Simple

Bryan Caplan weighs in on the cause of the crisis:

Arnold ably identifies two main narratives about the 2008 financial crisis.  But he neglects the simplest narrative of all: Once in a century, a once-in-a-century mistake happens. The end.

Bryan's half right. It is the simplest narrative. But I'm not sure what he means by "once in a century." We've had a whole bunch of financial crises around the world in the last 30 years. So his simple explanation has to be altered to:

"Once in a while, and...