A lot of economics bloggers have been discussing the merits and flaws of the paradigm of aggregate supply and demand. Mark Thoma linked to a couple of the more recent examples.
I think that, viewed on its own terms, AS-AD is confusing (or confused). That is, mainstream economists do not know what to put on the vertical axis. If you put the price level on the vertical axis, you get a nice textbook model, but no connection to the real world, where economists talk in terms of inflation rather than the price level. On the other hand, if you put inflation on the vertical axis, that does not work very well, either, as I explain in this 8-minute video. (Comments, other than about my handwriting, are welcome.)
I developed PSST as an alternative to AS-AD. PSST does not appear to explain nearly as much as AS-AD. That may look like a feature in AS-AD, but on closer inspection it is a bug. The explanatory “power” of AS-AD is a delusion.
1. The AS-AD paradigm is invoked to explain changes in the combination of inflation and unemployment. If one goes up while the other goes down, we call this an aggregate demand shift. If both move in the same direction, we call this an aggregate supply shift. Thus, we can explain anything. But there is a lot of hand-waving involved. The stories we tell about AD and AS are always post hoc, just-so stories.
2. In the 1970s, we had a huge rise in the combination of inflation and unemployment. What caused this? The rise in the price of imported oil is often cited, but it cannot possibly account for the large acceleration in inflation. At most, it would cause a temporary increase in inflation, followed by a decrease. Many (most?) macroeconomists attribute the 1970s disaster to a “rise in inflation expectations.” Prices rose because they were expected to rise. Through habit and repetition, we have come to accept this story. But how intellectually satisfying is that?
3. Turning to events since 2008, the typical macroeconomist describes the rise in unemployment as the result of a “demand shock.” However, they do not all agree on what the shock was. Scott Sumner says that it was a contraction by the Fed. Others say that it was the wealth effect of a decline in house prices. Others say that it was a credit crunch. Others say that it was the effect of inflation dropping too close to zero. All of these are just-so stories.
From April of 2003 through April of 2008, the rate of growth of the CPI averaged 3.2 percent. From April of 2008 through April of 2013, it averaged 1.6 percent. If in 2007 you had asked macroeconomists to predict the consequences of a decline in the inflation rate of that magnitude, how many would have told you to expect unemployment to rise above 7 percent? None of them would have foreseen it. My guess is that many of the macroeconomists would have regarded a drop in inflation of 1.6 percentage points as close to a non-event for unemployment. (Note to Scott Sumner: yes, the decline in nominal GDP growth was much bigger than the decline in inflation. But that only restates the mystery–it doesn’t solve it.)
The terms “aggregate demand” and “aggregate supply” are highly loaded. That is, they lead economists to imagine something analogous to supply and demand in microeconomics. But the analogy is mostly misleading, and economists who invoke AS and AD are the ones who are most misled.