The association between income inequality and all-cause mortality across urban communities in Korea

Data on household income were from Korean Community Health Survey (KCHS) and data on mortality rate and control variables (community population, percent of over 65 years in community population, ratio of social welfare expenditure to the general budget, number of physicians per 1,000 individuals, smoking rate, and drinking rate) in 2010–2012 were from KSIS.

Data on household income were obtained from the 2010–2012 KCHS conducted in 253 communities: 167 urban communities and 86 rural communities. KCHS is a nationwide health interview survey carried out by the Korean Centers for Disease Control and Prevention (KCDC) to estimate patterns of disease prevalence and morbidity, as well as to understand the lifestyle and health behaviors of adults aged 19 years and over [15]. The KCHS uses a multistage sampling design to obtain a representative sample. Within each of the 253 communities, 90 primary sampling units (PSUs) corresponding to smaller geographic entities were randomly selected; this was followed by the random selection of five to eight households within the PSU and in-person interviews with all adults in those households [15]. The KCHS database of surveys from 2010–2012 contained pooled data from 687,376 interviews (2010: 229,229; 2011: 229,226; 2012: 228,921). For this study to collect one household income data in each household, the first members of each household (338,045 total subjects) were considered, and among them, residents in cities (206,013) were selected. We excluded 16,908 individuals due to insufficient information on income and excluded 16,707 whose household income was less than the 5th percentile or higher than the 95th percentile in their community to remove outliers and extreme values. Finally, a total of 172,398 individuals were included in the analysis. The protocol of community health survey was reviewed and approved by the Institutional Review Board of KCDC (2010–02–CON–22–P). Written informed consent was obtained from all participants in the community health survey.

Data on mortality rate and other covariates in 2010–2012 were obtained from KSIS. KSIS collected various financial and social data on 250 communities: 167 urban communities and 83 rural communities. The data evaluated included: a) age-standardized all-cause mortality per 100,000 individuals, b) community population, c) percent of over 65 years in community population (%), d) ratio of social welfare expenditure to the general budget (%), e) number of physicians per 1,000 individuals, f) smoking rate, and g) drinking rate.

KCHS and KSIS definitions of urban communities differed; in at least one instance, a community considered a single entity by KCHS was defined as two by KSIS. Furthermore, boundaries and names of some administrative districts changed from 2010 to 2012. For example, Yeongi-gun became Sejong city, the administrative capital, and Changwon city was consolidated with Masan and Jinhae. After excluding communities whose definitions differed between databases or changed over the study period, a total of 157 urban communities were included.

Household income was measured through KCHS and was defined as income before payment of taxes and receipt of benefits. We used three alternative measures of income distribution: the Gini coefficient (GC), the Robin Hood index (RHI), and the income share ratio between the 80th and 20th percentiles of the distribution (80/20 ratio). The GC is calculated as the ratio of the area between the actual income distribution (the Lorenz curve) and the diagonal enclosing the distribution curve to the total area under the diagonal. Higher GCs mean greater income inequality and range from 0, meaning perfect equality, to 1, perfect inequality [3]. The RHI was estimated from state-specific data on share of total household income arranged by tenths of the distribution. This value approximates the share of total income that has to be taken from those above the mean and transferred to those below the mean to achieve equality in the distribution of incomes. The higher the value of the index, the less egalitarian is the distribution of income [16]. The 80/20 ratio is calculated as the income earned by the top 20 % of households divided by the income earned by the poorest 20 % of households. Decile ratios (90/10, 80/20 and 50/50) are known to be simple and effective measures of income inequality. We used the 80/20 ratio according to previous studies [17].

Table 1 shows general characteristics of the 157 urban communities. The median value of GC, RHI, and 80/20 ratio were 0.318, 0.233, and 6.570, respectively. The median value of all-cause mortality per 100,000 was 416.3 and that of the community population was 280,277.0.

When we graphed income inequality measures versus all-cause mortality as scatter plots, the R square values of the regression lines for GC, RHI, and 80/20 ratios relative to mortality were 0.230, 0.238, and 0.152, respectively (Fig. 1).

Table 2 lists all-cause mortality rates according to quartiles of income inequality measures. After adjusting for covariates such as community population, percent of over 65 years old in community population, ratio of social welfare expenditure to the general budget, number of physicians per 1,000 individuals, and smoking and drinking rates (Model 2), mean all-cause mortality increased significantly with increasing income inequality (GC: P for trend < 0.001, RHI: P for trend < 0.001, 80/20 ratio: P for trend < 0.001). After additionally adjusting for median household income (Model 3), mean all-cause mortality increased significantly with increasing GC (P for trend = 0.014) and RHI (P for trend = 0.031), and increased marginally with 80/20 ratio (P for trend = 0.067).