Why do central banks intervene?

Intervention by central banks, in terms of buying and selling foreign currency, has been a major activity in recent years. This paper investigates the motivations for such policy and the evidence for its effectiveness. We use high quality daily data on the dollar amounts of intervention by the central banks of the US and Germany. We also use information on agreed G7 target levels for the $/DM and $/Yen nominal exchange rates. Daily, nominal dollar exchange rate returns are well described as a Martingale-GARCH process, and we find little evidence that the different types of intervention have had much effect on the conditional mean of exchange rate returns. There is some evidence that intervention is associated with slight increases in the volatility of exchange rate returns. While little evidence is found for the effectiveness of intervention, the motivations are more clear. In particular, from the application of probit analysis we find that the probability of intervention is determined by the magnitude of the deviation of the nominal exchange rate from the agreed target level and, to a lesser extent, by the current volatility of exchange rates.