The following graph depicts inequality in income distribution using the Gini index, measuring household income inequality for counties in the United States, based on American Community Survey data pooled from 5 years (2006 to 2010).
Recall the Gini index varies between 0, indicating perfect equality, and 1, revealing perfect inequality. An absurd example might help to understand the Gini index. If 10 people lived in a county and each person had $100 in income, the Gini index would be equal to 0. On the other hand, if only one person had $1000 and the other nine people had 0 income the Gini index would be equal to 1. In the above graph the darker the color the greater the measured inequality.
The Census Bureau observes,
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.
In a separate post, Geographic Concentration of Inequality: Fifteen Counties Contributed All of the Rise in Inequality, I summarized James K. Galbraith’s study demonstrating the overall level of income inequality in America is concentrated in just a few counties.
“Basically, fifteen counties contributed all of the rise in inequality measured between counties from 1994 to 2000, meaning that if they had been removed from the dataset the rise in overall inequality would not have occurred. Of these, just five (New York; three counties in Northern California associated with the Silicon Valley; and King County, Washington) contributed about half of the rise in total inequality, again measured between counties, in the late 1990s.”
US Census Bureau. U.S. Department of Commerce. http://www.census.gov/prod/2012pubs/acsbr10-18.pdf