MILAN – In a recent set of studies, Carmen Reinhart and Kenneth Rogoff used a vast array of historical data to show that the accumulation of high levels of public (and private) debt relative to GDP has an extended negative effect on growth. The size of the effect incited debate about errors in their calculations. Few, however, doubt the validity of the pattern.This should not be surprising. Accumulating excessive debt usually entails moving some part of domestic aggregate demand forward in time, so the exit from that debt must include more savings and diminished demand. The negative shock adversely impacts the non-tradable sector, which is large (roughly two-thirds of an advanced economy) and wholly dependent on domestic demand. As a result, growth and employment rates fall during the deleveraging period.In an open economy, deleveraging does not necessarily impair the tradable sector so thoroughly. But, even in such an economy, years of debt-fuelled domestic demand may produce a loss of competitiveness and structural distortions. And the crises that often divide the leveraging and deleveraging phases cause additional balance-sheet damage and prolong the healing process.Thanks in part to research by Reinhart and Rogoff, we know that excessive leverage is unsustainable, and that restoring balance takes time. As a result, questions and doubts remain about an eventual return to the pre-crisis trend line for GDP, and especially for employment.What this line of research explicitly does not tell us is that deleveraging will restore growth by itself. No one believes that fiscal balance is the whole growth model anywhere.Consider southern Europe. From the standpoint of growth and employment, public and private debt masked an absence of productivity growth, declining competitiveness in the tradable sector, and a range of underlying structural shortcomings – including labour-market rigidities, deficiencies in education and skills training, and underinvestment in infrastructure. Debt drove growth, creating aggregate demand that would not have existed otherwise. (The same is true of the United States and Japan, though the details differ.)Government is not the sole actor in this. When the deleveraging cycle begins, the private sector starts to adjust structurally – a pattern clearly seen in the data on growth in the tradable side of the US economy. Muted wage growth increases competitiveness, and underutilized labor and capital are redeployed.How fast this happens partly depends on the private sector’s flexibility and dynamism. But it also depends on the ability and willingness of government to provide a bridging function for the deficiency in aggregate demand, and to pursue reforms and investments that boost long-term growth prospects.If public-sector deleveraging is not a complete growth policy – and it isn’t – why is there so much attention on fiscal austerity and so little action (as opposed to lip service) on growth and employment?Several possibilities – not mutually exclusive – come to mind. One is that some policymakers think that fiscal balance really is the main pillar in a growth strategy: Deleverage quickly and get on with it.The belief that the fiscal multiplier is usually low may have contributed to underestimation of the short-run economic costs of austerity policies – and thus to persistently optimistic forecasts of growth and employment. Recent research by the International Monetary Fund on the context-specific variability of fiscal multipliers has raised serious questions about the costs and effectiveness of rapid fiscal consolidation.Estimates of the fiscal multiplier must be based on an assumption or a model that says what would have happened in the absence of government spending of some type. If the assumption or the model is wrong, so is the estimate. The counterfactual needs to be made explicit and assessed carefully and in context.In some countries with high levels of debt and impaired growth, fiscal stimulus could raise the risk premium on sovereign debt and be counterproductive; others have more flexibility. Countries vary widely in terms of household balance-sheet damage, which clearly affects the propensity to save – and hence the multiplier effect. Uncertainty is a reality, and judgment is required.Policymakers (and perhaps financial markets) may have believed that central banks would provide an adequate bridging function through aggressive unconventional monetary policy designed to hold down short- and long-term interest rates. Certainly central banks have played a critical role. But central banks have stated that they do not have the policy instruments to accelerate the pace of economic recovery.All countries – and policymakers – face difficult choices concerning the timing of austerity, perceived sovereign-credit risk, growth-oriented reforms, and equitable sharing of the costs of restoring growth. So far, the burden-sharing challenge, along with naive and incomplete growth models, may have contributed to gridlock and inaction.Michael Spence, a Nobel laureate in economics, is Professor of Economics at NYU’s Stern School of BusinessCopyright: Project Syndicate