Time to beat up on really really bad news reporting. The stock market doesn't tell people why it does what it does. We have commentators who bloviate on what they think caused the market to rise or fall, but they don't really know and they could be completely wrong.
That is why it was incredibly irresponsible for NPR to tell listeners in its top of the hour news segment that the market plunged because Standard and Poor's downgrade of U.S. debt. NPR does not know this to be true and it certainly is not obviously the case.
The market that should have been most immediately affected by the S&P downgrade was the U.S. bond market. However bond prices soared in the trading immediately following the downgrade and continued to rise through Wednesday. If there was greater fear that the U.S. would default because of the downgrade, then bond prices should have plunged as investors demanded a higher risk premium. This did not happen.
The most obvious alternative explanation for the plunge in the market is the risk that the euro could break up as the debt crisis spread from relatively small countries like Greece and Ireland, to the euro zone giants, Spain and Italy. The prospect of a euro zone break-up raises a real risk of a Lehman-type freeze up of the world financial system. It is far more plausible that this prospect led to the plunge in the stock market than the downgrade by one of three major credit rating agencies.
This point is important because many political actors, including National Public Radio, are trying to use the debt downgrade as an argument for cutting Social Security and Medicare. Their argument will be furthered if they can claim that the downgrade had enormous consequences for the stock market, since so many people involved in political debates (i.e. columnists, policy wonks, reporters, congressional staffers) have substantial amounts of money invested in the stock market.