An estimated 4.5 million home owners owe 75 percent more than their homes are worth. That number is likely to peak at 5.1 million in June, affecting 10 percent of home owners and making them increasingly likely to just walk away.

''We're now at the point of maximum vulnerability,'' says Sam Khater, a senior economist with First American CoreLogic, the firm that conducted the recent research. ''People's emotional attachment to their property is melting into the air.''

Consultants at Oliver Wyman calculated that 17 percent of owners defaulting in 2008 -about 588,000- chose to default even though they could pay.

First American estimates that it would cost about $745 billion - about the same as the original 2008 bank bailout - to restore all underwater borrowers to the break-even point.

Doing so would be seen as highly unfair by many taxpayers, says Michael S. Barr, assistant Treasury secretary for financial institutions, but doing nothing would be another blow to a fragile economy.

Source: The New York Times, David Streitfeld (02/022010)