The term stagflation came into common use in the mid-1970s, when many advanced economies experienced higher inflation and slower growth than they had in the 1960s. At the time, the joint behavior of inflation and economic growth confused many economists. Throughout the 1950s and 1960s, growth and inflation generally moved in the same direction. Most important, inflation tended to fall during recessions and to rise in booms. Stagflation meant that these two key summary measures of macroeconomic performance moved in opposite directions. What caused this dramatic, painful, and persistent shift?

To understand the sources of stagflation in the 1970s—and how we subsequently avoided a repeat of that episode (at least so far)—we start with the simple premise that there are two types of disturbances hitting the economy: demand and supply. The first, changes in demand, moves inflation and growth in the same direction. The broad array of things that shift demand include fluctuations in consumer or business confidence, shifts in government tax and expenditure policy, and variation in the appeal of imports to domestic residents or of exports to foreigners. When any of these goes up or down, inflation and output rise and fall together.

Supply disturbances—which alter the cost of production—are fundamentally different. These stagflationary shocks move growth and inflation in opposite directions. For example, an adverse supply shock that raises the cost of production at least temporarily drives inflation up and growth down.

Importantly, these cost shocks cannot be the whole story behind a decade-long surge of inflation. Whether the consequences of a cost shock are one-off adjustments in the price level or an increase of the trend of inflation depends on the monetary policy response. Put differently, monetary policy determines whether we experience stagflation over any longer interval….