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