How high can public debt rise without compromising fiscal solvency? We answer this question using a stochastic model of sovereign default in which risk-neutral investors lend to a government that displays 'fiscal fatigue', whereby its ability to increase primary balances cannot keep pace with rising debt. As a result, the government faces an endogenous debt limit beyond which debt cannot be rolled over. Using data for 23 advanced economies over the period 1970-2007, we find evidence of a fiscal reaction function with these features, and use it to compute 'fiscal space', defined as the difference between current debt ratios and the estimated debt limits.

We thank Olivier Blanchard, Henning Bohn, David Romer, and participants at the IMF's Eleventh Annual Jacques Polak Research Conference for helpful comments and suggestions. Any errors are the authors' responsibility. The views expressed herein are those of the authors, and should not be attributed to the IMF, its Executive Board, its management, or 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 paper develops a cost-benefit approach that helps to quantify the optimal level of international reserves in low-income countries, focusing on the role of reserves in preventing and mitigating absorption drops triggered by large external shocks. The approach is applied to a sample of 49 LICs over the period 1980-2008 to yield estimates of the likelihood and severity of a crisis. The calibration results suggest that the standard metric of three months of imports is inadequate for countries with fixed exchange rate regimes. The results also highlight the role of overall policy frameworks and availability of Fund-support in determining optimal reserve levels, raising questions about the uniform applicability of standard rules of thumb across countries.

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. This paper introduces a methodology for assessing external balance in countries with large stocks of non-renewable resources based on oil stock data, and applies it to selected oil producing countries. The methodology uses a stock approach (instead of the more traditional flow approach) to estimate the equilibrium non-oil current account consistent with optimal consumption smoothing. One of the benefits of the stock approach is that geological data for oil reserves can be used to estimate oil wealth; however, the methodology makes the estimated non-oil current account norm very sensitive to oil price projections. Based on an oil price about US$70 per barrel prevailing in the summer of 2007, the baseline estimates indicate that the non-oil current accounts for most of the countries in the sample are broadly in equilibrium. By the same token, using oil price projections as of the summer of 2008 implies large disparities between the equilibrium non-oil current account position and the medium term forecast for all countries in the sample except for Malaysia.

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