The famous “Greenspan put” was based on the (empirically well- founded) belief that the U.S. central bank would resist raising interest rates in response to a sharp upward spike in asset prices (and therefore not undo them) but would react vigorously to any sharp fall in asset prices by cutting interest rates to prop them up.  Thus, markets believed, the Federal Reserve provided investors with a one way bet.  That the Federal reserve would resort to extraordinary measures once a collapse started has now been proven to be a fact. In hindsight, it is now clear that a single-minded focus on inflation can be justified only in an environment in which regulators are able to ensure that leverage does not become excessive.

From This Time is Different – Eight Centuries of Financial Folly by Carmen Reinhart and Kenneth Rogoff