The U.S. housing market is going through an adjustment of historic proportions. Before 2006, when the housing slump commenced, American home builders regularly built as many as 2 million new houses annually, rarely less than a million. This amount was needed to keep up with new household formation, immigration, homeowners moving up, and replacement due to obsolescence. Since then the number of new houses built has dropped drastically—the seasonally adjusted annual figure announced by the federal government in February 2011 was about 400,000! What's going on?

The recession, obviously. High unemployment and unease about the economy have made potential first-time homebuyers leery of entering the market, and many have decided to wait on the side lines. Although house prices have fallen, few are convinced that they have bottomed, and no one wants to buy a house and see its price decline. The large number of foreclosed (or about to be foreclosed) houses on the market, which account for no less than four out of 10 sales of existing homes, likewise dampens demand for new houses. And those willing to take the plunge discover that, despite low interest rates, lenders who were burned by the subprime mess now require large down payments. The other chief cause for weak demand is a slowdown in household formation—the U.S. Census reports that the rate of household formation is currently lower than at any time since 1947, as people put off getting married and starting a family. According to my colleague, real estate economistPeter Linneman, the marginal household size, which has historically hovered around two or three, shot up to more than six in 2009 and 2010, the result of doubling-up and moving in with relatives.