This report [PDF] presents measures of household income inequality for counties in the United States, based on data pooled from 5 years (2006 to 2010) of American Community Survey (ACS) data. For example, the data show that the more unequal counties were also more populous. Thirty-four percent of Americans lived in a county that ranked in the top 20 percent of U.S. counties by Gini index (the Gini index is a measure of income inequality).
In every region, the counties in the most unequal fifth of U.S. counties accounted for a disproportionately large share of that region's population. For example, only 8 percent of Midwestern counties had Gini indexes ranking among the top fifth of U.S. counties, but they contained 26 percent of the region's population. Also, the South region had a disproportionately large number of counties with high income inequality, while counties in the Midwest had lower levels of income inequality. Specifically, 32 percent of the counties in the South had Gini indexes ranking among the top fifth of U.S. counties, while 31 percent of Midwest counties ranked among the bottom fifth of U.S. counties.