The Economist The 90% question A seminal analysis of the relationship between debt and growth comes under attack April 20, 2013 GOVERNMENT indebtedness matters. Default and financial panic are the stuff of finance- minister nightmares. Government borrowing can crowd out private investment, dragging growth down. Yet economists have struggled to specify when a country needs to worry about its debt load. In a 2010 paper Carmen Reinhart, now a professor at Harvard Kennedy School, and Kenneth Rogoff, an economist at Harvard University, seemed to provide an answer. They argued that GDP growth slows to a snail's pace once government-debt levels exceed 90% of GDP. The 90% figure quickly became ammunition in political arguments over austerity. Paul Ryan, a Republican congressman, cited their "conclusive empirical evidence" in a budget plan calling for swingeing cuts to public spending. In a February letter to European Union finance ministers Olli Rehn, the vice-president of the European Commission, touted the "widely acknowledged" 90% limit as a reason to press on with European fiscal cuts. Such rhetoric has helped to make the Reinhart-Rogoff number the subject of bitter dispute. And this week a new piece of research poured fuel on the fire by calling the 90% finding into question. The 2010 calculation was a relatively simple one. The authors had already drawn on two centuries of public-debt data for their seminal 2009 financial history, "This Time is Different". In their paper Ms Reinhart and Mr Rogoff sorted the figures into four categories of indebtedness and took average growth rates for each. They found that public debt has little effect on growth rates until debt reaches 90% of GDP. Growth rates then drop sharply. Over the entire two- century sample (from 1790 to 2009), average growth sinks from more than 3% a year to just 1.7% once debt rises above the critical level. In a shorter post-war sample the decline is more dramatic; average growth drop