Dual Functionality in Later Life

Abstract

Gerontologists have long shown interest in both longevity and quality of life during later life, but considerable debate has ensued as scholars sought to integrate the two. Drawing from research on the topics of exceptional longevity, successful aging, and active life expectancy, we propose the concept of dual functionality to examine how humans reach advanced ages while maintaining physical and cognitive function. Dual functionality refers to being free of both physical and cognitive impairment. The loss of dual functionality challenges social networks to respond to the functional loss and is a harbinger of additional losses. Evidence of change in the percent of the older population maintaining dual functionality would be helpful for evaluating public health policies to aid quality of life during later life. This article is based on the first author’s Robert W. Kleemeier Award Lecture at The Gerontological Society of America 2022 Annual Scientific Meeting.

Population aging and rising life expectancy in developed nations over the past century have heightened scientific and public policy interest in the antecedents of quality of life during the later years. As President Kennedy argued in 1963, “It is not enough for a great nation merely to have added new years to life—our objective must also be to add new life to those years” (p. 189).

The aspirational goal of adding new life to years typically entails the older person maintaining both physical and cognitive function. There have been many studies of either physical (Morton, 2022; Sauerteig et al., 2022) or cognitive function (Alley et al., 2007; Ferraro et al., 2023), and a recent study examined the relationships between physical and cognitive function over time (Martin et al., 2022). Those investigations are advancing the literature in notable ways, but there also is utility in a more wholistic approach based on the confluence of both types of function. Gerontologists have developed concepts that summarize the “good life” during one’s later years such as healthy aging, successful aging, optimal aging, active life expectancy, and selective optimization with compensation. Scores of programs and university centers use such concepts in their names (e.g., healthy aging, successful longevity). Despite the widespread colloquial use of such terms, efforts to specify the meaning of these concepts precisely have not converged or, in some cases, sparked sharp criticism. The most prominent example is Rowe and Kahn’s (1987, 1997) articulation of successful aging, which continues to draw rebuttals on many grounds, including the implied competition (successful vs unsuccessful), the assertion that disease or disability disqualifies one from aging successfully, and the difficulty in applying the concept across cultures (Calasanti & King, 2021; Martin et al., 2015; Martinson & Berridge, 2015).

In this Feature article, we introduce the concept of dual functionality to identify older adults who maintain both physical and cognitive function, despite the challenges of senescence. Dual functionality during late middle age and early later life is highly desirable, but dual functionality during advanced ages is often judged as exceptional, in part, because it becomes less common. Who are these older adults who can maintain dual functionality into their later years? What early- and mid-life exposures are associated with the likelihood of dual functionality? These questions motivate and help frame this inquiry.

Unlike other conceptual frameworks, our concept of dual functionality is not predicated on the absence of a disease or syndrome. Many people may have a disease but nevertheless grow older without it compromising their physical and cognitive functioning (e.g., Raynaud’s disease). In describing the morbidity profiles of participants in the New England Centenarian Study, Evert et al. (2003) identified three sets of people who reached at least 100 years of age: survivors, who had an age-associated disease before age 80; delayers, who did not manifest a disease until at least age 80; and escapers, who reached 100 without disease. All three types represent remarkable human beings, and perhaps the persons who maintain dual functionality despite having an age-associated disease are those who are exemplars of adaptation and resilience when facing a disease.

In evaluating the compression of morbidity hypothesis, Crimmins et al. (2021) concluded that “there is little evidence that the decline in mortality and increase in life expectancy at older ages is primarily due to a delay in the morbidity process” (p. 412). Crimmins and Beltrán-Sánchez (2011) argued a decade earlier that many “diseases are both less lethal and less disabling; they have become more chronic but perhaps less progressive” (p. 82). Indeed, it could be argued that some of the most exceptional people have found ways to overcome a disease or delay a loss of function. We identify the conceptual roots of dual functionality and assert that the concept is useful for discussing trends in population aging, compression of morbidity, and quality of life during the later years. In addition, a focus on dual functionality—and positive images of aging—may act as a counterforce to ageism in many societies.

Conceptual Roots of Dual Functionality

Our conceptualization of dual functionality is related to and informed by many other concepts used to describe a favorable aging experience. Nevertheless, it is distinct, and we identify how in relation to three concepts: exceptional longevity, successful aging, and active life expectancy.

First, the term exceptional longevity has garnered currency in gerontology, but authors use the term to mean different things. Many scholars refer to exceptional longevity as simply reaching an advanced age, typically 85, 100, or more for humans (Barzilai et al., 2003; Perls & Terry, 2003). Others use the term for long-lived individuals but also specify that a measure of functional status should be a second criterion for exceptional longevity (Christensen et al., 2008; Pignolo, 2019). By doing so, the emphasis shifts from attaining a specific age to attaining that age while living independently, which is similar in some respects to dual functionality. A third category of scholars use the term exceptional longevity to describe outstanding function during later life. For instance, Gondo et al. (2006) examined three types of function (sensory, physical, and cognitive) among Japanese centenarians and concluded that only 2% were exceptional. Others have studied changes in physical and cognitive function to identify an “exceptional subgroup,” defined as remaining independent after 90 years of age (Santanasto et al. 2020). Each of these studies makes a notable contribution to research on longevity, but it is difficult to intuit the meaning of “exceptional longevity” because the term is used in different ways.

Second, successful aging is a term that has been widely used in gerontology (Martin et al., 2015; Pruchno et al., 2010). In the inaugural issue of The Gerontologist, Havighurst (1961) called for a theory of successful aging that emphasized life satisfaction and happiness. Rowe and Kahn (1987, 1997) used successful aging to integrate living to advanced age with other criteria: avoiding disease and disability, high cognitive and physical function, and engagement in social life. Their specification was much more encompassing and, therefore, exclusive than what we propose for dual functionality. In our view, having a disease does not necessarily lead to a loss of function; many people live with a disease for decades without major incident. Instead, we focus on function. Medications, surgeries, assistive devices, and supportive social relations enable many people with a disease to continue their lives without being dependent on others for daily living. As Maddox (1987) concluded in his Kleemeier Award Lecture, the aging process is, to some degree, modifiable. Baltes (1993) focused on cognitive function and described selective optimization with compensation as a model of successful aging. Age-related illnesses were important as a frame for social comparison, not as something that preempts flourishing, but as a trigger for compensation. Rowe and Kahn specified high cognitive and physical function for successful aging, whereas we are interested in a minimal threshold to be functional. High function is a desired quality but not a sine qua non for the independent living and quality of life to which most people aspire.

The third related concept is active life expectancy, pioneered by Katz et al. (1983) to estimate “expected remaining years of functional well-being” (p. 1218). Life expectancy is an invaluable measure of how many years people are expected to live, but it is silent on the quality of those years. Building on the extensive use of life-table methods to study mortality, Katz and colleagues addressed this limitation of life expectancy by integrating population-level health information on both mortality and activities of daily living (ADL) to estimate remaining years without an ADL limitation. Although their initial formulation estimated transitions from alive and fully independent to ADL-dependent, institutionalized, or death, subsequent articles have created separate estimates of active life expectancy and disabled life expectancy (Guralnik et al. 1993). More recent works examined additional transitions by terracing levels of ADL, instrumental ADL (IADL), and/or functional limitations to provide life-table population estimates of active life expectancy (Montez & Hayward, 2014; Zhang et al., 2019). The approach has been most informative for assessing trends in a population or major subgroups because it models the tandem risks of functional loss and mortality. For example, an ambitious collaborative project by leaders of the Global Burden of Diseases, Injuries, and Risk Factors Study (GBD, 2018) identified health-adjusted life expectancy based on hundreds of diseases and associated disability.

A parallel development to active life expectancy is life expectancy based on cognitive function. This includes studies of dementia-free life expectancy (Ritchie et al., 1994), life expectancy with cognitive impairment (Suthers et al., 2003), and cognitive health life expectancy (Garcia et al., 2021). We need continued research on each type of functionality—physical and cognitive—but also contend that research integrating these two streams of research can be notably useful for benchmarking public health progress. Instead of separate analyses of physical and cognitive function, studying humans who can maintain both types of function during later life provides a more complete picture of the quality of years associated with longevity.

There are some studies that examine “combined cognitive and physical decline” (Atkinson et al., 2005) or include some measure of cognitive function into estimates of active life expectancy. As an example of the latter type, Montez and Hayward (2014) capitalized on the respondent’s attribution of difficulty in ADL, IADL, or functional limitations “because of a health or memory problem” (p. 419). Given that memory problems are integral to completing IADLs (e.g., using a telephone, taking medications), this was an early step toward incorporating cognitive status into active life expectancy. These studies suggest that gerontology is ripe for a more substantial integration of streams of research on physical function and cognitive function by developing the concept of dual functionality.

What Do We Mean by Dual Functionality?

Dual functionality is a state of human existence in which people are free of both physical and cognitive impairment. This is typically assessed by the completion of age-appropriate tasks of everyday living. Given our focus on function, there is no requirement in this definition that people are free of disease. In addition, we do not posit that dual functionality is essential for quality of life. We speculate that there are many people who have dual functionality but nonetheless consider their quality of life to be poor, and many people have lost dual functionality, yet have high quality of life. Dual functionality is more closely related to independent living than quality of life, yet it has implications for how people evaluate quality of life.

It should be noted that there are young people who do not have dual functionality, most likely due to a congenital condition or severe injury. Following Verbrugge and Jette (1994), we refer to people who lost dual functionality early in life as those facing “lifelong” functional loss. In this essay, however, we focus on dual functionality beginning during adulthood.

It is also important to recognize that the loss of dual functionality may be temporary. People who have experienced a serious injury or major surgery may face the loss of dual functionality for a matter of weeks or months (e.g., joint replacement). Similarly, any hospitalization or acute care for a disease may leave patients with short-term delirium or cognitive confusion. If physical or cognitive function does not return within a year, however, it becomes increasingly likely that the problem is not a bout of impairment but an enduring one. Future investigations could capitalize on medical records or longitudinal survey data to analyze predictors of the duration of the loss of dual functionality or the probability of transitioning back to dual functionality.

Although growing older with both types of function is an aspirational goal, dual functionality is generally less prevalent in later life. Figure 1 displays three hypothetical Venn diagrams depicting how dual functionality becomes less prevalent during adulthood. Large swaths of the population have dual functionality during middle age, but this begins to reduce during late middle age and again during later life. Those persons who maintain dual functionality in later life are remarkable; consistent with prior studies, we might refer to them as manifesting exceptional longevity based on maintaining both types of function at advanced ages.

Figure 1.

Venn diagrams of dual functionality during middle age and later life.

We assert that the concept of dual functionality is a straightforward idea that bears resemblance to many laypersons’ views of the desirable years during later life. Instead of a long and abstract list of criteria for successful or healthy aging, dual functionality is built on two widely understood concepts. In everyday vernacular, people unwittingly allude to dual functionality with statements such as “he is fine physically, but his mind doesn’t work anymore” or “her mind is sharp, but she now requires a wheelchair to get around.” People prioritize these two domains of function.

In terms of aspirational aging, research on valuation of life clarifies that major health problems during later life are associated with a preference for a shorter life (Bowen & Skirbekk, 2017). As Lawton et al. (1999) observed decades ago, people “discount years of life marred by functional and cognitive impairment or pain” (p. 413). Most people desire to control the “when and how” of death (Lang et al., 2007), and the loss of either physical or cognitive function may reduce the desired length of life.

Measuring Dual Functionality

The concept of dual functionality requires a strategy to measure being free of both physical and cognitive limitations. Our approach is to identify established measures of each type of function, then examine who manifests both functions. We also were careful while selecting our measures to be sensitive to population diversity. We summarize our approach herein, with measurement details provided in our recent work (Bauldry et al., 2023).

To measure physical function, we used data from the Health and Retirement Study (HRS) to identify persons who did not report difficulty completing five ADLs: bathing, eating, dressing, walking across a room, and getting in/out of bed. The inability to perform these tasks may be reported by the participant or, in a small percentage of cases, a proxy respondent. Difficulty completing these five tasks reveals one or more bodily conditions that limit “people’s abilities to act in necessary, usual, expected and personally desired ways in their society” (Verbrugge & Jette, 1994, p. 3). Limitations in ADLs are widely used in research and for long-term care planning, making them a straightforward way to assess physical function. We acknowledge that ADLs are self-reported in most surveys, which may introduce bias if respondents misrepresent their physical function. Yet, research has shown that performance scores manifest “a very strong association with measures of self-reported disability” (Guralnik et al., 1994).

To measure cognitive function, we used the HRS series of tests from a modified version of the Telephone Interview for Cognitive Status. Tests include 10-word immediate and delayed recall tests of memory, a serial-seven subtraction test of working memory, and counting backwards to assess attention and processing speed. (We did not use the HRS measures of subjective memory or memory decline, relying instead on tests of cognitive performance.) We then used the Langa–Weir Classification to identify respondents in one of three categories: normal, cognitive impairment without dementia (CIND), and dementia (Crimmins et al., 2011; Langa et al., 2020). We identified persons who were free of dementia as the threshold for our examination of dual functionality.

Respondents were classified as having dual functionality if they had no difficulty in the five ADL tasks and were free of dementia. We also considered alternative indicators of dual functionality before finalizing our measurement. For instance, we estimated dual functionality with more selective cutoffs (i.e., excluding persons with CIND or those with an IADL limitation from being defined as having dual functionality). As one would expect, the more indicators one includes in the measurement of dual functionality, the more exclusive are the estimates of its prevalence. Also recognizing the controversy over the accuracy of CIND among minority adults, especially those with limited schooling, we were reluctant to use it for measuring dual functionality (Chin et al., 2011; White et al., 2022). Our approach is more inclusive because it identifies a minimal threshold for dual functionality consistent with studies of active life expectancy and dementia-free life expectancy. Given the widespread use of those measures, it is a logical next step to build on those literatures. Using these criteria, 88% of the HRS respondents manifest dual functionality for at least one wave.

Sequelae of the Loss of Dual Functionality

Multiple changes ensue after the loss of either physical or cognitive function. We highlight three main changes that follow either type of loss. First, the loss of dual functionality triggers social readjustment to address the functional loss. Someone will be needed to help address difficulty in ADLs, manage cognitive functioning, and advocate for the person experiencing the loss. Household members, if available, and family and friends in the person’s network may offer support. The person’s social network may adapt by directly providing, or purchasing, personal care and/or searching for alternative living arrangements. Although this adaptation often involves a primary caregiver, recent research reveals that a shared care network is another common approach, which may be beneficial for addressing the needs of individuals experiencing loss of function (Hu et al., 2022).

Second, the loss of dual functionality signals accumulating risks, including the loss of the second function (Ferraro & Shippee, 2009; Martin et al., 2022). The inability to perform ADLs or complete a battery of cognitive tests may mean that other tasks become more challenging for the individual, resulting in a cascade of additional concerns (e.g., falls, low morale). Compromised function in one domain also may spill over to other domains resulting in the accumulation of “secondary conditions and dysfunctions” (Verbrugge & Jette, 1994, p. 7). This may not be manifest as a rapid set of transitions. Good caregiving, use of assistive devices, and engaging activities may delay the onset of secondary conditions, and the primary caregiver or the shared care network may seek therapies and practices to delay and/or reduce the anticipated impact of accumulated risks.

Third, the loss of dual functionality may spark a reconsideration of existential questions and lead to revised life goals and priorities. What is the meaning of life after the loss of physical or cognitive function? What is most important now that dual functionality has been compromised? Carstensen’s (2021) development of socioemotional selectivity theory identified how “the approach of endings” leads one to reconsider emotionally meaningful goals. The loss of dual functionality may be perceived as such an “ending” because of one’s dependence on others as well as a growing awareness of finitude. The feeling that one’s time horizon is shrinking also may lead to reprioritizing life goals. For instance, one emotionally meaningful goal is to maintain social relations, albeit perhaps more selectively. In the convoy model of social relations, there are changes in the network structure, with the inner circle in later life involving more family members (Antonucci & Akiyama, 1987). Recent research on social connections in later life reinforces the idea of “rebalancing” social networks by adding new members to compensate for network losses (Cornwell et al., 2021). The will to revise meaningful goals and priorities is an act of human agency to seek the quality of life that the person values, thereby preserving some degree of autonomy despite the currents of functional loss.

Revisiting existential questions and aspirations during one’s later years is replete with choices. For instance, some people may consider using an assistive device in public to be an acceptable trade-off because it enables wider participation in social life, especially when undergirded by policy (e.g., Americans with Disabilities Act). Others may resist using an assistive device in public, despite it being clinically prescribed. Also, some people view an “independent” life as higher quality than one in which assistance is necessary, while others find high value in life regardless of the need for medical or assistive devices.

Although the three main sequelae of the loss of dual functionality were considered separately, there is ample overlap in them. Some changes are initiated by others to address the loss of function while other changes are initiated by the older person who seeks some level of control over life that may seem overwhelming. There will likely be some rebalancing of life goals and network members, and the conduct of those network members delivering the care is critical to well-being during a time rife with change.

Utility of Dual Functionality

Dual functionality is core to understanding how well a society’s older adults are faring during their later years. It provides a straightforward measure for research that equates well with how laypersons view a good life during their later years. It also shines a light on remarkable older adults. In discussing the utility of dual functionality, we also briefly suggest a few fruitful lines of inquiry.

First, we are accustomed to using life expectancy as an essential indicator of anticipated longevity, but dual functionality may be used as a complementary measure to identify those who are able to maintain both physical and cognitive function during their later years. If scientists observe that rates of dual functionality increase over time, this would be compelling evidence of improving functional health in the older adult population. If dual functionality decreases over time, however, that finding should stir public health initiatives to ask why it occurred and what are the most effective interventions to ameliorate the problem. A national goal should be to see rising rates of dual functionality.

Second, beyond the concept of dual functionality measured as whether one has both types of function, we developed a measure of age-50 dual-function life expectancy, which provides an estimate of the number of years remaining free of significant physical or cognitive limitations (Bauldry et al., 2023). If a person lives to age 50, how many additional years can a person expect to live with dual functionality? To address this question, we used three data sets: (a) the public-use National Health Interview Surveys (NHIS), (b) the NHIS Linked Mortality Files, and (c) the HRS. We used the first two data sets to generate estimates of mortality in 5-year age intervals sufficient to estimate life expectancy at age 50. We then used the HRS to obtain estimates of the prevalence of dual functionality in 5-year age intervals and Sullivan life tables to estimate dual-function life expectancy at age 50 as well as the percent of remaining life with dual functionality (Bauldry et al., 2023).

Third, one may use dual functionality and dual-function life expectancy as critical indicators of health inequality within and across societies. Both may be used to assess “health gaps” between groups of people in a society. In our recent work, we also found a substantial racial–ethnic gap in dual-function life expectancy, especially for women. White women in their mid-70s experience comparable levels of dual functionality as Black and Hispanic women in their early 50s (Bauldry et al., 2023). In addition to assessing change over time, dual functionality is a useful indicator for identifying health inequalities on multiple axes of stratification (race–ethnicity, gender, socioeconomic status) with the goal of reducing and eliminating those inequalities. Gender and race-ethnic disparities are based on ascribed characteristics, but the need for assistance with ADLs or memory among older people is an emergent phenomenon, meaning that there may be substantial variability in how people respond to assistance, both in terms of identity and function.

Finally, although dual functionality is less prevalent in later life, we do not assume that the loss of dual functionality is an absorbing state. As noted earlier, some people may temporarily lose physical or cognitive functioning for a season, perhaps after major surgery, but return to dual functionality thereafter. We propose that even the long-term loss of dual functionality may be reversed and call for research examining the predictors and consequences of the duration of dual functionality and transitions in and out of it.

Implications

The concept of dual functionality is one way to integrate parallel streams of research on exceptional longevity, optimal aging, and active life expectancy. We do not propose that dual functionality is a replacement for research on any of those topics but that it is a useful complement to those existing inquiries. Dual functionality opens a new way of studying remarkable people who have been able to retain physical and cognitive function. The concept also raises the question as to why some individuals experience so few years of dual functionality and what are the contextual factors that explain gaps in the outcome across subpopulations.

Dual functionality is a more wholistic measure of health than life expectancy and bears a resemblance to layperson views of desirable later years. The concept is positioned on the “big two” types of function. Others may want to integrate additional criteria to create a more abstract concept, but it may end up being less inclusive. As Strawbridge et al. (2002) documented decades ago with the Alameda County Study, fewer than 20% of adults 65 years or older met the Rowe and Kahn (1997) criteria to be classified as successful aging. Instead, we integrate two major types of function for a minimal threshold of dual functionality, which is observed in 84% of the women and 88% of the men 65 years or older in the HRS.

Making higher rates of dual functionality a national goal is one way to respond to the call of President John Kennedy to “add new life” to the later years. One specific way to do so is incorporate dual functionality and/or dual-function life expectancy as indicators of “healthy longevity” in the National Academy of Medicine (NAM) Global Roadmap. Most gerontologists enthusiastically endorse NAM’s (2022) first overarching principle that “people of all ages, particularly older adults, reach their full potential to live life with good health, functioning, meaning, purpose, and dignity” (p. 3). Yet, the question of how to measure “healthy longevity” remains. By focusing on people who maintain dual functionality in their later years, we connect a national goal to an aspiration held by most humans to retain function sufficient to participate in and contribute to society.

Funding

Support for this research was provided by grants from the National Institute on Aging to K. F. Ferraro (AG043544 and AG068388).

Conflict of Interest

None declared.

Data Availability

The authors do not report data and therefore the preregistration and data availability requirements are not applicable.