The optimal timing of real investment is studied under the assumptions that investment is irreversible and that new information about returns is arriving over time. Investment should be undertaken in this case only when the costs of deferring the project exceed the expected value of inforrnation gained by waiting. Uncertainty, because it increases the value of waiting for new information, retards the current rate of investment. The nature of investor's optimal reactions to events whose implications are resolved over time is a possible explanation of the instability of aggregate investment over the business cycle.

To overcome the limits on traditional monetary policy imposed by the effective lower bound on short-term interest rates, in recent years the Federal Reserve and other advanced-economy central banks have deployed new policy tools. This lecture reviews what we know about the new monetary tools, focusing on quantitative easing (QE) and forward guidance, the principal new tools used by the Fed. I argue that the new tools have proven effective at easing financial conditions when policy rates are constrained by the lower bound, even when financial markets are functioning normally, and that they can be made even more effective in the future. Accordingly, the new tools should become part of the standard central bank toolkit. Simulations of the Fed’s FRB/US model suggest that, if the nominal neutral interest rate is in the range of 2–3 percent, consistent with most estimates for the United States, then a combination of QE and forward guidance can provide the equivalent of roughly 3 percentage points of policy space, largely offsetting the effects of the lower bound. If the neutral rate is much lower, however, then overcoming the effects of the lower bound may require additional measures, such as a moderate increase in the inflation target or greater reliance on fiscal policy for economic stabilization.

Adverse shocks to the economy may be amplified by worsening credit-market conditions-- the financial 'accelerator'. Theoretically, we interpret the financial accelerator as resulting from endogenous changes over the business cycle in the agency costs of lending. An implication of the theory is that, at the onset of a recession, borrowers facing high agency costs should receive a relatively lower share of credit extended (the flight to quality) and hence should account for a proportionally greater part of the decline in economic activity. We review the evidence for these predictions and present new evidence drawn from a panel of large and small manufacturing firms.

We study the phenomenon of short-run increasing returns to labor (SRIRL) in a sample of 10 interwar U.S. manufacturing industries. Our main findings are that SRIRL was common in the interwar period and that the pattern of SRIRL across industries was similar to that observed in the postwar period. We argue that, since presumably the Depression was not caused by technical regress, these findings are inconsistent with the claim of real business cycle theorists that SRIRL is in general due to procyclical technical shocks. We propose tests for discriminating between two other leading explanations of SRIRL but find that our conclusions differ by industry.

This paper analyzes the impact of changes in monetary policy on equity prices, with the objectives of both measuring the average reaction of the stock market and understanding the economic sources of that reaction. We find that, on average, a hypothetical unanticipated 25‐basis‐point cut in the Federal funds rate target is associated with about a 1% increase in broad stock indexes. Adapting a methodology due to Campbell and Ammer, we find that the effects of unanticipated monetary policy actions on expected excess returns account for the largest part of the response of stock prices.