Is There a Core of Usable Macroeconomics We Should All Believe In
探讨宏观经济学中是否存在一个公认的实用核心，认为长期趋势由供给面驱动，短期波动主要由总需求驱动，并质疑实际商业周期学派对短期波动的供给面解释。
Real output in most advanced capitalist economies fluctuates around a rising trend. One can argue about whether it is best to think about that trend as passing through successive cyclical averages, defined in one way or another, or best to think of it as passing through cyclical peaks, or some other measure of output. While the outcome of that argument has consequences for macroeconomic theory, I will bypass it for now. The important observation is that, on the whole, the observed fluctuations around trend are contained within a moderately narrow corridor. Unemployment rates tend to run between, say, 5 percent and 10 percent in the United States. (Other countries have different typical ranges, and in each of them, the range can shift from time to time. It is important, theoretically and practically, to understand why; but that remains an open question.) There are notable exceptions to this generalization, of course, the most famous being the depression of the 1930's; but they are exceptions. Again it is important to know why fluctuations are so contained. This could reflect some natural equilibrating process, or it could reflect the intervention of automatic or discretionary government policy, or it could be a mixture of both. That is another issue on which opinions differ. I think it is part of the usable common core of macroeconomics that the trend movement is predominantly driven by the supply side of the economy (the supply of factors of production and total factor productivity) and that the appropriate vehicle for analyzing the trend motion is some sort of growth model, preferably mine. Now, what about those fluctuations around the trend of potential output? A moment ago I put the normal range of unemployment rates at 5-10 percent. By Okun's law I am talking about fluctuations of real GDP with an amplitude of 8-10 percent or so from peak to trough-contained, but not trivial. In my picture of the usable common core of macroeconomics, those fluctuations are predominantly driven by aggregate demand impulses, and the appropriate vehicle for analyzing them is some model of the various sources of expenditure. I am not so obtuse as not to have observed that the whole point of theory is the assertion that these short-run motions of the economy are in fact supplydriven. But my view is that this explanation has been an empirical failure, or at best a nonsuccess. There are now two possibilities. As for the first, I entertain the hope that flexible, observant members of the real-business-cycle school, like Martin Eichenbaum and his coworkers, have come more or less to the same conclusion, and they have found ways to open up the fabric of their underlying model so that it will allow-or insist-that demand-side impulses play the dominant role in short-run macroeconomic fluctuations. Then this proposition is indeed part of the usable core of macroeconomics, and economists can go on to argue back and forth about the best way of modeling those demand-side forces. The other case is that the situation is as before, and the real-business-cycle school holds monolithically to the view that short-run fluctuations are just optimal supply-side adjustments to unforeseeable shocks to tastes and * Department of Economics, Massachusetts Institute of Technology, Cambridge, MA 02139.