Life in the middle class ain’t what it used to be. As a case in point, there are fewer Americans in the middle-income bracket than there were four decades ago, or so says a study released in mid-November by the Russell Sage Foundation.

In 1970, Richard Nixon was in the White House, US military forces were in Vietnam, the Beatles were in the process of disbanding and 65% of Americans were in middle-class neighborhoods. By 2007, the most recent year covered by the study, that figure had dropped to 44%, partly a result of fewer jobs in manufacturing and other traditionally middle-class industries. During the same time period, the percentage of Americans living in either poor (that is, lower income than low-income) or affluent neighborhoods doubled from 15% to 31%. These trends slowed in the 1990s and accelerated from 2000 onward, the study says.

Those figures represent averages across the 117 largest SMSAs nationwide; the actual numbers vary widely from metro area to metro area. The market with the greatest income segregation is Fairfield County in Connecticut; the one with the largest percentage of either poor or affluent families is New York City, including its suburbs in Westchester County in New York State and Bergen and Passaic counties in New Jersey. The Philadelphia metro area saw the largest increase in segregation of income over the 37-year period.

This separation has occurred not only because the affluent (i.e. those whose household income is greater than 150% of the median for the metro area) have gotten more so. It’s also because we’ve been seeing a decrease in the number of mixed-income neighborhoods over the past four decades. Birds of a feather flock together, the old saying goes—and it applies to neighborhoods, too.

Mostly, it seems to apply to wealthy enclaves, which increasingly are pockets of gentrification or exurbia that the less well-off may never even pass through. “The increasing isolation of the affluent from low- and moderate-income families means that a significant proportion of society’s resources are concentrated in a smaller and smaller proportion of neighborhoods,” write study authors Sean F. Reardon and Kendra Bischoff of Stanford University. The result, they add, may be “lower public and private investments in resources, services and amenities that benefit large shares of the population,” including schools and parks.

Bear in mind that a study that stops with ’07 doesn’t take into account the full effects of the housing market collapse, and the authors acknowledge this. “The enormous number of housing foreclosures in the last few years has likely led many low-income families to move to lower-income neighborhoods, which would lead to increased income segregation,” write Reardon and Bischoff. “Conversely, declining incomes and income volatility among the middle-class may lead to lowered income segregation, because it may widen the income distribution within previously middle-class neighborhoods, or force these families to move into lower-income neighborhoods.”

We’ve heard a lot of debate lately about the consequences of income inequality. It’s one of the points (I think) of Occupy Wall Street, and one of the reasons this loosely organized movement has spread across the country. Whether or not income inequality matters, income segregation has long-term implications for commercial real estate, especially development.

Obviously, this study examines patterns that have formed over the course of decades, as do recent ones on income distribution from the Brookings Institution and the Congressional Budget Office. And it’s equally clear that all of these studies are as much about local conditions as about broad national trends—just as anyone planning to invest or develop in a particular market needs to study the fundamentals on a micro as well as macro scale, looking at where they’ve been and where they’re going.

Still, you have to ask: what does long-term development look like in a neighborhood where the demographics may not be so constant, and the evolution of those demographics may not be for the better?