Using an estimated DSGE model that features monetary and fiscal policy interactions and allows for equilibrium indeterminacy, we find that a passive monetary and passive fiscal policy regime prevailed in the pre-Volcker period while an active monetary and passive fiscal policy regime prevailed post-Volcker. Since both monetary and fiscal policies were passive pre-Volcker, there was equilibrium indeterminacy that gave rise to self-fulfilling beliefs and resulted in substantially different transmission mechanisms of policy as compared to conventional models: unanticipated increases in interest rates increased inflation and output while unanticipated increases in lump-sum taxes decreased inflation and output. Unanticipated shifts in monetary and fiscal policies however, played no substantial role in explaining the variation of inflation and output at any horizon in either of the time periods. Pre-Volcker, in sharp contrast to post-Volcker, we find that a time-varying inflation target does not explain low-frequency movements in inflation. Finally, in a counterfactual exercise, we show that had the monetary policy regime of the post-Volcker era been in place pre-Volcker, inflation volatility would have been lower by 36% and the rise of inflation in the 1970s would not have occurred.