Will Chicago, the city of slaughterhouses and skyscrapers, soon look like haute bourgeois 19th-century Vienna?

In the past three decades, Chicago has undergone changes that are routinely described as gentrification but are, in fact, more complicated and more profound than the process that term suggests. A better description would be "demographic inversion." Chicago is gradually coming to resemble a traditional European city. The poor and the newcomers are living on the outskirts. The people who live near the center – some of them black or Hispanic but most of them white – are those who can afford to do so.

Developments like this rarely occur in one city at a time, and indeed demographic inversion is taking place, albeit more slowly than in Chicago, in metropolitan areas throughout the country.

We are not witnessing the abandonment of the suburbs or a movement of millions of people back to the city all at once. But we are living at a moment in which the massive outward migration of the affluent that characterized the second half of the 20th century is coming to an end.

For several decades now, cities in the United States have wished for a "24/7" downtown, a place where people live as well as work and keep the streets busy, interesting and safe at all times of day. This is what urbanist Jane Jacobs preached in the 1960s, and it has long since become the accepted goal of urban planners. Only when significant numbers of people lived downtown, planners believed, could central cities regain their historic role as magnets for culture and as a source of identity and pride for the metropolitan areas they served.

Now that's starting to happen, fueled by the changing mores of the young and by soaring gasoline prices. In many of its urbanized regions, an America that seemed destined for ever-increasing individualization and sprawl is experimenting with new versions of community and sociability.

Why has demographic inversion begun? For one thing, the deindustrialization of the central city, for all the tragic human dislocations it caused, has eliminated many of the things that made affluent people want to move away from it. Nothing much is manufactured downtown anymore (or anywhere near it), and that means the noise and grime that prevailed for most of the 20th century have gone away.

Urban historian Robert Bruegmann goes so far as to claim that deindustrialization has, on the whole, been good for downtowns because it has permitted so many opportunities for creative reuse of the buildings.

I wouldn't go quite that far, and, given the massive job losses of recent years, I doubt most residents of Detroit would, either. But it is true that the environmental factors that made middle-class people leave the central city for streetcar suburbs in the 1900s and for station-wagon suburbs in the 1950s do not apply anymore.

Nor, in general, does the scourge of urban life in the 1970s and '80s: random street violence. For the most part, middle-class people of all colors began to feel safe on the streets of urban America in the 1990s, and they still feel that way. The paralyzing fear that anyone of middle age can still recall vividly from the 1970s – that the shadowy figure passing by on a dark city street at night stands a good chance of being a mugger – is rare these days, almost nonexistent among young people.

This is the generation that grew up watching Seinfeld, Friends and Sex and the City, mostly from the comfort of suburban sofas. We have gone from a sitcom world defined by Leave It to Beaver and Father Knows Best to one that offers a whole range of urban experiences and enticements.

I do not claim that a handful of TV shows has somehow produced a new urbanist generation, but it is striking how pervasive the pro-city sensibility is within this generation, particularly among its elite.

In recent years, teaching undergraduates at the University of Richmond, the majority of them from affluent suburban backgrounds, I made a point of asking where they would prefer to live in 15 years – in a suburb or in a neighborhood close to the center of the city. Few ever voted for suburban life.

I can't say that they had necessarily devoted a great deal of thought to the question: When I asked whether they would want to live in an urban neighborhood without a car, many seemed puzzled and said no. Clearly, we are a long way from producing a generation for whom urban life and automobile ownership are mutually exclusive.

Not that cars and the demographic inversion aren't closely related; they are. In Atlanta, where the middle-class return to the city is occurring with more suddenness than perhaps anywhere in the United States, the most frequently cited reason is traffic. People who did not object to a 20-mile commute from the suburbs a decade ago are objecting to it now, in part because the same commute takes quite a bit longer.

To this, we can add the prospect of $5-a-gallon gasoline. It's impossible at this point to say with any certainty just what energy costs will do to American living patterns over the next decade. Urbanists predicted a return to the city during previous spikes in the cost of gasoline, notably during shortages in the 1970s. They were wrong. Gas prices came down, and the suburbs expanded dramatically.

But today's prices at the pump are not the result of political pressures by angry sheiks in the Persian Gulf. They are the result of increased worldwide demand that is only going to continue to increase. Some suburbanites will simply stay where they are and accept the cost. But many will decide to stop paying $100 every few days for a tank of gasoline that will allow them to commute 40 or 50 miles a day, round-trip.

Ultimately, though, the current inversion is less the result of middle-aged people changing their minds than of young adults expressing different values, habits and living preferences than their parents.

The demographic changes that have taken place in America over the past generation – the increased propensity to remain single, the rise of cohabitation, the much later age at first marriage for those who do marry, the smaller size of families for those who have children and, at the other end, the rapidly growing number of healthy and active adults in their 60s, 70s and 80s – have combined virtually all of the significant elements that make a demographic inversion not only possible but likely.

We are moving toward a society in which millions of people with substantial earning power or ample savings can live wherever they want, and many will choose central cities over distant suburbs.

As they do this, others will find themselves forced to live in less desirable places – now defined as those further from the center of the metropolis. And, as this happens, suburbs that never dreamed of being entry points for immigrants will have to cope with new realities. It should come as no surprise that the most intense arguments about hiring and educating the undocumented have occurred in the relatively distant reaches of American suburbia.

There are responsible critics who look at all this and see a lot being made out of very little. They argue that, in absolute numbers, the return to the urban center remains a minor demographic event.

They have a point. In most metropolitan areas, in the first few years of the 21st century, many more people have moved to the suburbs than have moved downtown. The bulk of the married-with-children middle class has not only been living in the suburbs, it has been moving to the suburbs.

Joel Kotkin, perhaps the most prominent of the downtown debunkers, declares flatly that, until families begin turning up in significant numbers on downtown streets, we are talking about a blip rather than a major cultural phenomenon.

But it's not just a blip. The evidence from most American cities – carefully presented by Christopher Leinberger, the real estate developer and University of Michigan urban planning professor, in his recent book, The Option of Urbanism – suggests that the number of downtown residents these days depends more on supply than demand.

But, even if the critics are mostly right – even if the vast majority of cities never see a downtown residential boom of massive proportions – there is no doubt that a demographic inversion, in which the rich are moving inside and the poor are moving outside, is taking place. The crucial issue is not the number of people living downtown, although that matters. The crucial issue is who they are, and the ways in which urban life is changing as a result.

What would a post-inversion American city look like? In the most extreme scenario, it would look like many of the European capitals of the 1890s.

Take Vienna, for example. In the mid-19th century, the medieval wall that had surrounded the city's central core for hundreds of years was torn down. In its place there appeared the Ringstrasse, the circle of fashionable boulevards where opera was sung and plays performed, where rich merchants and minor noblemen lived in spacious apartments, where gentlemen and ladies promenaded in the evening under the gaslights, where Freud, Mahler and their friends held long conversations about death over coffee and pastry in sidewalk cafes.

By contrast, if you were part of the servant class, odds were you lived far beyond the center, in a neighborhood called Ottakring, a concentration of more than 30, 000 cramped one- and two-bedroom apartments, whose residents – largely immigrant Czechs, Slovaks and Slovenes – endured a long horse-car ride to get to work in the heart of the city.

In the worst case, demographic inversion would result in the poor living out of sight and largely forgotten in some new kind of high-rise projects beyond the city border, with the wealthy huddled in gated enclaves in the center.

But I think this is an unlikely scenario. The people who are moving to the downtowns of American cities today are doing so in part to escape the real or virtual "gated-ness" of suburban life. The condos that house them in the coming years may feature elaborate security systems, but the inhabitants will not be walled off from the street. They want to be in contact with the street.

Nor do we have to worry about the return of the idea of warehousing the poor in vertical Corbusian ghettoes. That is one beast we have managed to slay.

Less dystopian are the prophecies of Mr. Leinberger, who believes that a dramatic increase in middle-class central-city population will in fact take place, and that one consequence will be the deterioration of today's car-dependent, suburban tract homes into the slums of 2030.

I don't think this will happen either, at least not in such extreme form. There simply are not enough lofts and townhouses to double or triple the number of people living in the center of a mid-sized American city.

Nor does it seem likely that exurbia will turn into a wasteland. The price of the houses will go down and render them more attractive for newcomers trying to rise in the U.S. economy and society.

Urbanists have complained for years that immigrants and poor people in the inner city have a hard time commuting to the service jobs that are available to them in the suburbs. If they live in the suburbs, they will be closer to the jobs. Transportation will remain a problem, but not one that can't be solved.

Somewhere in between, there lies the vision of Jane Jacobs, who idealized the Greenwich Village of the 1950s and the casual everyday relationships that made living there comfortable, stimulating and safe.

Much of what Ms. Jacobs loved and wrote about will not reappear: The era of the mom-and-pop grocer, the shoemaker and the candy store has ended for good. We live in a big-box, big-chain century.

But I think the youthful urban elites of the 21st century are looking, in some sense, for the things Ms. Jacobs valued, whether they have heard of her or not. They are drawn to the densely packed urban life that they saw on television and found vastly more interesting than the cul-de-sac world they grew up in. And, by and large, I believe central cities will give it to them.

Not only that, but much of suburbia, in an effort to stay afloat, will seek to urbanize itself to some extent.

That reinvention is already taking place: Look at all the car-created suburbs built in the 1970s and '80s that have created "town centers" in the past five years, with sidewalks and as much of a street grid as they can manage to impose on a faded strip-mall landscape. None of these retrofit efforts look much like a real city. But they are a clue to the direction in which we are heading.

In the 1990s, a flurry of academics and journalists (me among them) wrote books lamenting the decline of community and predicting that it would reappear in some fashion in the new century. I think that is beginning to happen now in the downtowns of America, and I believe, for all its imperfections and inequalities, that the demographic inversion ultimately will do more good than harm.

We will never return – nor would most of us want to return – to the close-knit but frequently constricting form of community life that prevailed 50 years ago. But, as we rearrange ourselves in and around many of our big cities, we are groping toward the new communities of the twenty-first century.

Alan Ehrenhalt is executive editor of Governing Magazine and author of "The United States of Ambition and The Lost City." A longer version of this essay appears in The New Republic.

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