The past 15 years have seen total RMBS rating upgrades outpace downgrades by a two to one ratio, and in each of the past 10 years more deals have been upgraded than downgraded, according to an historical study by Moody's Investors Service. The only thing that is likely to put a stop to that trend is a rise in mortgage rates, which so far, unexpectedly, has not happened.

If and when mortgage rates begin to rise appreciably, as has been expected since the Federal Open Market Committee began tightening last year, Moody's expects the pace of RMBS rating upgrades and downgrades to even out. "It's been more of an anomaly these past few years," said Julia Tung, senior analyst with Moody's. If the housing market cools and mortgage rates rise, "there should be a return to a more normal state," she added.

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