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Kai Ryssdal: Christmas trees already going up across the country, so I guess it's not too early to bring a little Charles Dickens into the conversation. You remember in "A Christmas Carol" how Ebenezer Scrooge was visited by the ghosts of Christmases past, present and future? Well, we got a little past, present and future ourselves today.

First, the past. One of the big housing market indicators revealed this morning that home prices were still going up as of September. The S&P/Case-Shiller Index showed prices in 20 major cities rose about a third of 1 percent last month. That is the fourth straight month of increases.

Then there's the present. The real estate information company First American CoreLogic reported today one in four homeowners are underwater. That is, they owe more on their mortgage than the house is worth.

That, in turn, brings us to the future and Marketplace's Jeremy Hobson.

JEREMY HOBSON: Not surprisingly, the underwater mortgages are concentrated in the states that have felt the housing bust the most. Arizona, California, Florida and Nevada -- where 65 percent of borrowers are underwater.

Former Fed economist Morris Davis teaches in the real-estate department at the University of Wisconsin. He says about 10 percent of the underwater homeowners are likely to default. But the rest will probably stick it out.

MORRIS DAVIS: Because a lot of people don't want to walk away from their house, and even the people that are tempted to walk away from their house, they don't want to face the market punishment that walking away would inflict on them.

Like a visit from a collection agency, or a low credit score. But Davis says even homeowners who plan to keep paying their mortgage may act differently toward their homes.

DAVIS: For example you have less incentive now to maintain your property, because any dollar you spend maintaining your property is a dollar that goes to your mortgage company, not to you as a homeowner.

And that'll send home prices down further. Lenders don't want that to happen any more than homeowners do.

Andrew Jakobovics is a housing expert at the Center for American Progress in Washington. He says many lenders are racing to modify mortgages so homeowners don't give up on their properties.

ANDREW JAKOBOVICS: If you in fact take the properties, those are losses that you have to realize today. Whereas if you're modifying the loan, you're taking a much smaller loss today, especially if it's a permanent modification. But the expected risk that exists with that property is smaller, because you've got now what's an affordable payment for the homeowner.

And since homes are usually the biggest single expense for Americans, mortgage modifications can ripple across the economy. Homeowners who feel confident about their mortgage may be more likely to spend on everything from a new paint job to a new TV.

In New York, I'm Jeremy Hobson for Marketplace.