Via Mark Thoma:
This is why I have always thought that the argument that "Fannie and Freddie did it" is a non-starter. Two reasons:
Loans owned by Fannie and Freddie can be a problem for the government, but they are not a problem for the financial system--nobody thinks that they could create a linked domino chain of destabilizing bankruptcies. Securitized loans without a public backstop owned by banks, on the other hand...
Fannie, Freddie, and Ginnie were losing market share as the housing bubble grew--not gaining it. They did not force more lax lending standards on the private market, they followed the private market down.