From the Wall Street Journal:

‘Underwater’ Need Not Mean Foreclosure

What does being “underwater” in your house really mean? Probably not that you’re drowning.

The number of underwater homeowners — those who owe more on their mortgages than their home is now worth — has been growing sharply since 2006 as real-estate prices have tumbled. By some estimates, between one in six and one in eight homeowners are in that position, most of them people who bought homes in the past few years or who put down small or no down payments.

This worries economists and policy makers, since owing more than your home is worth is the first step toward foreclosure. And it’s a concern to the rest of us because foreclosures are roiling the financial markets and, closer to home, they drag down our neighborhoods. (Most people who still have equity, by contrast, would rather sell their houses at a loss than lose what’s left of their investment.)
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But experts who have studied previous sharp housing downturns in Texas, California, New York and Massachusetts say that being underwater, while unpleasant, doesn’t lead huge numbers of homeowners to default on their mortgages and end up in foreclosure.

Christopher L. Foote, Kristopher Gerardi and Paul S. Willen of the Boston Federal Reserve Bank studied more than 100,000 homeowners who were underwater in Massachusetts in 1991 and found that just 6.4% of them lost their homes to foreclosure over the next three years, according to a paper published in the September Journal of Urban Economics. The vast majority of homeowners simply continued paying as usual because they focused on the affordability of their payments, not on what they owed, and they believed home values would eventually recover.
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The economists found that homeowners typically lost their homes only after at least two things happened: Their home values dropped and they either couldn’t afford the payments or stopped making payments after losing hope that prices would eventually recover.
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Typically, homeowners fall behind after a job loss, divorce or serious illness. In the current downturn, foreclosures are higher than in previous cycles because more homeowners reached beyond their means to buy their homes and simply can’t keep up the payments. As a result, the Boston economists project that up to 8% of underwater Massachusetts homeowners could lose their homes between now and 2010 — a significant amount, but still not catastrophic.