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the drift of risk from simple loans in traditional banks to structured products in leveraged nonbanks increased the danger of a “ ‘positive feedback’ dynamic,” a vicious cycle that could amplify a crisis. If asset prices fell, firms and investors would need money to meet margin calls, prompting fire sales that would drive asset prices even lower, and so on.
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Timothy F. Geithner (Stress Test: Reflections on Financial Crises)