ONE great puzzle about the recent housing bubble is why even most experts didn’t recognize the bubble as it was forming.

Alan Greenspan, a very serious student of the markets, didn’t see it, and, moreover, he didn’t see the stock market bubble of the 1990s, either. In his 2007 autobiography, “The Age of Turbulence: Adventures in a New World,” he talks at some length about his suspicions in the 1990s that there was irrational exuberance in the stock market. But in the end, he says, he just couldn’t figure it out: “I’d come to realize that we’d never be able to identify irrational exuberance with certainty, much less act on it, until after the fact.”

With the housing bubble, Mr. Greenspan didn’t seem to have any doubt: “I would tell audiences that we were facing not a bubble but a froth  lots of small local bubbles that never grew to a scale that could threaten the health of the overall economy.”

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.