Why has the natural rate of interest fallen to zero?
Core argument: In a capital-market equilibrium framework, the model-implied steady-state safe r* across advanced economies falls from roughly 5% in the 1970 steady state to about 0% in the new long-run steady state, reflecting shifts in capital demand and especially capital supply.
I develop and quantify a capital-market equilibrium framework to study the joint evolution of r∗ and wealth across advanced economies. This bloc [saw] shifts in capital demand and especially in capital supply. As a result, the model-implied steady state safe r∗ has fallen from ~ 5% in the 1970 steady state to ~0% in the new long-run steady state. A decomposition of the dynamics of r∗ shows the role of the growth slowdown and demographics [as well as] the rise in mark-ups, which temporarily depress r∗ before pushing it up, and of the rising risk premia, which push down on safe r∗ close to 1-to-1. Decomposition of the rise in the wealth-to-GDP ratio [which ~doubled from 3 to 6], [shows] longer expected retirement and slower growth [but not risk premia] as dominant. [Finally, because] current long-term real yields stand above the baseline r∗ estimate, I consider the possibility that new shifts in the environment have acted to raise the natural rate sharply higher∗. I find that AI and inflation risk scenarios are most powerful quantitatively, each raising r∗ by circa 1 percentage point. The heightened inflation risk raises the safe rate very quickly, while the effects of AI-driven growth boom build up over several years.

