The "Greenspan Put" refers to the monetary policy approach that Alan Greenspan, the former Chairman of the United States Federal Reserve Board, and other Fed members exercised from the late 1987 to 2000.
The term "Put" refers to a put option, in which the buyer of the put acquires the right to sell an asset at a particular price to a counterparty; it can be exercised if prices decline below that price. During Greenspan's chairmanship, when a crisis arose and the stock market fell more than about 20%, the Fed would lower the Fed Funds rate, often resulting in a negative real yield. In essence, the Fed added monetary liquidity and encouraged risk taking in the financial markets to avert further deterioration.
The Fed did so after the 1987 stock market crash, which prompted traders to coin the term Greenspan Put, later termed Moral hazard. In 2000, Alan Greenspan raised interest rates several times. These actions were believed by some to have caused the bursting of the dot-com bubble. The Fed also injected funds to avert further market declines associated with the Savings and loan crisis and Gulf War, the Mexican crisis, the Asian crisis, the LTCM crisis, Y2K, the burst of the internet bubble, the 9/11 attacks and repeatedly from the early stages of the Global Financial Crisis to the present.
The Fed's pattern of providing ample liquidity resulted in the investor perception of put protection on asset prices. Investors increasingly believed that in a crisis or downturn, the Fed would step in and inject liquidity until the problem got better. Invariably, the Fed did so each time, and the perception became firmly embedded in asset pricing in the form of higher valuation, narrower credit spreads, and excess risk taking. Joseph Stiglitz criticized the put as privatizing profits and socializing losses and implicates it in inflating a speculative bubble in the lead-up to the 2008 financial crisis.
Bernanke Put 
In 2007 and early 2008, the financial press had begun discussing the Bernanke Put, as new Federal Reserve Board chairman, Ben Bernanke continued the practice of reducing interest rates to fight market falls. The decision by the Fed to lower short-term interest rates to 50 basis points (0.5%) on October 8, 2008, and thereafter a range from 0.00-0.25% rate in December 2008 suggests attempts to create a Bernanke put similar to the Greenspan put. New steps in quantitative easing further illustrate the Fed's attempt to moderate the business cycle. Recent (post March 2011) declines in measures of velocity and related declines in monetary growth measures suggest there is a limit to market manipulation, though interest rates have not risen and the economy continues to show little sign of serious improvement.
See also 
- Credit cycle
- Liquidity trap
- Privatizing profits and socializing losses
- Speculative bubble
- Too big to fail
- Zero interest-rate policy (ZIRP)
- Greenspan "put" may be encouraging complacency - Financial Times
- Stiglitz, Joseph E. (2010). Freefall: America, Free Markets, and the Sinking of the World Economy. New York and London: W. W. Norton & Company. p. 135. ISBN 9780393075960.
- Stevenson, Tom (2007-09-19). "History won't treat 'Bernanke put' kindly". The Daily Telegraph (London). Retrieved 2010-05-04.
- The 'Bernanke Put'—with a Currency Kicker
- "When Markets Are Too Big to Fail". The New York Times. 2007-09-22. Retrieved 2010-05-04.
- "Paint it black". The Economist. 2007-10-18.
- David Leonhardt and Catherine Rampell (2008-10-08). "Q&A: The Fed’s Rate Cut". New York Times. Retrieved 2008-10-08.
- CARTER DOUGHERTY and EDMUND L. ANDREWS (2008-10-08). "Central Banks Coordinate Global Cut in Interest Rates". New York Times. Retrieved 2008-10-08.
- Greenspan Put, Investopedia.