Frank Manzo IV is the Policy Director of the Illinois Economic Policy Institute (ILEPI). Visit ILEPI at www.illinoisepi.org or follow ILEPI on Twitter @illinoisEPI. This is part of the “Frankonomics” series.

Illinois’ middle class is relatively strong in 93 of the state’s 102 counties, according to the Measure of America by the Social Science Research Council.

One of the many outcomes included in the Measure of America is the Gini coefficient for every county across America. Gini coefficients are the most commonly used measure of income inequality. The coefficient ranges from 0, which represents complete equality, to 1, which reflects complete inequality (i.e., one person has all the income).

For the United States, the national average Gini coefficient is 0.469. Fully 93 out of 102 counties in Illinois have a Gini coefficient below the national average.

With assistance from diymaps.net, I quickly categorized Illinois’ counties into groups, from those with the highest Gini coefficient to those with the lowest Gini coefficient. The 26 counties in Illinois with the highest income inequality are further broken down into two groups: those in red are the nine counties with higher Gini coefficients than the rest of the nation while those in yellow fall below. The 25 light green (0.420-0.439) and 25 dark green (0.400-0.419) are relatively more equal, while the 26 blue counties have the least income inequality in Illinois – with Gini coefficients under 0.4.

Counties with large universities (Champaign, Coles, Jackson, Peoria) tend to have higher Gini coefficients than the U.S. average. This is probably because professors and administrators with advanced degrees push the income distribution in one direction, while poor college students push it in the opposite direction.

However, perhaps the most important finding is the fact that Cook County, which contains 40.7% of the state’s residents (including yours truly), is one of the 9 counties that is more unequal than the rest of the nation. This is due to a litany of factors, including racial segregation in Chicago and a market concentration of securities/commodities and legal services jobs which respectively pay $194,022 and $82,319 to employees on average.

On the whole, the Chicago area has greater income inequality than the rest of Illinois– although workers in the Chicago region earn higher hourly incomes.

The table below summarizes data from the Current Population Survey (Merged Outgoing Rotation Groups), published annually by the U.S. Census and U.S. Bureau of Labor Statistics. In 2014, the average hourly income of workers in the Chicago area was $24.17 compared to $20.56 outside of Chicago. The median worker earned $18.75 per hour in Chicagoland and $16.83 per hour in the rest of Illinois. However, in order to be part of the Top 1 Percent in Cook County and the surrounding areas, a worker would have to earn at least $99.90 an hour. For the rest of the state, the equivalent amount is $79.92 per hour in wage and salary income.

Folks in the Top 1 Percent earned at least 5.3 times as much every hour as the median worker in the Chicago area. Outside of the Chicago area, someone in the Top 1 Percent “only” earned 4.7 times as much per hour as the typical worker.

Lower levels of income inequality are also likely explained in part by the fact that the share of workers represented by a labor union is actually higher outside of the Chicago area. Greater unionization has been found to be highly correlated with less income inequality. In the Chicagoland economy, 15.6% of workers are covered by a labor union. In the rest of the state, unions represent 17.3% of the workforce.

Illinois is one of the most equal states in the nation, except for 9 important counties. Below is the Measure of America map generated for all U.S. counties by Gini coefficient. Darker shades of green are the most unequal counties in America, while lighter shades are the most equal. While 93 out of 102 Illinois counties fit right in with the low Gini coefficients throughout the Midwest, Cook County “looks” like it belongs in the Deep South where inequality is highest.

There are strong economic reasons to address income inequality. Policies aimed at reducing income inequality in Illinois must start by strengthening and rebuilding the middle class in the state’s most populous county.