CAMBRIDGE, Mass. - IRONICALLY, the recent recession has encouraged a macabre optimism. Panglossian souls expect that housing will prove a cyclical phenomenon and that eventually housing prices will fall. After all, for other economic phenomena unemployment, inflation, interest rates economists draw fluctuating trend curves. The sanguine prophets expect the housing market to correct itself.

It won't happen.

The housing market at first glance is no different from markets for other commodities, where supply intersects with demand. Generally, rising demand will initially drive up prices for the commodity, until suppliers, seeing the money to be made in that commodity, gear up to increase supply. This is the classic widget model of Economics 101. And generally it explains why prices have fallen for many "necessities" once considered luxuries, such as television sets and microwave ovens.

Housing, though, is different. One reason: We have sharply constrained supply. The large-lot restrictions, the building codes, the aesthetic requirements, the bans on manufactured homes, all have shaped the landscape, guaranteeing that scenic spots stay rooted in a scenic status quo. If we suddenly erased all the constraints on supply, builders could construct more dwellings smaller, on smaller lots, with fewer amenities.

In upping the quality, the size and the amenities of homes, we curtailed supply, thereby keeping prices up.

Early in the last century, builders built three-decker tenements in New England mill towns, expressly to house the swarms of immigrants. We don't build triple-deckers any- more. Ditto for the Sears & Roebuck cottages on postage-stamp lots.

Visit those idyllic enclaves where schoolteachers, nurses and police officers cannot afford to buy (sometimes even to rent) homes. Then read those enclaves' zoning/building restrictions. The limited supply represents one barrier to "affordable" housing, yet it is not the sole barrier.

The second barrier is income. Our labor force is bifurcated: low- income service workers, earning close to the minimum wage, and the middle- to upper-income managerial workers, earning far more.

The middle manufacturing class has shrunk. Since mid-2000, 2.7 million manufacturing jobs have been lost. Today, 11 percent of our work force labors in manufacturing, down from 30 to 40 percent years ago.

Our demand for housing reflects that bifurcation in income. The growing cadre of low-income service workers in some markets earns too little to constitute an effective demand. From 1976 to 2001, the median income (in real terms) of 18- to 35-year-olds with less than a high-school education declined almost 20 percent.

Consider the full-time Wal-Mart employee. (Wal-Mart is the largest private-sector employer in the United States.) That worker earns $9 an hour. At that wage, he cannot afford a moderately priced two-bedroom apartment in any U.S. city. He constitutes a "demand" but a demand that will not spur "supply" because the people who control the supply of housing the builders, the landlords, the property owners see no profit in housing him. Without federal subsidies (e.g., the Low-Income Housing Tax Credit), they cannot make a sufficient profit in serving low-income tenants, or low- income homeowners. So while the growing population suggests a burgeoning demand for housing, low incomes shut the Wal-Mart households out of the market.

Instead, builders, landlords and property owners build for the other part of the labor force: middle- to upper-income Americans, whose numbers are also growing. The past decade's spurt of new housing is expensive, geared to people who can afford it. Although it is easy to blame the "crisis of affordability" on inequities and glitches in the housing market, this crisis is not primarily one of housing, but of incomes.

People earning very low incomes cannot afford the housing we are building. Indeed, even if we eased all the restrictions on supply, it is not clear that we could house the Wal-Mart worker in a "safe, decent home," by 21st Century standards. And a return to the 19th Century poorhouses is neither likely nor desirable.

We accept that by virtue of income, many Americans are shut out of many markets: the Wal-Mart clerk, the McDonald's cook, the hospital orderly can't buy a new car, take a cruise vacation, or send his children to private schools. Housing, though, is essential, and a just society should not let its low-income workers live at the edge of subsistence.

Unfortunately, the "affordability" crisis won't disappear as the economy cycles into its next phase. Poor workers will continue to pay too much for housing they can't afford.

Nicolas Retsinas, an occasional contributor, is director of the Joint Center for Housing Studies at Harvard.