The percentage of middle-income neighborhoods in the 100 largest metropolitan areas across the country dropped from 58% in 1970 to 41% in 2000, according to the Brookings Institution. The study, which defined moderate-income families as those with incomes between 80% and 120% of the local median, found that these neighborhoods are disappearing faster than the proportion of metropolitan families earning middle incomes, which in three decades has fallen from 28% to 22%. The trend suggests that people are moving out of economically diverse neighborhoods, and the resulting disparities between high- and low-income neighborhoods make it harder for lower-income homeowners to move up the residential ladder.