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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 means that idiosyncratic risk can create contagion and snowball into systemic risk. Furthermore, in a Diamond and Dybvig (1983) setting, we show that, surprisingly, uncertainty aversion causes investors to be less prone to run individual banks, but runs will be systemic. In addition, we show that bank runs are associated with stock market crashes and flight to quality. Finally, we argue that increasing uncertainty makes the financial system more fragile and more prone to crises. We conclude with implications for the current public policy debate on the management of financial crisis

We present a novel source of disagreement grounded in decision theory: ambiguity aversion. We show that ambiguity aversion generates endogenous disagreement between a firm's insider and outside shareholders, creating a new rationale for corporate governance systems. In our paper, optimal corporate governance depends on both firm characteristics and the composition of the outsiders' overall portfolio. A strong governance system is desirable when the value of the firm's assets in place, relative to the growth opportunity, is sufficiently small or is sufficiently large, suggesting a corporate governance life cycle. In addition, more diversified outsiders (such as generalist mutual funds) prefer stronger governance, while outsiders with a portfolio heavily invested in the same asset class as the firm (such as venture capitalists or private equity investors) are more willing to tolerate a weak governance system, where the portfolio companies' insiders have more leeway in determining corporate policies. Finally, we find that ambiguity aversion introduces a direct link between the strength of the corporate governance system and firm transparency, whereby firms with weaker governance should also optimally be more opaque.