The most ignored and maybe most important report to be issued this year has a startling finding, and you don’t have to be a Jeffersonian steeped in the romance of the agrarian life to recognize its danger: Sometime in the next year, more than half the world’s population will live in cities.

This report, produced by the much-overlooked United Nations Human Settlements Program, provides ample reason for the lack of celebration over this remarkable demographic landmark – and over the accompanying projection that nearly two-thirds of the world’s population will live in cities by the year 2030.

Cities have long been the collision points of two important elements of human folklore represented, in literature, in two colliding strains. There are, of course, the great novels of urban sophistication, where the innocents of the farms are introduced to the cultural attributes of the city; one American example is Willa Cather’s Thea Kronberg, who in “The Song of the Lark” went from the great West of the 19th century to the Metropolitan Opera, though not without cost. The other strain emphasizes the corrupting influence of the city; that theme screams from the pages of Theodore Dreiser’s “Sister Carrie,” even though it was written by a man who, like Carl Sandburg, regarded Chicago as a glittery destination of destiny.

Dreiser and Sandburg were born in the late 19th century. In the early 21st century, the city is a symbol for oppression as much as opportunity, and the booming new cities of the developing world are toxic stews of overcrowding, unemployment, disease and crime. The U.N. study reports that one-third of city dwellers live in slums, which despite their value historically as fertile seedbeds of great literature are hopeless centers of social distress and problems. Here’s a statistic that blurs the traditional images of the city and the country: About a quarter of urban dwellers lack decent toilets.

Indeed, the U.N. study calls inadequate sanitation the “silent tsunami,” arguing that five times as many people die annually from poor sanitation and hygiene than perished in the 2004 tsunami in the Indian Ocean. The study cites the Mbare neighborhood of Harare, in Zimbabwe, where as many as 1,300 people share one communal toilet with six squatting holes, and the Kenyan port city of Mombasa, where water is sometimes available only three hours a day.

Then there is the overcrowding. The study raises the possibility that slums could become “the predominant type of settlement in the 21st century,” pointing out that 90 percent of the world’s slums are in the developing world, “where urbanization has become virtually synonymous with slum formation.”

It’s hard to resist the conclusion that, once again, the developed world and the developing world are going in two contradictory directions.

In the developing world, the stream of people into the cities runs unabated; cities in the developing world will account for 95 percent of urban expansion in the next 20 years. But here in the developed world, the population is becoming more suburban. Here the bright lights still have their appeal, and the big jobs and the big money are in the cities, but suburbs and exurbs are booming.

The United States is a remarkable society because, for all our urban woes and for all the sterility of some of our suburbs, Americans have melded the two experiences fairly gracefully. It helps, of course, to be a wealthy society with plumbing, highways and telecommunications, and in truth the United States didn’t set out to accomplish this.

No planning here. It just happened.

“The American pioneers moved west, and then they wanted to move to the cities, and then they wanted more land and more control and not to be under the heels of others, so they moved to the suburbs,” says William H. Frey, a demographer at the Population Studies Center at the University of Michigan. “It’s the general tendency here to try to get a little more of a house and a yard and your own space. It’s nice to be in a somewhat dense environment and not to be on top of each other. We have that luxury, which is why we call this the American dream.”

Americans haven’t accomplished this as well as we might have, of course. We’re still too dependent on the automobile, and we have a patchwork of Balkanized suburban communities. For all the worship of planning in the nation’s graduate schools of business, the notion of planning in American civic life still has a stigma to it, a whiff of Soviet excess and central control. You see the result every time you drive from one strip plaza to another.

The cost of attending to all of the woes this race to urban areas will cause could rise to the hundreds of billions of dollars – that is, if you think money is the way to address these problems. Upgrading slums is costly, and the cost rises daily because the slums grow daily. Ordinarily U.N. reports call for massive spending, usually by the developed world. This one is different. It suggests that 80 percent of the money required to upgrade slums can be provided by local residents. What’s needed, the report says, is “public sector borrowing to fund slum upgrades and support for slum dwellers savings associations and credit schemes.”

What’s needed, too, is a way to stem the tide of urban immigration. At the end of World War I, Americans wondered, to the strains of an infantry band, How ‘ya gonna keep ’em down on the farm after they’ve seen Paree? Nearly a century later, one glimpse of Mombasa could do the trick.