Phrase still popular today that was used to characterize the market mood in December 1996. It was uttered by Alan Greenspan, the then Federal Reserve Chairman, during a talk on monetary policy to admonish investors that the stock market was overvalued. These were his words: "How do we know when irrational exuberance has unduly escalated asset values, which then become subject to unexpected and prolonged contraction...?" It immediately caused a stir. Right after he spoke, the stock market in the U.S. fell by 2 percent, in Tokyo and Hong Kong by 3 percent, and in Frankfurt and London by 4 percent. In 2000, the Yale economics professor Robert Shiller titled his book with the phrase to argue that the stock market was indeed dangerously overvalued, and the phrase is still used by many economists and analysts today.