In our earlier discussion of the uses of aggregate demand and aggregate supply analysis in textbooks, the proposal was made that these concepts make it easy, or possible, to represent the effects of an aggregate demand or aggregate supply shock. The textbook and professional terminology nowadays defines shock as a shift of aggregate demand or supply that is not caused by government. I had not kept up with the literature on "shocks." So I made a point to pay attention to this in my recent readings. Consider the following four examples of shocks that are taken from a Journal of Economic Perspectives article by Lars Swensson (The Journal of Economic Perspectives, Vol. 17, No. 4. (Autumn, 2003), pp. 145-166.) 1. Bursting an asset bubble: This cannot be an exogenous shock. The money that disappears from the asset market and that causes the asset price to fall must be put somewhere else. It must cause other asset prices to rise. So whence the shift in aggregate supply or aggregate demand? 2. A correction of over optimistic growth and productivity expectations: This cannot be an exogenous shock. If entrepreneurs begin to expect slower growth and productivity, they will reduce the demands for funds. Savers will respond by either increasing or decreasing their saving, leading to the opposite effect on consumer spending. Whatever change in aggregate investment spending occurs will be offset by a change in aggregate consumption spending, as we know from the crowding out effect. So whence the shift in aggregate supply or aggregate demand? 3. Increased doubts about future pensions and benefits due to demographic developments and/or reckless fiscal policy: If consumers begin to be concerned that pension promises will not be kept, they presumably would reduce consumption spending and increase saving and investment. So whence the shift in aggregate supply or aggregate demand? Could it come from increased liquidity or gambling on asset prices? It is hard to believe that prudent savers would use their increased savings in these ways, at least as a general rule. 4. Increased uncertainty for geopolitical or other reasons. I have no idea how to handle this one. Perhaps Professor Svennson means this to refer to the common idea of an oil price shock. So let's ask about that case. 5. Oil price shock: The oil price shock is one example of a much larger class of possible events that, other things equal, would reduce the outputs of many goods that could be produced with given amounts of other inputs. Both aggregate demand-aggregate supply analysis and the simple quantity theory can be used to predict that real output would fall and the price level would rise. But what is the advantage of AD-AS analysis? I suggest that it has a disadvantage. The whole framework is constructed with idea in mind that real output is related to employment of work. Yet there is no reason to think that a decrease in the amount of some other input besides work would reduce (or increase) the amount of work. In the standard textbook model, we might propose for simplicity that, beginning with a full employment macro equilibrium, a decrease in aggregate supply would raise prices and reduce real output. But we would have to add that it would change the full employment level of real output. It would be an error to say that the supply shock could cause greater unemployment without some further, more complicated analysis. For the same reason it would be an error to say that some kind of macro policy might be in order to offset the effects. If you don't buy the reasoning, perhaps you will buy the empirical research. It has been argued that international oil price hikes were responsible for stagflation. Indeed, AD-AS analysis has been defended on this list as a means of explaining stagflation. But research on this issue suggest that this is a misuse of the analysis. (Not to mention, of course, the absence of stagflation at the present moment in history, following a recent hike in international oil prices.) Systematic Monetary Policy and the Effects of Oil Price Shocks Ben S. Bernanke; Mark Gertler; Mark Watson; Christopher A. Sims; Benjamin M. Friedman Brookings Papers on Economic Activity, Vol. 1997, No. 1. (1997), pp. 91-157. Barsky, Robert, and Lutz Killian. 2001. "Do We Really Know That Oil Caused the Great Stagflation?" NBER Working Paper 8389. Pat Gunning