Thanks to the nearly 200 Dishheads who wrote in with feedback, ranging from simple web links to expert analyses into the thousands of words.

The overwhelming consensus from readers and the economic writers they cite is that the government-sponsored enterprises (GSEs) Fannie Mae and Freddie Mac did not solely cause the financial crisis, contrary to Mayor Bloomberg's recent assertion. The most cited debunking of Bloomberg was written by Mike Konczal:

For some data, start here: ”More than 84 percent of the subprime mortgages in 2006 were issued by private lending institutions [not GSEs]….Private firms made nearly 83 percent of the subprime loans to low- and moderate-income borrowers that year."

As Center For American Progress’ David Min pointed out to me, the timing doesn’t work at all: "But from 2002-2005, [GSEs] saw a fairly precipitous drop in market share, going from about 50% to just under 30% of all mortgage originations. Conversely, private label securitization [PLS] shot up from about 10% to about 40% over the same period. This is, to state the obvious, a very radical shift in mortgage originations that overlapped neatly with the origination of the most toxic home loans."

This paragraph from Min is pretty devastating to Bloomberg's position:

Did Fannie and Freddie buy high-risk mortgage-backed securities? Yes. But they did not buy enough of them to be blamed for the mortgage crisis. Highly respected analysts who have looked at these data in much greater detail than [American Enterprise Institute's Peter] Wallison, [AEI consultant Ed] Pinto, or myself, including the nonpartisan Government Accountability Office [pdf], the Harvard Joint Center for Housing Studies [pdf], the Financial Crisis Inquiry Commission majority [pdf], the Federal Housing Finance Agency [pdf], and virtually all academics, including the University of North Carolina [pdf], Glaeser et al at Harvard [pdf], and the St. Louis Federal Reserve [pdf], have all rejected the Wallison/Pinto argument that federal affordable housing policies were responsible for the proliferation of actual high-risk mortgages over the past decade.

A chart from Min:

Konzcal's caption:

Even this "high risk" category [devised by Pinto] isn’t risky compared to subprime, and looks like the national average. When you slice it by private-label, the numbers are even worse.  Private label loans "have defaulted at over 6x the rate of GSE loans, as well as the fact that private label securitization is responsible for 42% of all delinquencies despite accounting for only 13% of all outstanding loans (as compared to the GSEs being responsible for 22% of all delinquencies despite accounting for 57% of all outstanding loans)."

Konzcal's bottom line:

The GSEs had a serious corruption problem and were flawed in design - Jeff Madrick and Frank Partnoy had a good column about the GSEs in the NYRB recently that you should check out about all this – but they were not the culprits of the bubble.

Dean Baker of the Center for Economic and Policy Research takes that same position - Fannie and Freddie were participants but not instigators of the crisis - in his takedowns of George Will and David Brooks earlier this year. The aforementioned Pinto and Wallison were involved in the bipartisan Financial Crisis Inquiry Commission, which released a January 2011 report on the cause of the crisis. In an analysis of that report, Jennifer S. Taub tackles the "myth" that Fannie and Freddie were to blame:

Both the Report and the primary dissenting statement agree that on their own Fannie and Freddie did not cause the financial crisis. [Of the ten commission members, including four Republicans, Wallison was the lone dissenter on that point.] ... The Report states that, "Affordable housing goals imposed by the Department of Housing and Urban Development (HUD) did contribute marginally" to Fannie and Freddie’s collapse.  However, it was the voluntary, profit, not mission-motivated decision by the management teams of the GSEs to load up on Wall-Street and other private bank created securities, coupled with a 75-1 leverage ratio that brought them to the brink. It was clear that the "private-sector, publicly traded, profit-making companies with implicit government backing and a public mission was fundamentally flawed."

A Krugman post from 2009 adds more compelling evidence against Bloomberg:

[R]eading this scary piece about commercial real estate, I realized that [Commercial Real Estate] offers yet another debunking. After all, there was no federal act driving banks to lend money for office parks and shopping malls; Fannie and Freddie weren’t in the CRE loan business; yet 55 percent — 55 percent! — of commercial mortgages that will come due before 2014 are underwater. The lenders didn’t need government urging to dive deep into a property bubble, and drown.

In a subsequent post, Krugman illustrated that point with a chart:

A reader adds more data to the debate:

Far from being a cause of the subprime meltdown, the Community Reinvestment Act actually facilitated more prudent lending from the small segment of the mortgage lending industry actually subject to the CRA (i.e. FDIC-insured banks, as opposed to non-depositary mortgage lenders like Countrywide). Take a look at this study [pdf] of publically available loan data, released before the meltdown. It concludes that lenders subject to the CRA (CRA Banks) were substantially less likely than other lenders to make the kinds of risky home purchase loans that helped fuel the foreclosure crisis, specifically that:

(1) CRA Banks were significantly less likely than other lenders to make a subprime loan;
(2) The average APR on subprime loans originated by CRA Banks was appreciably lower than the average APR on subprime loans originated by other lenders;
(3) CRA Banks were more than twice as likely as other lenders to retain originated loans in their portfolio rather than selling them in the secondary market ; and
(4) Foreclosure rates were lower in MSAs with greater concentrations of bank branches.

Another reader crafts an excellent narrative of Fannie and Freddie's role in the crisis: