We build a tractable heterogeneous-firm business cycle model where firms face Knightian uncertainty about their profitability and learn it through production. The cross-sectional mean of firm-level uncertainty is high in recessions because firms invest and hire less. The higher uncertainty reduces agents' confidence and further discourages economic activity. We characterize this feedback mechanism in linear, workhorse macroeconomic models and find that it endogenously generates empirically desirable cross-equation restrictions such as: amplified and hump-shaped dynamics, co-movement driven by demand shocks and countercyclical correlated wedges in the equilibrium conditions for labor, risk-free and risky assets. In a rich model estimated on US macroeconomic and financial data, the information friction changes inference and significantly reduces the empirical need for standard real and nominal rigidities. Furthermore, endogenous idiosyncratic uncertainty propagates shocks to financial conditions, disciplined by observed spreads, as key drivers of fluctuations, and magnifies the aggregate activity's response to monetary and fiscal policies.

I study a business cycle model where agents learn about the state of the economy by accumulating capital. During recessions, agents invest less, and this generates noisier estimates of macroeconomic conditions and an increase in uncertainty. The endogenous increase in aggregate uncertainty further reduces economic activity, which in turn leads to more uncertainty, and so on. Thus, through changes in uncertainty, learning gives rise to a multiplier effect that amplies business cycles. I use the calibrated model to measure the size of this uncertainty multiplier.

Annual Meeting, the SITE conference on Macroeconomics of Uncertainty and Volatility, and Wharton for helpful comments. The views expressed herein are those of the authors and do not necessarily reflect the views of the National Bureau of Economic Research. NBER working papers are circulated for discussion and comment purposes. They have not been peer-reviewed or been subject to the review by the NBER Board of Directors that accompanies official NBER publications.

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