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Recent vector autoregression (VAR) studies have shown that monetary policy shocks have had a reduced effect on the economy since the beginning of the 1980s. This paper investigates the causes of this change. First, we estimate an identified VAR over the pre- and post-1980 periods, and corroborate the existing results suggesting a stronger systematic response of monetary policy to the economy in the later period. Second, we present and estimate a fully specified model that replicates well the dynamic response of output, inflation, and the federal funds rate to monetary policy shocks in both periods. Using the estimated structural model, we perform counterfactual experiments to quantify the relative importance of changes in monetary policy and changes in the private sector in explaining the reduced effect of monetary policy shocks. The main finding is that changes in the systematic elements of monetary policy are consistent with a more stabilizing monetary policy in the post-1980 period and largely account for the reduced effect of unexpected exogenous interest rate shocks. Consequently, there is little evidence that monetary policy has become less powerful.

Under rational expectations, monetary policy is generally highly effective in stabilizing the economy. Aggregate demand management operates through the expectations hypothesis of the term structure: Anticipated movements in future short-term interest rates control current demand. This paper explores the effects of monetary policy under imperfect knowledge and incomplete markets. In this environment, the expectations hypothesis of the yield curve need not hold, a situation called unanchored fi nancial market expectations. Whether or not financial market expectations are anchored, the private sector's imperfect knowledge mitigates the efficacy of optimal monetary policy. Under anchored expectations, slow adjustment of interest rate beliefs limits scope to adjust current interest rate policy in response to evolving macroeconomic conditions. Imperfect knowledge represents an additional distortion confronting policy, leading to greater inflation and output volatility relative to rational expectations. Under unanchored expectations, current interest rate policy is divorced from interest rate expectations. This permits aggressive adjustment in current interest rate policy to stabilize inflation and output. However, unanchored expectations are shown to raise significantly the probability of encountering the zero lower bound constraint on nominal interest rates. The longer the average maturity structure of the public debt, the more severe is the constraint.