Financial markets operate on the assumption that US sovereign debt is “risk-free”. It is thus understandable that stock markets reacted with shock when Standard & Poor’s said that it was putting US debt on to negative outlook.
It is a big moment. S&P has rated Uncle Sam continuously since 1941, and through war and peace it has always had the top triple-A rating. Since it introduced its outlook system in 1991, it has never put the US on negative outlook. (Moody’s did so once, in 1996).
The chances that S&P follows through with a downgrade are not trivial. Over history, downgrades happen to a third of credits within six to 24 months after they go on negative outlook.
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