Objectives: This study uses the counties of Texas to empirically test the predictions of Wilkinson's theory on the role of income and inequality in explaining health differentials in populations. Wilkinson predicts (1) that health is affected more by income inequality than average income in areas with large population, and (2) that health is affected more by average income than income inequality in areas with small population. We investigate how large the population of a unit must be for income inequality within the unit to affect mortality.
Methods: Measures of income inequality were computed from the 1990 U.S. census data and mortality was computed from Vital Statistics data. Poisson regressions estimated the age-adjusted relative risk of the top quintile relative to the bottom quintile for equality and for income among selections of Texas counties based on population size. County ethnic composition, educational level, and health care access were controlled for.
Results: Among counties with populations greater than 150,000, the risk of death was lower in counties with more equal income distribution than in counties with less equal income distribution. Among counties with population less than 150,000, median income affected relative risk in counties with less than 30 percent Hispanics, but not in those with more than 30 percent Hispanics.
Conclusions: This study provides some support for the predictions of Wilkinson's theory.