Can governments count on favorable interest-rate-minus-growth trends for debt sustainability?
Core argument: Duration-matched r-g for a weighted average of four key economies rose by an annual average of 7bps on a linear trend basis over 1931–2023, a qualitative change from the -2.7bps trend measured over 1507–1931, though the later period exhibits substantial volatility.
Over the observation period of 1931-2023, "Global AW [average weighted over 4 key economies]" duration-matched r-g [the difference between long-term interest rates and trend growth rates] increases by an annual average of 7bps when measured on a linear [trend] basis, but the period exhibits substantial volatility. The post-1932 era represents a substantial qualitative change compared to the downward trend of 2.7bps observed for the same series over 1507-1931, but If one looks at the longer sample, the idea that the world today can likely count on favorable trends in r-g and public debt sustainability dynamics, as much recent literature has done, becomes questionable. Our reconstructions reveal many historical examples of fairly sharp (visual) reversals in duration-matched r-g, for instance during the 1550s or during the 1690s (the Glorious Revolution and Louis XIV’s European Wars), the Napoleonic Wars, or the 1860s (the Crimean and Prussian Wars). In all these cases, fiscal outlays surged rapidly and r-g increases of more than 4% were observed within a decade of a new multi-decade r-g low.





