Between 1990 and 2016, over 180 countries and territories enjoyed an increase in life expectancy (LE) (1). However, we are not sure whether the added years of life are spent in health (2, 3). Investigating the relationship between morbidity, disability, and mortality could help us understand the complicated process of health changes and its impact on the health and social care needs (4). Many studies have made effort to analyze the health changes by examining three alternative theoretical hypotheses: compression of morbidity, expansion of morbidity, and dynamic equilibrium hypothesis (2, 5–7). The majority are conducted in developed countries and comparative studies are largely lacking (8). Meantime, our knowledge about the relationship between income inequality and health changes is very limited.
The compression of morbidity hypothesis argues that there is a natural limit to life span and, by adopting healthy lifestyle, the onset of chronic diseases can be delayed or even prevented, which results in a shorter life with morbidity (9). The expansion of morbidity hypothesis states that, medical interventions might increase years lived with morbidity by saving those with serious chronic conditions from dying and making more people survive to the oldest-old stage when the risks of morbidity are high (10, 11). Manton (1982) proposed an intermediate alternative, dynamic equilibrium hypothesis (12). When disability is regarded as severe morbidity, the life years with morbidity increase as mortality decreases and those with disability keep relatively constant as the medical interventions or life style changes reduce the progression rate of chronic diseases. Theoretically, this process will prolong the life years without disability.
The scenarios proposed by these hypotheses seem to coexist in the real world, although most evidences are from developed countries (2, 13). For example, in the United States, the disability-free LE at age 65 increased from 6.6 years in 1970 to 9.3 years in 2010, with its proportion in total LE rising from 50.8–52.5% (4). On the other hand, the expansion of morbidity appears in Germany and England (8, 14). Moreover, Jagger et al. (2016) reported mixed results for the same population: absolute compression of cognitive impairment, relative compression of self-perceived health, and dynamic equilibrium of disability (2).
The variation in health changes across populations is partly due to the different study designs, such as the data sources, health measurements, and estimation methods (2, 15). In other words, we cannot capture the global distribution of health changes by reviewing existing literatures, as their conclusions are not completely comparable. Thus, a comparative study covering a wide range of countries and territories is necessary. However, with some exceptions(16), existing studies mainly focus on single country or several countries, especially high-income countries (2, 4–6).
Is income inequality associated with health changes? The level of income affects the relationship between morbidity, disability, and mortality. People exposed to harsh economic conditions live a shorter disease-free and disability-free LE compared with their counterparts, especially for older women(17). Moreover, education attainment, a factor closely related to income, affects the health changes. In the United States between 1980 and 1990, people with higher education attainment enjoy compressed morbidity, while their counterparts experience the opposite (18). However, there is no direct evidence that the degree of income inequality matters for health changes.
Furthermore, although a large body of literature has examined the income inequality hypothesis that income distribution within a society influences population health with greater inequality associated with worse outcomes (19, 20), it is a different research question from the one raised in this study. The health outcomes in these literatures are commonly for single health domains, e.g., life expectancy and obesity prevalence (20). By contrast, the health changes, e.g., the compression and expansion of morbidity, reflect the complex relationship between the changes in morbidity and mortality.
Therefore, this study aims (1) to analyze the global distribution of compression and expansion of morbidity and (2) to investigate the relationship between income inequality and the compression and expansions of morbidity. Due to data limitation, this study does not examine dynamic equilibrium hypothesis.