Morality aside, there are other factors deterring “strategic defaults,” whether in recourse or nonrecourse states. These include the economic and emotional costs of giving up one’s home and moving, the perceived social stigma of defaulting, and a serious hit to a borrower’s credit rating. Still, if they added up these costs, many households might find them to be far less than the cost of paying off an underwater mortgage.

An important implication is that we could be facing another wave of foreclosures, spurred less by spells of unemployment and more by strategic thinking. Research shows that bankruptcies and foreclosures are “contagious.” People are less likely to think it’s immoral to walk away from their home if they know others who have done so. And if enough people do it, the stigma begins to erode.

A spurt of strategic defaults in a neighborhood might also reduce some other psychic costs. For example, defaulting is more attractive if I can rent a nearby house that is much like mine (whose owner has also defaulted) without taking my children away from their friends and their school.

So far, lenders have been reluctant to renegotiate mortgages, and government programs to stimulate renegotiation have not gained much traction.

Eric Posner, a law professor, and Luigi Zingales, an economist, both from the University of Chicago, have made an interesting suggestion: Any homeowner whose mortgage is underwater and who lives in a ZIP code where home prices have fallen at least 20 percent should be eligible for a loan modification. The bank would be required to reduce the mortgage by the average price reduction of homes in the neighborhood. In return, it would get 50 percent of the average gain in neighborhood prices  if there is one  when the house is eventually sold.

Because their homes would no longer be underwater, many people would no longer have a reason to default. And they would be motivated to maintain their homes because, if they later sold for more than the average price increase, they would keep all the extra profit.

Banks are unlikely to endorse this if they think people will keep paying off their mortgages. But if a new wave of foreclosures begins, the banks, too, would be better off under this plan. Rather than getting only the house’s foreclosure value, they would also get part of the eventual upside when the owner voluntarily sold the house.

This plan, which would require Congressional action, would not cost the government anything. It may not be perfect, but something like it may be necessary to head off a tsunami of strategic defaults.