The 90% Public Debt Threshold: The Rise & Fall of a Stylised Fact

This paper analyses the original Reinhart-Rogoff dataset, made public by Herndon et al. (2013), on the basis of descriptive statistics and formal econometric testing. First, based on the public debt thresholds (30%, 60% and 90%) proposed by Reinhart and Rogoff (2010), descriptive statistics reveal that real GDP growth slows considerably as the central government debt-to-GDP ratio goes beyond the 30% threshold and that no further slowdown can be observed in the data as the debt-to-GDP ratio rises above 60% and 90% during the periods 1790-2009 and 1946-2009. For the United States (1946-2009), the negative nonlinear finding completely disappears for any level of public debt, once reverse causality and influential outliers are accounted for. Looking at general (and central) government debt during the more recent period of 1960-2009 suggests that economic slowdown occurs when public debt moves above 60% or 90% of GDP. But it seems more appropriate to determine nonlinearity and the associated debt threshold endogenously. Therefore, in a second stage, we put the Reinhart-Rogoff dataset to a formal econometric test by employing nonlinear threshold models. Overall, our estimation results indicate that the nonlinear relation from debt to growth is not very robust. Taken with a pinch of salt, our results suggest, however, that there may be a tipping point at around 20% of GDP, beyond which central government debt has a negative influence on growth. Further (and greater) thresholds may exist but their magnitude is highly uncertain. For general government debt (1960-2009), the threshold beyond which negative growth effects kick in is considerably higher at about 50%. Finally, individual country estimates reveal a large amount of cross-country heterogeneity. For some countries including the United States, a nonlinear negative link can be detected at about 30% of GDP. For others, the thresholds are surrounded by a great amount of uncertainty or no nonlinearities can be established. This instability may be a result of threshold effects changing over time within countries and depending on economic conditions, not captured in our estimations. Overall, our results can be seen as a formal econometric confirmation that the 90% public debt threshold is not in the data. But our results also seem to suggest that public debt might have a negative effect on economic performance kicking in at already fairly moderate public debt levels. Furthermore, the absence of threshold effects or low estimated thresholds may not preclude the emergence of further threshold effects, especially as public debt levels are rising to unprecedentedly high levels.