THE mortgage crisis has put “for sale” signs in front of houses across the country  including so-called McMansions  the large, expensive, often tasteless homes that have taken up so much suburban space over the last couple of decades.

But frantic selling is just the beginning, according to Christopher B. Leinberger, a professor of urban planning at the University of Michigan. “Signs of physical and social disorder are spreading” thro'gh cul-de-sac suburbia, he writes in the March issue of The Atlantic. And it is not just because of the mortgage mess. A “structural change” is occurring in the housing market  a “major shift in the way many Americans want to live and work,” moving social problems out of the city and into the suburban fringe.

Mr. Leinberger cites the work of Arthur C. Nelson, the director of Virginia Tech’s Metropolitan Institute, who has predicted that, by 2025, there will be a surplus of 22 million large-lot homes (those built on at least one-sixth of an acre). This, Mr. Leinberger writes, is a result of “the pendulum swinging back toward urban living,” thanks to a set of economic, social, and demographic trends.

The result, he says, could be that low-density suburbs “may become what inner cities became in the 1960’s and 1970’s  slums characterized by poverty, crime, and decay.”