Trajectory of functional status among older Taiwanese: Gender and age variations
Introduction
Extensive research has focused on gender differences in aging and health (Arber and Cooper, 1999, Macintyre et al., 1996, Verbrugge et al., 1989). Nonetheless, there is very limited understanding of how men and women differ in health dynamics in old age. In cross-sectional studies of health states, intrapersonal change cannot be distinguished from interpersonal differences. Studies of health transitions between two points in time have suggested that women are more likely to experience functional decline than men (e.g., Anderson et al., 1998, Mor et al., 1994). However, these studies provide little information on the underlying growth curve or trajectory, which consists of multiple health transitions over time (Singer & Willet, 2003). A more complete understanding dictates an analysis of how men and women differ in the level as well as the rate of change associated with a health trajectory. This is consistent with the notion of gendered health careers, which calls for the delineation of gender differences in the nature and range of health pathways over the life course (Moen & Chermack, 2005).
Health trajectories in late life, on the other hand, may differ significantly across age groups. Health transition studies have documented that older age is associated with a greater probability of functional decline and decreased odds of stability and improvement over time (Anderson et al., 1998, Crimmins and Saito, 1993). In addition, there is emerging evidence in the U.S. that younger cohorts enjoy better health. For instance, on average functional limitations in the U.S. declined by about 40% from the early 1900s to the 1990s, attributable to reduction in the debilitating effect of chronic conditions and the reduced rates of chronic conditions (Costa, 2003).
Although there is increasing research on gender differences in the trajectory of functional health, empirical findings are equivocal. Whereas a higher level of disability among older women than men was observed by many (Newman & Brach, 2001), this was not replicated consistently (Maddox and Clark, 1992, Mendes de Leon et al., 2005). Findings concerning gender differences in the rate of change are equally inconclusive. For instance, some investigators have observed that women experienced greater odds of functional decline than men (Anderson et al., 1998, Leveille et al., 2000, Taylor and Lynch, 2004), whereas others have reported the reverse (Maddox and Clark, 1992, Mendes de Leon et al., 2005). In addition, there is some evidence that gender differences are greater among the old–old than the young–old (Liang et al., 2008). Further research is clearly needed to clarify gender and age differences in the trajectory of functional health.
Moreover, the vast majority of studies is based on data collected in Western developed nations, particularly the United States. Because of substantial differences in demographic, epidemiological, and economic transitions, the generalizability of current findings across nations remains to be established. For instance, there is some indication that rates of disability and functional limitations across Asia and Latin America are on the rise (Ofstedal et al., 2007, Palloni and McEniry, 2007), which diverge substantially from observations of declining disability in old age in the United States (Freeman & Martin, 2000).
Replications across nations are indispensible for establishing the generality of findings and the validity of interpretations derived from single-nation studies (Kohn, 1989). In no other way can we be certain that existing empirical observations are merely the product of some limited set of historical, cultural, and political circumstances such as those existed in the Western developed nations. In addition, replications are very valuable in motivating the investigators to revise their interpretations to take into account of differences or inconsistencies that could never be uncovered in research done in a single nation.
In this regard, Taiwan serves as an ideal setting for further research on gender and age differences in health dynamics. Since the early 1950s, Taiwan has evolved from a poor agricultural economy to a newly industrialized society, with a gross domestic product (GDP) per capita of $31,900 (adjusted for purchasing power parity) in 2007 (Central Intelligence Agency [CIA], 2009). Largely because of substantial declines in fertility, Taiwan is aging very rapidly. Those aged 65 and older accounted for 10.2% of the total population in 2007, and it is expected to grow to 21.7% by 2027 (Council for Economic Planning and Development, 2008). Life expectancy at birth in Taiwan was 77.9 years in 2009, similar to that of the United States (CIA, 2009).
In comparison with older people in the Western developed countries, older Taiwanese are significantly less educated. As of 2008, 14.7% of the 65 and over population in Taiwan was illiterate (Ministry of Education, 2008). This is further exacerbated by substantial gender disparities, with 25.1% and 3.7% of older women and men being illiterate respectively (Ministry of Education, 2008). In addition, older men outnumbered older women in Taiwan until 2004, a legacy of the massive migration including a large number of troops brought by the Nationalist government during the civil war in 1949 (National Statistics, 2009, Ofstedal et al., 2007).
The current cohort of older Taiwanese have more children and are far more likely to reside with their children than their counterparts in Western developed nations. In 2005, 60% of those 65 of age and over lived with their children (Tsai, 2008). As old age pension programs are still not well developed, there is greater poverty among older persons in Taiwan than Western developed nations, with many of them depending on their children for significant financial assistance (Jesuit & Smeeding, 2003).
The present study aims to extend current knowledge of aging and health in three respects. We first chart the trajectory of functional health by using longitudinal data from a national sample of Taiwanese over 60 years of age for a period of 10 years (1993–2003). Second, we examine how the level and rate of change associated with the trajectory of functional status differ between men and women and across age groups. Finally, we explore whether the gender gap in functional status trajectory varies by age.
We offer several hypotheses grounded in the perspective of social stratification of aging and health (Crimmins and Seeman, 2001, House et al., 2005). In particular, social economic status (SES) is viewed as a function of age, gender, and ethnicity as they have profound implications for the distribution of wealth, prestige, and power. As a fundamental cause, SES shapes people’s exposure to many risk factors including social relations, health behaviors, and personality dispositions (Link & Phelan, 1995). More importantly, age, gender, and ethnic differences on health and well-being often cannot be fully explained by SES. Examples of these perspectives include age stratification (Riley, 1987) and gender stratification (Huber, 1990). Accordingly, gender and age are hypothesized to affect functional status directly and indirectly via SES, social support, and prior health. Social relations and health status are assumed to be time-varying, as they may change substantially if an extended period of observation is involved.
Hypothesis 1: Functional status worsens over time among older Taiwanese (H1).
Most of the studies of older Taiwanese relied on cross-sectional data (Chang and Zimmer, 2006, Chiu et al., 2005, Ofstedal et al., 2007, Wang et al., 2006). The limited number of longitudinal studies tended to focus on health transitions between two points in time, yielding little information on health trajectory (Beckett et al., 2002, Zimmer et al., 1998). According to recent longitudinal studies in the United States, functional status in old age deteriorates over time in a linear or quadratic fashion (Kahng et al., 2004, Kim and Durden, 2007, Liang et al., 2008). However, parallel observations in Taiwan are lacking. Extrapolating from observations made in the U.S., we hypothesize that functional status worsens over time among older Taiwanese (H1).
Hypothesis 2: Older Taiwanese women not only experience higher functional impairment than older Taiwanese men but also tend to decline functionally at a greater rate (H2).
There is a fair amount of research suggesting that older Taiwanese women experience more functional disability than men (Chang and Zimmer, 2006, Ofstedal et al., 2007, Zimmer et al., 2002). However, evidence on gender differences in changes in functional status is mixed. For instance, Hsu (2005) found older Taiwanese women more likely to develop difficulties with activities of daily living (ADL) than men over a six-year period, whereas others observed no gender differences in the risk of developing disability over time (Chiu et al., 2005, Wu et al., 1999). All of these studies focused on health transitions between two points in time. Little is known about how elderly Taiwanese men and women differ in the trajectory of functional status. In the U.S., findings regarding gender differences in changes in functional status are similarly equivocal (Anderson et al., 1998, Kahng et al., 2004, Mendes de Leon et al., 2005) and further research is required. We hypothesize that Taiwanese women not only experience higher functional impairment than men but also tend to decline functionally at a greater rate (H2).
Hypotheses 3: The level of disability and its rate of change are higher among the old–old Taiwanese than the young–old Taiwanese (H3).
Cross-sectional data suggest greater disability in Taiwanese seniors at advanced ages than the young old (Department of Statistics, 2006). Several studies of health transitions also showed that older age was associated with increased risk of functional decline and a lower likelihood of functional improvement among elderly Taiwanese (Chiu et al., 2005, Zimmer et al., 1998). Although there is some indication that the prevalence of disability is increasing among older Taiwanese (Zimmer, Martin, & Chang, 2002), there has been no direct observation of age differences in the rate of change in disability trajectory.
Age differences in the trajectories of functional impairment observed over time reflect a combination of cohort and age effects. According to Gruenberg (1977), even with increasing life expectancy, there is little change in the ages of onset of morbidity and disability. Hence there is little or no difference in disability between older cohorts and younger cohorts. In contrast, Fries (1983) proposed the notion of compression of morbidity in that the onsets of morbidity and disability have been delayed significantly in younger cohorts relative to older cohorts. Finally, even though declines in mortality may increase the prevalence of chronic diseases, the rates of progression for these diseases and thus disability may fall (Manton, 1982). Thus, there would be greater functional impairment among members of older cohorts compared to younger cohorts at the same ages. This is supported by recent research by Costa (2003) that age-specific prevalence rates of chronic diseases have declined substantially with a significant delay in the onset of chronic diseases during the 20th century.
In addition, within a given birth cohort, age plays an important role. Research on health transitions strongly suggests that age is associated positively with the risk of functional decline and negatively with functional improvement (Anderson et al., 1998, Crimmins and Saito, 1993, Mor et al., 1994). Accordingly, we hypothesize that compared to the young–old, old–old persons have greater functional impairment and experience more rapid functional decline over time (H3).
Hypothesis 4: Gender differences in both the level and rate of change in disability are greater among the old–old in Taiwan than the young–old (H4).
A key factor underlying the gender gap in disability is that women have more comorbidity and chronic health problems than men (Newman and Brach, 2001, Verbrugge et al., 1989). However, Crimmins and Saito (2000) found a significantly greater increase in most of the diseases among men than women in the United States. In addition, there is evidence that the impact of chronic diseases on disability has been reduced over the years (Freeman & Martin, 2000), suggesting a decreased gender gap among members of younger cohorts than older cohorts. This is supported by recent research in the U.S. that gender differences in functional decline were more substantial among the old–old than the young–old (Liang et al., 2008). This may be a result of improved education, occupation, and income among women in recent decades (Guralnik et al., 1993, House et al., 2005). There, however, has been no parallel research on older people in Taiwan. Extrapolating from observations from the U.S., we hypothesize that gender differences in both the level and rate of change in disability are greater among the old–old in Taiwan than the young–old (H4).
Section snippets
Data
Data for this research came from the Health and Living Status of the Elderly in Taiwan (HLSET) survey. Initiated in 1989, it involved a nationally representative 3-stage equal probability sample of 4049 Taiwanese 60 years of age or older (response rate = 91.8%). The respondents were followed in 1993, 1996, 1999, and 2003 with a response rate ranging from 87% to 92%. Whenever possible, proxy interviews were conducted for individuals who were unable to complete the survey themselves. Deaths
Results
To evaluate the hypotheses, we took the following steps. First, we charted the trajectory of disability over time (H1) without any covariates (M0 in Table 2). Second, we examined the disability trajectory by adjusting for proxy interview, mortality, and attrition as confounding variables (M1). Third, we added measures of social stratification including age differences, gender, age by gender interaction, and ethnicity (M2). The intent was to evaluate their effects on disability trajectory as
Discussion
A key contribution of this research lies in its focus on the dynamics of functional status in old age in Taiwan, a non-Western society. Approximated by a quadratic function, disability increases over time with an accelerated rate among older Taiwanese. Women and the old–old experience not only greater disability but also more rapid functional decline. Moreover, old–old women bear a disproportionately larger burden of disability. Finally, gender and age differences in the trajectory of
Acknowledgements
This research was supported by grants R01-AG015124 and R01-AG028116 (Jersey Liang, PI) from the U.S. National Institute on Aging. The National Bureau of Health Promotion in Taiwan and the Michigan Claude D. Pepper Older Americans Independence Center (P60-AG08808) provided additional support.
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