????標普（Standard & Poor）近期下調(diào)美國主權(quán)信用評級后，各界再次開始熱議評級機構(gòu)是否值得信賴：它們是否能夠公平地評估各類債券？一份新調(diào)查可能火上澆油：印第安納大學（Indiana University）、美利堅大學（American University）和萊斯大學（Rice）的教授發(fā)現(xiàn)，過去三十年里，三大評級機構(gòu)中至少有一家——穆迪投資者服務(wù)（Moody's Investors Service）給私營部門債券的信用評分相對來說高于政府債券，有“注水”之嫌。

????更為致命的結(jié)論是：為評級機構(gòu)帶來更多收益的資產(chǎn)所獲評級比那些無利可圖的證券更為有利。“某種資產(chǎn)給評級機構(gòu)帶來的收入越多，評級樂觀主義（寬大或注水）也就越明顯，”研究報告的執(zhí)筆者們寫道，“證據(jù)強有力地表明，結(jié)構(gòu)性產(chǎn)品的評級比公司債券要寬大得多（樂觀），而市政或主權(quán)債券的評級顯然更為嚴厲（更加悲觀）。”

????盡管標普、穆迪和惠譽（Fitch）使用不同評級體系來衡量各種類型債券的信用，但所有資產(chǎn)類別都適用其自定標準。也就是說，當穆迪將一種債務(wù)擔保證券、一種公司債券和一種主權(quán)債券均評為AA級，那就意味著穆迪認為這三種債券的信用程度是相同的。

????可是，喬?巴萊斯特里諾稱，專業(yè)投資者長期以來對這些評級的共通性一直持懷疑態(tài)度。他效力于管理著3,550億美元資產(chǎn)的投資管理公司Federated Investors，擔任首席固定收益策略師，他指出：“各類資產(chǎn)之間存在不平衡或差別待遇，市政債券是最突出的例子。”

????幾十年來，穆迪給市政債券適用了一套單獨的標尺，使這些債券的評級頗有被低估之嫌。該機構(gòu)還稱，經(jīng)驗豐富的投資者理解市政債券的違約可能性要比評級相同的公司債券低。

????政府官員對此很不認同。2008年，康涅狄格州總檢察長理查德?布盧門撒爾起訴了三大評級機構(gòu)，援引一些內(nèi)部研究，稱這三家公司明知市政債券的違約率遠低于企業(yè)債券。加利福尼亞州財政部長比爾?洛克耶則發(fā)起一場運動，試圖迫使評級機構(gòu)修改評級體系。

????最后，穆迪和惠譽（后者也為市政債券適用了一種單獨的標準）作了讓步。2010年，這兩家機構(gòu)調(diào)整了市政債券評級體系，使其與公司債券的評級標準更一致，引發(fā)數(shù)千種政府債券的評級上調(diào)。標普則堅持其統(tǒng)一評級標準并不歧視市政債券。

????不過，上述研究的結(jié)論不僅涵蓋了市政債券。“這一事實沒有變，主權(quán)債券相比公司債券而言遭到了不公待遇，而公司債券又不如結(jié)構(gòu)性產(chǎn)品那么受優(yōu)，”報告作者之一杰斯?科爾納賈如是說。（盡管該研究只使用了穆迪的數(shù)據(jù)，但科爾納賈認為其結(jié)論很可能也適用于標普和惠譽，因為三家機構(gòu)的評級結(jié)果通常是相似的。）

????科爾納賈及報告的其他作者分析了穆迪在1980-2010年間給出的信用評級，然后比較獲得同等評級的不同種類資產(chǎn)的違約率，他們發(fā)現(xiàn)了巨大的不一致性。例如，被評為“A”級的資產(chǎn)中，只有0.49%的市政債券違約，主權(quán)債券從未違約，而獲得同種評級的公司債券中有1.83%違約，金融債券有4.92%違約，結(jié)構(gòu)性產(chǎn)品的違約率更是高達27.2%。

????The recent downgrade of U.S. debt by Standard & Poor's reignited the debate over whether ratings agencies can be trusted to fairly assess different types of debt. A new study could add fuel to the fire: Professors at Indiana University, American University and Rice found that, over the last 30 years, at least one of the big three agencies, Moody's Investors Service, inflated the credit scores of private debt relative to public bonds.

????Even more damning is their conclusion that assets that generated greater proceeds for the ratings agency were rated more leniently than less lucrative securities. "Ratings optimism (leniency or inflation) increases in the revenue generation by asset class," wrote the authors. "The evidence overwhelmingly suggests that while ratings of structured products were significantly more generous (optimistic) than those assigned to corporate issues, those assigned to municipals and sovereign issuers were significantly less generous (more pessimistic)."

????Although S&P, Moody's and Fitch use different letter systems to gauge the health of various types of bonds, each applies its own individual criteria across asset categories. So when Moody's says a collateralized debt obligation, a corporate bond and a sovereign issue are all rated AA, the implication is that the three types of bonds are equally healthy.

????But professional investors have long been dubious of the ratings' commonality, according to Joe Balestrino, the chief fixed income strategist at Federated Investors, an investment management firm with $355 billion in assets. "There are inconsistencies or discrepancies from one asset class to another," he says. "Munis are the most glaring example."

????For decades, Moody's used a separate scale for municipal debt, which gave the bonds the appearance of being underrated. The agency argued that sophisticated investors understood that municipal issues were less likely to default than similarly rated corporate issues.

????Public officials disagreed. In 2008, Conn. Attorney General Richard Blumenthal sued the big three ratings agencies, citing several internal studies that suggested the companies were aware that municipal bonds were defaulting at a much lower rate than private issues. Bill Lockyer, the treasurer of the state of California, led a campaign to force the agencies to modify their scales.

????Eventually, Moody's -- and Fitch Ratings, which also used different criteria for munis -- caved. In 2010, both agencies recalibrated their municipal ratings systems so that they better corresponded to their corporate rating scales, which resulted in upgrades for thousands of public bonds. S&P maintains that its general ratings scale does not discriminate against municipal debt.

????But the study's findings go beyond municipal bonds. "The fact remains that sovereigns are getting the shaft relative to corporates, and corporates are getting the shaft relative to structured products," says Jess Cornaggia, one of the authors of the paper. (Though the study only used data from Moody's, Cornaggia says the results likely apply to S&P and Fitch, which tend to produce similar ratings).

????Cornaggia and his co-authors looked at the credit ratings Moody's assigned between 1980 and 2010, then compared the frequency at which different assets that received the same letter grade defaulted. They found large disparities. For example, while just 0.49% of municipal bonds and 0% of sovereign bonds that received an "A" rating defaulted, the rate was 1.83% for analogous corporate bonds, 4.92% for financial bonds and 27.2% for structured products.