Behind the Great Stock Rally of 2009

The U.S. economy is coping with alarmingly high double-digit unemployment, a widening commercial real estate bust, and over-indebted consumers. Few think the economic recovery now under way will be a spectacular one in 2010. So why has the stock market surprised skeptics by powering higher in recent weeks? One explanation being bandied about by equity strategists and portfolio managers is that the stock market may be in the midst of a momentum-driven trading phenomenon known as a "melt up" that has precious little to do with economic fundamentals.

A melt up is a rapid and mass rush by investors into an asset class after a belated realization by market players that worthwhile gains are to be had there. Part herd mentality, part self-fulfilling prophecy, this trading behavior is amplified by the age-old tendency of fund managers and retail investors to chase returns in the hopes of making up for lost time and lagging performance. The U.S. stock market is enjoying an explosive rally that has humbled plenty of bears, who have been predicting a deep correction for several months now. Instead, the Standard & Poor's 500-stock index has soared 63% since its Mar.�9 low and is up 22% for 2009.