Why are Americans hoarding $3.5 trillion in cash? An interesting question. Let's dig deeper and see if we can find out why.

Here are some interesting facts about the $3.5 trillion:

• After reducing money market accounts by 11% this year, investors hold cash equal to 73% of S&P 500 Index. At its peak in 2007, the buying power was at 62%.
• Estimates are for GDP to increase sixfold to 2.9% in the third quarter.
• In 2007 and 2008, investors placed $1.45 trillion in money market accounts. As of the week ending January 14, that number reached a record $3.92 trillion.
• Investors have added $15.8 billion to domestic equity funds since March.

As one of the few people to publicly call the internet and real estate bubbles for what they were, he has more credibility than just about anyone. And he's saying the stimulus package is too small and we need more if we are to avert another depression.

In an op-ed piece for Bloomberg, Shiller writes that "In the face of a similar Depression-era psychology today, we are in need of massive pump-priming again. We appear to be in a much better situation due to the stronger efforts to date. Still, there is a danger that, because of a combination of faulty economic theory and inadequate appreciation of human psychology, as well as deep public anger, we will not continue with such stimulus on a high enough level."

Economist Robert J. Shiller -- the one who identified the internet bubble and the housing bubble as bubbles before pretty much anyone else did -- will introduced a pair of new ETFs later this month in partnership MacroShares.

The MacroShares' Major Metro Housing product will consist of two kinds of shares: "up" shares and "down" shares. If you want to bet that housing prices will go up, buy the "up" shares. If you're feeling bearish, buy the "down" shares. But the fund won't be investing in housing at all.

Robert J. Shiller's Irrational Exuberance is the classic book for understanding the stock market bubble of the late 1990s and early 2000s. His contribution to the study of real estate is equally compelling. The House Price Index used to track our real estate market was co-developed by Mr. Shiller -- and is innovative in that it adjusts for the quality of homes involved in transactions.

So given his expertise in bubbles and real estate, he is probably the guy to listen to when it comes to the topic of the real estate bubble.

In a column in this Sunday's New York Times, Shiller gives an interesting possible explanation for a question that hasn't gotten a lot of attention: Why were Alan Greenspan -- and a lot of other presumably intelligent people -- unable to see that real estate bubble for what it was given that, in retrospect, it seems so obvious?

The answer may lie in a psychological phenomenon known as information cascade. Be sure to read Shiller's column for an explanation of how this may have applied to the real estate market. It's fascinating stuff.

And understanding why the bubble wasn't widely detectable is key to understanding why it happened. As Shiller writes, "The failure to recognize the housing bubble is the core reason for the collapsing house of cards we are seeing in financial markets in the United States and around the world. If people do not see any risk, and see only the prospect of outsized investment returns, they will pursue those returns with disregard for the risks."