When people say Americans are dividing along class lines, they usually mean it as a metaphor. But it's also true in a literal sense. When it comes to where we live, the rich, poor, and shrinking middle class have been moving further apart. Cornell University's Kendra Bischoff and Stanford University's Sean Reardon report that in 1970, 65 percent of U.S. families resided in middle-income neighborhoods. By 2009, that number was down to 42 percent. Meanwhile, the fraction of households in very affluent or very poor neighborhoods more than doubled.