How a spreadsheet formula error altered global austerity policy
In 2010, Harvard economists Carmen Reinhart and Kenneth Rogoff published an influential paper arguing that countries with public debt exceeding 90% of GDP experience negative economic growth. Politicians globally cited this figure to justify severe budget cuts. In 2013, University of Massachusetts Amherst graduate student Thomas Herndon replicated their Excel spreadsheet and found an accidental formula glitch: an average calculation omitted five key countries entirely, completely invalidating the dramatic 90% debt cliff claim.
The Emergence of the 90 Percent Cliff
In the wake of the 2008 global financial crisis, governments across the world found themselves facing collapsing tax revenues, costly bank bailouts, and emergency stimulus measures. As public debt soared, economists and policymakers engaged in an urgent debate over whether countries should continue spending to support aggregate demand or pivot aggressively toward deficit reduction. Into this contested environment stepped Carmen Reinhart and Kenneth Rogoff, two prominent Harvard economists whose earlier book on financial crises, 'This Time Is Different', had established them as preeminent authorities on sovereign debt dynamics.
In May 2010, the authors published a paper in the American Economic Review Papers and Proceedings entitled 'Growth in a Time of Debt'. Analyzing historical economic series across twenty advanced economies spanning roughly two centuries, Reinhart and Rogoff sought to establish how varying levels of gross central government debt correlated with real annual gross domestic product growth. They sorted country-years into four distinct debt-to-GDP brackets: below 30 percent, 30 to 60 percent, 60 to 90 percent, and above 90 percent.
The paper's headline finding appeared clear and stark. While median and average growth rates remained relatively steady across the first three brackets, the authors reported that average real GDP growth turned negative, contracting at minus 0.1 percent, once debt exceeded 90 percent of GDP. This specific numerical tipping point presented the global policy community with a tangible boundary. Rather than a gentle slowdown, crossing the 90 percent line appeared to trigger an economic precipice, turning debt from an ongoing management problem into an acute threat to national prosperity.