How a Bubble Stayed Under the Radar By ROBERT J. SHILLER 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...