For years now, people have been talking about the insulated world of the top 1 percent of Americans, but the top 20 percent of the income distribution is also steadily separating itself — by geography and by education as well as by income.

This self-segregation of a privileged fifth of the population is changing the American social order and the American political system, creating a self-perpetuating class at the top, which is ever more difficult to break into.

The accompanying chart, taken from “The Continuing Increase in Income Segregation,” a March 2016 paper by Sean F. Reardon, a professor of education at Stanford, and Kendra Bischoff, a professor of sociology at Cornell, demonstrates the accelerating geographic isolation of the well-to-do — the upper middle and upper classes (a pattern of isolation that also applies to the poor, with devastating effect).

In hard numbers, the percentage of families with children living in very affluent neighborhoods more than doubled between 1970 and 2012, from 6.6 percent to 15.7 percent.