accurately the nature of the aging process but can also provide the basis for effective interventions to improve the daily lives of older adults.

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