Harvard Economist:

The current mess would never have occurred in the absence of ill-conceived federal policies. The federal government chartered Fannie Mae in 1938 and Freddie Mac in 1970; these two mortgage lending institutions are at the center of the crisis. The government implicitly promised these institutions that it would make good on their debts, so Fannie and Freddie took on huge amounts of excessive risk.

Worse, beginning in 1977 and even more in the 1990s and the early part of this century, Congress pushed mortgage lenders and Fannie/Freddie to expand subprime lending. The industry was happy to oblige, given the implicit promise of federal backing, and subprime lending soared.