Question

If the Phillips curve is vertical in the long run, what will happen if the central bank tries to push
unemployment rate below its natural rate?

Answer 1

The phenomenon of stagflation and the break down in the Phillips curve led economists to look more deeply at the role of expectations in the relationship between unemployment and inflation. Because workers and consumers can adapt their expectations about future inflation rates based on current rates of inflation and unemployment, the inverse relationship between inflation and unemployment could only hold over the short run.

When the central bank increases inflation in order to push unemployment lower, it may cause an initial shift along the short run Phillips curve, but as worker and consumer expectations about inflation adapt to the new environment, in the long run the the Phillips curve itself can shift outward. This is especially thought to be the case around the natural rate of unemployment or NAIRU (Non Accelerating Inflation Rate of Unemployment), which essentially represents the normal rate of frictional and institutional unemployment in the economy. So in the long run, if expectations can adapt to changes in inflation rates then the long run Phillips curve resembles and vertical line at the NAIRU; monetary policy simply raises or lowers the inflation rate after market expectations have worked them selves out.

In the period of stagflation, workers and consumers may even begin to rationally expect inflation rates to increase as soon as they become aware that the monetary authority plans to embark on expansionary monetary policy. This can cause an outward shift in the short run Phillips curve even before the expansionary monetary policy has been carried out, so that even in the short run the policy has little effect on lowering unemployment, and in effect the short run Phillips curve also becomes a vertical line at the NAIRU.

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