What determines the long-run evolution of the real long-term interest rate?

We are interested in the long-run determinants of the evolution of the real long-term interest rate in the United States from 1950 to 2018. The different possible explanatory variables are: Growth in GDP or in productivity (at a given point in time or as a trend); Inflation or money supply growth; Capital intensity; The national savings rate; Demographics (the proportion of the population aged 40 to 64 or over 65); The current-account balance and external debt; The public debt or the fiscal deficit. We find that t he following variables play a significant role in explaining the evolution of the real long-term interest rate in the United States since 1950: Capital intensity; The fiscal deficit; Demographics , b ut not growth, inflation, money supply growth, the savings rate, the current-account balance or external debt. The slowdown in potential growth, monetary policy choices and the growing external debt related to low savings in the United States do not seem to have an effect on the real interest rate, which depends on fiscal policy in the short term and highly structural variables in the long term (the age structure of the population, capital intensity).