Over the past 30 years, mutual funds have become the dominant vehicle through which individual investors prepare for retirement via defined contribution plans. Further, money market mutual funds, which hold $2.7 trillion as of September 2013, are now a major part of the cash economy in the U.S....

We propose a new theory of systemic risk based on Knightian uncertainty (or "ambiguity"). We show that, due to uncertainty aversion, beliefs on future asset returns are endogenous, and bad news on one asset class induces investors to be more pessimistic about other asset classes as well. This...