Both fiscal and monetary policies affect aggregate demand. But because discretionary fiscal policy changes in the U.S. are often difficult to enact in a timely fashion, automatic fiscal stabilizers and discretionary monetary policy are commonly viewed as the primary policy tools for macroeconomic stabilization. However, there are situations in which monetary policy might be unable to stimulate the economy, and discretionary fiscal policy would be needed to combat a recession. In the face of a recession, central banks reduce interest rates, but no central bank can lower interest rates below zero. If interest rates fall to zero, as occurred in the U.S. during the Great Depression and in Japan in recent years, monetary policy may be unable to stimulate the economy further, and discretionary fiscal policy would be needed to expand the economy.