In 1980, 23% of U.S. lower-income households lived in majority low-income neighborhoods; in 2010, that had risen to 28%. At the other end of the economic scale, the share of upper-income households living in majority upper-income neighborhoods doubled, to 18% in 2010 from 9% in 1980.

The increase in income segregation is a contrast to the long decline in black-white racial segregation in neighborhoods. Why would income segregation be rising? The major reason is that the share of neighborhoods that are predominantly middle income or mixed income has declined, from 85% in 1980 to 76% in 2010. In turn, the shares that are majority lower income or upper income have grown.

The idea behind neighborhood-level segregation analysis is to study the potential exposure of people to others from different groups. But census tracts located outside metropolitan and micropolitan areas can include so much land that they would not resemble what most people would consider a neighborhood.