Credit constraints linking debt to market-determined prices embody a systemic credit externality that drives a wedge between competitive and constrained socially optimal equilibria, inducing private agents to "overborrow." This externality arises because private agents fail to internalize the financial amplification effects of carrying a large amount of debt when credit constraints bind. We conduct a quantitative analysis of this externality in a two-sector dynamic stochastic general equilibrium (DSGE) model of a small open economy calibrated to emerging markets. Raising the cost of borrowing during tranquil times restores constrained efficiency and significantly reduces the incidence and severity of financial crises.

Collateral constraints widely used in models of financial crises feature a pecuniary externality: Agents do not internalize how borrowing decisions taken in "good times" affect collateral prices during a crisis. We show that agents in a competitive equilibrium borrow more than a financial regulator who internalizes this externality. We also find, however, that under commitment the regulator's plans are time-inconsistent, and hence focus on studying optimal, time-consistent policy without commitment. This policy features a state-contingent macroprudential debt tax that is strictly positive at date t if a crisis has positive probability at t + 1. Quantitatively, this policy reduces sharply the frequency and magnitude of crises, removes fat tails from the distribution of returns, and increases social welfare. In contrast, constant debt taxes are ineffective and can be welfare-reducing, while an optimized "macroprudential Taylor rule" is effective but less so than the optimal policy.

Bank, and the National Bank of Belgium. 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 peerreviewed or been subject to the review by the NBER Board of Directors that accompanies official NBER publications.

This Working Paper should not be reported as representing the views of the IMF. The views expressed in this Working Paper are those of the author(s) and do not necessarily represent those of the IMF or IMF policy. Working Papers describe research in progress by the author(s) and are published to elicit comments and to further debate.Two striking facts about international capital flows in emerging economies motivate this paper: (1) Governments hold large amounts of international reserves, for which they obtain a return lower than their borrowing cost. (2) Purchases of domestic assets by nonresidents and purchases of foreign assets by residents are both procyclical and collapse during crises. We propose a dynamic model of endogenous default that can account for these facts. The government faces a trade-off between the benefits of keeping reserves as a buffer against rollover risk and the cost of having larger gross debt positions. Long-duration bonds, the countercyclical default premium, and sudden stops are important for the quantitative success of the model. JEL Classification Numbers:F32, F34, F41.