Does the gap between interest rates and growth determine debt sustainability?
Core argument: With debt at 300% of GDP and an interest rate–growth differential of 1.5%, a primary surplus of 4.5% of GDP is required to stabilize the debt ratio.
The sustainability of debt hinges on the relationship between interest rates and economic growth. If the interest rate (r) exceeds the growth rate (g), debt can snowball, with the debt-to-GDP ratio accelerating. Debt cannot surpass total wealth, and as it rises, investors demand higher returns, necessitating large primary surpluses to curb debt growth. For instance, with debt at 300% of GDP and r-g at 1.5%, a primary surplus of 4.5% of GDP is needed to stabilize debt levels. Historical precedents exist, but achieving such surpluses today poses significant political challenges, potentially leading to financial repression or restructuring.





