We study the optimal accumulation of international reserves in a quantitative model of sovereign default with long-term debt and a risk-free asset. Keeping higher levels of reserves provides a hedge against rollover risk, but this is costly because using reserves to pay down debt allows the government to reduce sovereign spreads. Our model, parameterized to mimic salient features of a typical emerging economy, can account for a significant fraction of the holdings of international reserves, and the larger accumulation of both debt and reserves in periods of low spreads and high income. We also show that income windfalls, improved policy frameworks, larger contingent liabilities, and an increase in the importance of rollover risk imply increases in the optimal holdings of reserves that are consistent with the upward trend in reserves in emerging economies. It is essential for our results that debt maturity exceeds one period.

This paper proposes a theory of foreign reserves as macroprudential policy. We study an open economy model of financial crises, in which pecuniary externalities lead to over-borrowing, and show that by accumulating international reserves, the government can achieve the constrained-efficient allocation. The optimal reserve accumulation policy leans against the wind and significantly reduces the exposure to financial crises. The theory is consistent with the joint dynamics of private and official capital flows, both over time and in the cross section, and can quantitatively account for the recent upward trend in international reserves.

Recently, several economies with interest rates close to zero have received large capital inflows while their central banks accumulated large foreign reserves. Concurrently, significant deviations from covered interest parity have appeared. We show that, with limited international arbitrage, a central bank's pursuit of an exchange rate policy at the ZLB can explain these facts. We provide a measure of the costs associated with this policy and show they can be sizable. Changes in external conditions that increase capital inflows are detrimental, even when they are beneficial away from the ZLB. Negative nominal rates and capital controls can reduce the costs.