Northwest of Richmond, Virginia, in the generally comfortable confines of suburban Henrico County, there's a 15-mile residential corridor along Interstate 64 that posted discouraging numbers in the 2000 Census. Without much warning, it had become economically stagnant. The area had been comparatively prosperous in previous decades, but by 2000, the median family income in this part of the county had shrunk to less than 70 percent of the average for Henrico as a whole.

Those numbers were intriguing to William Lucy and David Phillips, a pair of urban planning professors at the University of Virginia. The easy explanations were all just a bit too easy. The area borders on Richmond's city limits, but its problems didn't seem to be a case of poverty spreading outward from the city, because Richmond itself had actually fared better in the 1990s than in previous decades. It wasn't a simple matter of the housing getting too old, because nearby areas with much older houses had seen their income go up significantly.

Age was part of the answer, though. The more time Lucy and Phillips spent with the numbers, the more one fact stared out at them: The neighborhoods that were having the most trouble were those in which the houses were built during a particular period--the 1950s and '60s.

As they pored over Census data from other parts of the country, the two scholars found that a similar pattern had developed in dozens of metropolitan areas. Some suburbs were getting richer, and some were getting poorer. But the ones losing ground the fastest had a common characteristic: middle age. They were composed largely of homes built after World War II but before 1980.

By the time they had finished their research, Lucy and Phillips had studied a total of 2,586 suburban communities in every region of the United States. All they needed to know was the decade in which most of the houses were built, and they could pretty much predict what had been happening to income.

Among the newest suburbs--defined as those with at least 2.5 times as many 1990s houses as the metropolitan average--78 percent gained in median family income over the metropolitan average in the 2000 Census. Among suburbs specializing in pre-war houses--again using the same definition--65 percent were outpacing their metro areas in income growth. The middle-aged suburbs, those with a disproportionate number of houses from the '50s and '60s, were invariably the weak performers. About 70 percent of them lost relative ground in median family income during the 1990s.

As in Richmond, this was not a simple matter of geography. In general, it was true that many of the struggling middle-aged suburbs were close to a central city. But the correlation was weak. In the Chicago area, Lucy and Phillips found suburbs as far as 40 miles out that had plummeted in relative family income during the 1990s. And they found others right on the border of the city that were thriving. The one factor that seemed to explain it all was middle age. Suburbs born in the '50s or '60s were all but certain to be in trouble in the opening decade of the new century.

Why was that? Well, as they looked at middle-aged suburbs across the nation, Lucy and Phillips found a variety of problems. The schools weren't particularly good. The commercial districts, dominated by aging strip malls with vacant storefronts, were unappealing. But these were almost certainly symptoms of decline, not causes. The real issue was something remarkably simple and easy to measure: the size of the houses themselves.

In 1950, the average size of a new home built in America was a little more than 1,100 square feet. In some of the suburbs sprouting up on farmland just beyond the big cities, it was even smaller. The first houses in Levittown, on Long Island, all built in 1950, had an average size of less than 800 square feet.

Those numbers changed relatively slowly over the next couple of decades. Throughout the 1970s, the average size for new homes was still just 1,375 feet. But then it began to take off. In the 1990s, it passed the 2,000-square-foot milestone. By 2002, it was up to 2,114.

The American middle class simply wants more space. And the suburban landscape is burdened with a huge supply of undersized, middle-aged houses that don't match the lifestyle choices of families in the 21st century. As Lucy and Phillips say in their new book, Tomorrow's Cities, Tomorrow's Suburbs, "the more extensive these small-house areas, the more at risk these neighborhoods were to deterioration."

The newest suburbs are attractive right now because the big homes being built there match current living choices. And at the other end of the spectrum, the older, pre-war suburbs held up reasonably well in the 1990s because the homes in those places, for all their creaks and repair problems, are significantly roomier than those of the '50s and '60s.

So house size turns out to be the primary driver of the middle-aged suburban syndrome. From it flow most of the other consequences: first, population decline, then loss of retail business, weakened public schools, even crime. Lucy and Phillips put it rather succinctly: "Everything is worn--houses, schools, streets and commercial districts. Many residents experience these conditions and leave. Many prospective residents anticipate them getting worse and they don't buy."

What are middle-aged suburbs supposed to do? How do they go about reinventing themselves?

Well, there's one obvious way. If the houses are too small, you can make them bigger. Almost any 1,200-square-foot house from the 1950s in any suburb in America can be doubled in size with some ingenuity and a sufficient investment of funds. Expanding a middle-aged house isn't cheap, but it's almost always less expensive than buying a new one. Many of the middle-aged suburbs are geographically well-situated, close to jobs and central city commerce and more attractive from a commuting standpoint than the distant newer ones. If enough two- bedroom bungalows could be turned into two-story houses with four bedrooms, plenty of suburbs that stagnated in the 1990s could come to life again.

Unfortunately, it's not that simple. Financing can be hard to obtain for expansion of a small house in an aging suburb; sometimes, even home-equity loans don't cover the full cost of the job. It isn't always easy to find a contractor for a project of that sort. Then there's the problem of moving out while the work is being done. Families add those things up and decide it's just less of a hassle to move into a new subdivision on the edge of the metropolitan area.

Beyond those concerns, though, there's a tipping point issue. No homeowner in a neighborhood of 1950s bungalows relishes being the first one to lift the roof and create a fancy new house. If nobody else does it, the neighborhood won't improve, property values won't go up and the investment will have been a waste of money. In order to regain their youthful attractiveness, middle-aged suburbs need momentum. And momentum can be difficult to generate.

It's especially difficult when the local government doesn't appreciate the problem--or refuses to face it. While most struggling middle-aged suburbs understand that they need help, the leaders tend to focus on improving the schools and keeping the retail businesses that generate tax revenue. Not too many of them seem to realize that they have a small-house problem, and that if this could be addressed and middle-class residents induced to move there, the other problems might gradually take care of themselves.

It's easier said than done, of course. But as they looked around the country, Lucy and Phillips found several creative efforts to deal with the small-house problem and the accompanying signs of middle-aged suburban decay.

Some of them are modest and simple, such as distributing guidebooks to show homeowners exactly what they need to do to turn a small house into a bigger one. Some local governments offer lists of contractors and lenders willing to take them on. Others have waived the standard permit fees for the house expansions they particularly want to encourage.

In the Kansas City area, the metropolitan regional council publishes what it calls the First Suburbs Coalition Idea Book, with design ideas for renovation of almost every common middle-aged suburban house, along with practical lending advice. Chicago has its "bungalow initiative," through which small-house owners are given free permit assistance, discounts from vendors, as well as financing assistance.

None of these programs are guaranteed solutions for any middle-aged neighborhood that has begun to decline. But compared to resuscitating a school system or reviving a retail district, they hold out at least the prospect of tangible benefit at modest expense. One thing, at least, is certain: Very few American families who have options will be interested in buying Levittown-size houses in the coming decades. That leaves thousands of middle-aged suburbs with little choice but to redesign themselves if they are going to survive and prosper.Almost any 1,200-square-foot house from the 1950s in any suburb in America can be doubled in size with some ingenuity and a sufficient investment of funds.