Hence it is very important to think about how policy influences the expectations of the private sector. Consider a simple and stark example. Suppose that a central bank managed to control inflation perfectly by responding to all shocks instantaneously. The outcome would be a constant inflation rate. Households and firms would know that potential movements in inflation would never emerge because all future shocks would be instantly offset by changes in interest rates. Interest rates would change with no apparent link to or effect on inflation. To an observer – whether journalist or econometrician – interest rate changes would appear to have little to do with inflation. The central bank would appear to be behaving almost randomly. But that inference would be false. Indeed, if people did expect the central bank to behave randomly, then the behaviour of households and firms would change and inflation would no longer be stable. This is what I call the Maradona theory of interest rates. The great Argentine footballer, Diego Maradona, is not usually associated with the theory of monetary policy. But his performance against England in the World Cup in Mexico City in June 1986 when he scored twice is a perfect illustration of my point. Maradona’s first “hand of God” goal was an exercise of the old “mystery and mystique” approach to central banking. His action was unexpected, time-inconsistent and against the rules. He was lucky to get away with it.
Chart 3 shows that the slope of the short-run Phillips curve has moved around during the post-war period, apparently in response to changes in the monetary policy regime. In the 1970s labour market pressure was not offset by tighter monetary policy, leading to a spiral of wage and price inflation. The short-run Phillips curve steepened, with larger inflationary consequences of any deviation from the natural rate of unemployment. As monetary policy became more focused on controlling inflation, the Phillips curve flattened in the latter part of the 1980s and 1990s. Such changes in the monetary policy regime can also be detected in the behaviour of inflation over time. Table 2 shows that the persistence of inflation – measured by the estimated explanatory power of past inflation in predicting current inflation – has fallen quite markedly since the inflation target was introduced in 1992. Was this  4  BIS Review 36/2005  because the failure of monetary policy to react quickly to an inflationary shock in the 1970s meant that inflation remained high for some time? And has the prompt response of monetary policy meant that movements in inflation more recently have proved short-lived? The answers to these questions matter for monetary policy. But the economy is continually evolving, and we can never definitively conclude that one answer is right and the others wrong. So learning about changes in the structure of the economy lies at the heart of the daily work of central banks.