Unlike the aggregate demand curve, the aggregate supply curve does not usually shift independently. This is because the equation for the aggregate supply curve contains no terms that are indirectly related to either the price level or output. Instead, the equation for aggregate supply contains only terms derived from the AS-AD model. For this reason, to understand how the aggregate supply curve shifts, we must work from the AS-AD model as a whole.
depicts the AS-AD model. The intersection of the short-run aggregate supply curve, the long-run aggregate supply curve, and the aggregate demand curve gives the equilibrium price level and the equilibrium level of output. This is the starting point for all problems dealing with the AS- AD model.
The primary cause of shifts in the economy is aggregate demand. Recall that aggregate demand can be affected by consumers both domestic and foreign, the Fed, and the government. For a review of the shifters of aggregate demand, see the SparkNote on aggregate demand. In general, any expansionary policy shifts the aggregate demand curve to the right while any contractionary policy shifts the aggregate demand curve to the left. In the long run, though, since long-term aggregate supply is fixed by the factors of production, short-term aggregate supply shifts to the left so that the only effect of a change in aggregate demand is a change in the price level.
Let's work through an example. For this example, refer to . Notice that we begin at point A where short-run aggregate supply curve 1 meets the long-run aggregate supply curve and aggregate demand curve 1. The point where the short-run aggregate supply curve and the aggregate demand curve meet is always the short-run equilibrium. The point where the long-run aggregate supply curve and the aggregate demand curve meet is always the long-run equilibrium. Thus, we are in long-run equilibrium to begin.
Now say that the Fed pursues expansionary monetary policy. In this case, the aggregate demand curve shifts to the right from aggregate demand curve 1 to aggregate demand curve 2. The intersection of short- run aggregate supply curve 1 and aggregate demand curve 2 has now shifted to the upper right from point A to point B. At point B, both output and the price level have increased. This is the new short-run equilibrium.
But, as we move to the long run, the expected price level comes into line with the actual price level as firms, producers, and workers adjust their expectations. When this occurs, the short-run aggregate supply curve shifts along the aggregate demand curve until the long-run aggregate supply curve, the short-run aggregate supply curve, and the aggregate demand curve all intersect. This is represented by point C and is the new equilibrium where short-run aggregate supply curve 2 equals the long-run aggregate supply curve and aggregate demand curve 2. Thus, expansionary policy causes output and the price level to increase in the short run, but only the price level to increase in the long run.