Homeowners with the best credit are the next big risk for the U.S. housing market.

An increase in mortgage defaults among prime borrowers in 2009 is likely to accelerate this year, slowing the real estate recovery even as Americans become more optimistic about the economy, said Robert Shiller and Karl Case, the economists who created the S&P/Case-Shiller Home Price Index.

"There will be continuing foreclosures, and not just subprime, it will be prime mortgages," said Shiller, a professor at Yale University. "This is creating a huge shadow inventory of homes that are still owned, but they're going to be on the market in the next year or so."

The number of prime mortgages overdue by at least 60 days more than doubled in the third quarter from a year earlier to 838,000, according to a report from the Office of the Comptroller of the Currency and the Office of Thrift Supervision. Unemployed homeowners struggling to pay their bills will default on their home loans and increase foreclosures, Shiller and Wellesley College's Case said.

Employers have cut more than 7.2 million jobs in the past two years, the biggest employment loss since the Great Depression. Measured annually, the U.S. jobless rate probably will average 10 percent in 2010, according to the median estimate of economists surveyed by Bloomberg. That would be the highest rate in government records dating to 1948, after it was at a 26-year high of 9.3 percent last year.

"Unemployment is not respecting income boundaries," Case said. "It's affecting rich people, poor people and middle-income people and they all have mortgages." The U.S. may begin to see some signs of a housing recovery this year, he said.

While an increase in prime foreclosures will slow the housing recovery that began in September, it won't be enough to knock it entirely off track, Case said.

Foreclosures are declining for the type of subprime mortgages that sparked the global financial meltdown in 2008. New foreclosure starts among subprime adjustable-rate mortgages fell to 4.92 percent in the third quarter from 6.47 percent a year earlier, after the bulk of loans were either modified by lenders or the properties repossessed and sold, according to the MBA.

"What makes the rising default rates on prime loans so insidious is these are not folks who took out some crazy new type of mortgage," said Brad Hunter, chief economist at Metro- Study real estate research in West Palm Beach, Florida. "These are people who probably took out what would ordinarily be a responsible mortgage."

The increase in unemployment and the lackluster housing market have been at the center of the worst economic contraction since the 1930s and remain a challenge for President Obama as he enters his second year in office. Foreclosures and price declines continue, even after the government spent $230 billion in fiscal 2009 to support homeownership, according to a tally by the Congressional Budget Office in Washington.

"Even if the jobs start coming back, where are they coming back? If it's in Texas or Oklahoma, it's not helping people in California or Rhode Island," said Jay Brinkmann, chief economist of the Mortgage Bankers Association in Washington.

Michigan had the highest U.S. unemployment rate in November, at 14.7 percent, followed by Rhode Island at 12.7 percent, according to the Bureau of Labor Statistics. California, Nevada and South Carolina tied for third place, with a 12.3 percent jobless rate.

Confidence is the key ingredient to a sustainable economic recovery, Shiller and Nobel Laureate George A. Akerlof said in their 2009 book "Animal Spirits: How Human Psychology Drives the Economy, and Why It Matters for Global Capitalism." The book expands on a John Maynard Keynes macroeconomic theory by the same name that says emotion, rather than logic, drives consumer decisions that lead to economic change.

"I do see some signs of animal spirits, but it's a mixture," Shiller said last week of the housing market. In some areas of the U.S., such as California, home prices are going up at an "amazing" pace, he said. At the same time, "It would be entirely plausible that we would have a weak housing market for many years."