Barry Ritholtz: "The Big Lie Goes Viral."

Wall Street has its own version: Its Big Lie is that banks and investment houses are merely victims of the crash. You see, the entire boom and bust was caused by misguided government policies. It was not irresponsible lending or derivative or excess leverage or misguided compensation packages, but rather long-standing housing policies that were at fault.

Indeed, the arguments these folks make fail to withstand even casual scrutiny. But that has not stopped people who should know better from repeating them.

The Big Lie made a surprise appearance Tuesday when New York Mayor Michael Bloomberg, responding to a question about Occupy Wall Street, stunned observers by exonerating Wall Street: “It was not the banks that created the mortgage crisis. It was, plain and simple, Congress who forced everybody to go and give mortgages to people who were on the cusp.”

What made his comments so stunning is that he built Bloomberg Data Services on the notion that data are what matter most to investors. The terminals are found on nearly 400,000 trading desks around the world, at a cost of $1,500 a month. (Do the math — that’s over half a billion dollars a month.) Perhaps the fact that Wall Street was the source of his vast wealth biased him. But the key principle of the business that made the mayor a billionaire is that fund managers, economists, researchers and traders should ignore the squishy narrative and, instead, focus on facts. Yet he ignored his own principles to repeat statements he should have known were false.