No association of income inequality with adult mortality within New Zealand: a multi-level study of 1.4 million 25-64 year olds

Study objective: To determine the association of regional income inequality within New Zealand with mortality among 25-64 year olds. Design: Individual census and mortality records were linked over the 1991-94 period. Income inequality (Gini coefficients) and average household income variables were calculated for 35 regions. "Individual level" variables were sex, age, ethnicity, household income, rurality, and small area socioeconomic deprivation. Logistic regression was used for the analyses. Sensitivity analyses for the level of regional aggregation were conducted. Participants: 1.4 million New Zealand census respondents aged 25-64 years followed up for mortality for three years. Main results: Controlling for age, ethnicity, rurality, household income, and regional mean income, there was no association of income inequality with all cause mortality for either men (OR= 1.007 for a 0.01 increase in the Gini, 95% confidence intervals 0.989 to 1.024) or women (OR= 1.004, 0.983 to 1.026). By cause of death (cancer, cardiovascular disease, unintentional injury, and suicide) there was some suggestion of a positive association for female unintentional injury (OR= 1.068, 0.952 to 1.198) and suicide (OR= 1.087, 0.957 to 1.234) but the 95% confidence intervals all included 1.0. Failure to control for ethnicity at the individual level resulted in some association of increasing regional income inequality with increasing mortality risk. Using fewer (n= 14) or more (n=73) regional divisions did not substantially change the findings. Conclusion: There is no convincing evidence of an association of income inequality within New Zealand with adult mortality. Previous ecological analyses within New Zealand suggesting an association of income inequality with mortality were confounded by ethnicity at the individual level. However, this study does not refute the possibility that income inequality at the national level affects health.