"In 1970, 65 percent of families lived in middle-income neighborhoods; by 2009, only 42 percent of families lived in such neighborhoods".
This is from a report recetly published by Kendra Bischoff and Sean Reardon showing the trend in residential segregation by income in the US between 1970 and 2009. (via Tyler Cowen)
[click on the image to enlarge it]
[Image Credit: Bischoff and Reardon, via Mother Jones]