The new moral hazard in financial markets has its source in what can be best described as the Greenspan Doctrine. It was clearly enunciated by Greenspan in his December 19, 2002, speech, in which he made an asymmetric argument leading to an asymmetric monetary policy. He argued that asset bubbles cannot be detected and monetary policy ought not in any case to be used to offset them. The collapse of bubbles can be detected, however, and monetary policy ought to be used to offset the fallout.
Two months earlier Ben Bernanke had made a similar argument. He endorsed the Greenspan Doctrine, arguing against the use of monetary policy to prevent asset bubbles: "First, the Fed cannot reliably identify bubbles in asset prices. Second, even if it could identify bubbles, monetary policy is far too blunt a tool for effective use against them." Since Bernanke is now Fed chairman, it is reasonable for market participants to assume that the Greenspan Doctrine still governs current Fed policy.
~ Gerald P. O'Driscoll Jr., "Subprime Monetary Policy," The Freeman, November 2007