At the same time, the percentage of families with children living in traditional middle class neighborhoods with median incomes between 80 and 125 percent of the surrounding metropolitan area fell from 64.7 percent in 1970 to 40.5 percent.

Reardon and Bischoff write:

Segregation of affluence not only concentrates income and wealth in a small number of communities, but also concentrates social capital and political power. As a result, any self-interested investment the rich make in their own communities has little chance of “spilling over” to benefit middle‐ and low-income families. In addition, it is increasingly unlikely that high‐income families interact with middle‐ and low‐income families, eroding some of the social empathy that might lead to support for broader public investment in social programs to help the poor and middle class.

Geographic segregation dovetails with the growing economic spread between the top 20 percent and the bottom 80 percent: The top quintile is, in effect, disengaging from everyone with lower incomes.

Timothy Smeeding, a professor of public affairs and economics at the University of Wisconsin, has explored how the top quintile is pulling away from the rest of society. In an essay published earlier this year, “Gates, Gaps, and Intergenerational Mobility: The Importance of an Even Start,” Smeeding finds that the gap between the average income of households with children in the top quintile and households with children in the middle quintile has grown, in inflation-adjusted dollars, from $68,600 to $169,300 — that’s 147 percent.

In an earlier paper, Smeeding and two co-authors wrote that

we have seen a threefold increase between 1972 and 2007 in top-decile spending on children, an increase that suggests that parents at the top may be investing in ever more high-quality day care and babysitting, private schooling, books and tutoring, and college tuition and fees.

The bottom line, Smeeding wrote in an email, is this:

The well-to-do are isolated from the day to day struggles of the middle class and below to provide these key services (health, education, job search and other opportunities) to aid the upward mobility of their children. But the upper middle class are happy to take advantage of tax subsidies for their own housing, preschool for their kids, and saving for college which benefit them.

Political leverage is another factor separating the top 20 percent from the rest of America. The top quintile is equipped to exercise much more influence over politics and policy than its share of the electorate would suggest. Although by definition this group represents 20 percent of all Americans, it represents about 30 percent of the electorate, in part because of high turnout levels. The accompanying chart, which shows voting patterns by income in the 2012 and 2014 elections, illustrates this phenomenon (it was created by Sean McElwee, a policy analyst at Demos, a liberal think tank).