Reinhart and Rogoff’s empirical result posited a clear threshold. Reinhart and Rogoff were clear that debt-to-GDP ratio above 90% spelled doom for growth. The actual data is far less clear:
There is some correlation, but that correlation was loose enough to suggest that this was just one factor of many, and it never said anything at all about whether high debt caused low growth, or low growth caused high debt, or whether some exogenous factor was causing both. The real questions are all about causation.
Far from being a magical no-growth threshold, the UK experienced some of its strongest growth at a public debt level above 90% of GDP, suggesting very strongly that there are many other factors in play. In general, I would tend to caution against the use of arbitrary thresholds to establish principles in economics, whether that is the debt level necessary to lower growth, or the leverage level necessary to trigger a bank run, etc. The evidence suggests these almost certainly vary on a case-by-case basis.
[I]t is widely acknowledged, based on serious research, that when public debt levels rise about 90% they tend to have a negative economic dynamism, which translates into low growth for many years.
Paul Ryan defended austerity using the same criteria:
Economists who have studied sovereign debt tell us that letting total debt rise above 90 percent of GDP creates a drag on economic growth and intensifies the risk of a debt-fueled economic crisis.
It’s an excellent study, although in some ways what you’ve summarized understates the risks.
[W]e would soon get to a situation in which a debt-to-GDP ratio would be 100%. As economists such as Reinhart and Rogoff have argued, that is the level at which the overall stock of debt becomes dangerous for the long-term growth of an economy. They would argue that that is why Japan has had such a bad time for such a long period. If deficits really solved long-term economic growth, Japan would not have been stranded in the situation in which it has been for such a long time.
The debt hurts the economy already. The canonical work of Carmen Reinhart and Kenneth Rogoff and its successors carry a clear message: countries that have gross government debt in excess of 90% of Gross Domestic Product (GDP) are in the debt danger zone. Entering the zone means slower economic growth.
This all feels very much like a case of the Emperor’s New Clothes. Those shining robes that cloaked the austerian case for austerity now and at-all-costs were based on serious methodological errors — as opposed to more nuanced criteria for fiscal consolidation during the boomtime, when interest rates on government debt exceed the unemployment rate. All those serious people who praised Reinhart and Rogoff’s seriousness clearly didn’t read it very well, or study the underlying data. Much more like they formed an opinion on the necessity of austerity now, and looked around for whatever evidence they could find for their preconception, whether Reinhart and Rogoff, or Alessina and Ardagna.
The fact that Reinhart and Rogoff did not, and are still not prepared to issue some clarification to their study to prevent its abuse by austerity-obsessed policymakers is sad given the copious evidence that austerity under present conditions is self-defeating. The fact that their response has so far consisted of defending their very weak conclusions — in full knowledge of the political implications of their work, and how it has been used to justify harsh austerity in very slack economic conditions — is very sad indeed.