A dynamic IS–LM model with two time delays in the public sector.

Some recent contributions to Economic Dynamics have shown an increasing interest on the impact that fiscal policy lags may have on the income adjustment processes. Lags dealing with fiscal policy come from delays either in the government expenditure or in the tax revenues. These two lags yield jointly their macroeconomic effects. They are such that to make traditional fiscal policy rules ineffectual to control, and stabilize the GDP dynamics. Here we study a dynamic IS–LM model where the public expenditure and the tax revenues have a delayed functional form. We show that the equilibrium of the system may lose or gain its local stability depending either on the length of the lags or on their particular combinations. When instability arises, very complicated dynamics may characterize the national income time path.