Real estate loans have caused more than half of U.S. banks' loan losses during the financial crisis — far more than the 40% they caused in the last banking crisis in the 1990s, and far more than banks had reserved for during the boom years.

But bankers couldn't have set aside enough money to cover those losses before the Great Recession, says a research report from the Federal Reserve Bank of San Francisco, because they never could have seen them coming.

The study goes on to endorse a long-term fix: The Obama administration's call for higher capital at lenders, as determined by the stress tests given to banks last year.

Over the past year — with the benefit of hindsight — a number of bankers, economists and investors have lined up to praise the Treasury Department's stress tests of banks last year.

"Even if banks had better forecasts and more discretion in setting reserves, they would probably still be unable to adequately provision against unexpected large economic shocks," Fed researchers Fred Furlong and Zena Knight wrote in the report released Monday.

By contrast, they wrote, "the stress tests focused on what would happen in cases of significant unexpected losses."

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