Keynesian-Monetarist Dynamics and the Corridor

The idea that for small disturbances the full employment equilibrium is stable while for large disturbances it is unstable was coined by Leijonhufvud in the notion of a "corridor". We discuss the existence of a corridor in the standard Keynesian-Monetarist textbook macro-model. It turns out that though the full employment steady state of this model may be locally stable - which is the case when the well-known Cagan condition holds - the model is never globally stable. This is due to the inherent non-linearity in the demand for money function, arising from non-negativity of the nominal rate of interest. Thus, perhaps surprisingly, the Cagan condition is both necessary and sufficient for the existence of a corridor in the Keynesian-Monetarist model.