We propose a theory of financial intermediaries operating in markets influenced by investor sentiment. In our model, banks make, securitize, distribute, and trade loans, or they hold cash. They also borrow money, using their security holdings as collateral. Banks maximize profits, and there are no conflicts of interest between bank shareholders and creditors. The theory predicts that bank credit and real investment will be volatile when market prices of loans are volatile, but it also points to the instability of banks, especially leveraged banks, participating in markets. Profit-maximizing behavior by banks creates systemic risk.

In a January 2009 lecture on the financial crisis, Federal Reserve Chairman Bernanke advocated a new Fed policy of credit easing, defined as a combination of lending to financial institutions, providing liquidity directly to key credit markets, and buying of long term securities. We show that Bernanke's analysis and recommendations can be naturally considered in a model of "unstable banking," which relies on two mechanisms: 1) fire sales reduce asset prices below fundamental values, and 2) financial institutions prefer speculation to new lending when markets are dislocated. We analyze credit easing and compare it to alternative government interventions during the crisis.