It is a front-page WSJ article, read it here. Excerpt:
In a 2000 academic paper, Mr. Geanakoplos offered a theory. He said that when banks set margins very low, lending more against a given amount of collateral, they have a powerful effect on a specific group of investors. These are buyers, whether hedge funds or aspiring homeowners, who for various reasons place a higher value on a given type of collateral. He called them "natural buyers."
Using large amounts of borrowed money, or leverage, these buyers push up prices to extreme levels. Because those prices are far above what would make sense for investors using less borrowed money, they violate the idea of efficient markets. But if a jolt of bad news makes lenders uncertain about the immediate future, they raise margins, forcing the leveraged optimists to sell. That triggers a downward spiral as falling prices and rising margins reinforce one another. Banks can stifle the economy as they become wary of lending under any circumstances.
I thank Daniel Lippman for the pointer.