One of these daysI'm going to do a post on why I think the Keynesian cross model is worthless: sheer disinformation. Upon reviewing basic Keynesian macro before teaching it, however, I was relieved to notice that the aggregate demand-aggregate supply (AD-AS) model, though traditionally derived from the Keynesian cross, seems to be separable from it. AD-AS doesn't seem as desperately flawed as the Keynesian cross.
Still, the concept of "aggregate demand" puzzles me. It seems like an attempt to import into macro a brilliant model from micro-- the supply-and-demand model (not aggregate)-- that doesn't really have a clear meaning in its new environment. In AD-AS, the horizontal axis, which in micro is Q (quantity), is called "GDP," which is sort of like Q since GDP is sort of the quantity of stuff being made, while the vertical axis, which in micro is P, is interpreted as "the price level." As in the micro model, demand-- only now it's "aggregate demand"-- slopes down, and supply-- "aggregate supply"-- slopes up, and the place where the curves cross is called "equilibrium." The interaction of AD and AS is supposed to jointly determine the price level and GDP.
Now, to my mind, this whole analysis is thick with fallacies of composition. The P in a micro model is really a relative price. That is generally kept implicit, but the ceteris paribus assumption makes it so. What the Law of Demand (i.e., demand curves always slope down, never mind Giffen goods for now) really says is that if the relative price of a good falls, quantity demanded rises. But it doesn't make sense to speak of all relative prices rising. It's not clear to me why aggregate demand should slope down. I know the textbook reasons but they can easily be countered. It's not clear to me how the ceteris paribus assumption, which is the soul of microeconomics, can be applied here, or why the supply-and-demand model should have any validity without it. In micro, ceteris paribus locks demand and supply in place. The supply and demand schedules depend on other factors. In macro, there are no other factors for them to depend on, so I don't see how you justify treating them as exogenous. I might grant that a price level and/or GDP not at equilibrium is unsustainable, because it will cause inventories to pile up or run down. But I don't see why equilibration should take the form of price level and/or GDP moving, rather than AD and AS moving.
And yet so many economists that I like, and not just from any particular camp, i.e., left-Keynesians like Krugman, but Arnold Kling and Scott Sumner too, and also old Robert Lucas papers, use the concept of "aggregate demand." Why? Why the resilience of the AD concept in view of the valid critiques of it? I think the reason is that it's intuitive. In any recession, what a typical business sees is that demand is weak. They would like to sell more but can't. And it's not just one industry: lots of different industries see that at the same time. Not just one demand curve is moving unfavorably; lots of them are; and that's the cause of, or maybe constitutive of, the downturn. That's the kind of half-empirical-half-intuitive story that people want to find a way to tell.
OK. So let's say we want to salvage aggregate demand somehow. I'm still skeptical about drawing a downward-sloping AD curve. (Or an upward-sloping AS curve; I'll talk about AD but the argument cross-applies.) Where's the empirical evidence that AD slopes down in GDP/price level space? Yes, each individual demand curve slopes down. But products are substitutes for each other: a rise in the price of x boosts demand for product y. So if all prices rise, sellers lose demand from the rise in their own price, but benefit from the rise in price of competing goods. Also, every seller is also a buyer, and a higher price level should mean that customers have more money to spend. This is where the inapplicability of the ceteris paribus assumption is lethal. In the micro demand-supply model, you can raise the price but hold consumer incomes/assets constant. In the macro AD-AS model, that doesn't make sense.
So I was thinking about how to reformulate this, and here's why I start. AD exists, but forget the price level for the moment. AD can drive fluctuations. But AD depends on NNPVFGDP. In case you can't immediately decipher that never-before-seen acronym, it means nominal net present value of future GDP. It's the answer to the question "How much does everyone expected to earn (or spend) over the infinite future?" discounted by the relevant interest rate. The graph I want to draw is this:
No, that's not a mistake: AD is an upward-sloping function of NNPVFGDP, and AS is a downward-sloping function of it. And on the horizontal axis is spending / sales / real GDP. I'm not sure whether the quantity on the horizontal axis should include investment / saving or not.
Why NNPVFGDP? Well, because if people are rational and not (in the aggregate) credit-constrained, that's what AD should really depend on. (If some are credit-constrained, that might be partly offset by other non-credit-constrained people taking the effects of those credit constraints into account and adapting, though I'd have to think more about that one too.) Also, my hunch is that NNPVFGDP is actually to a considerable extent an exogenous factor in business cycles, a source of "shocks." Another hunch is that a substantial amount of the volatility in NNPVFGDP comes from political developments, stuff like war, revolution, or in democracies, public opinion-driven policy revolutions with large redistributive and efficiency ramifications, like the New Deal. But large NNPVFGDP shifts can also emerge from spontaneous market processes, from corporate groupthink and consumer fads and financial bubbles. By the way, NNPVFGDP is all about expectations, and it's not really measurable. This is theory. It might not lend itself easily to being taken to the data. NNPVFGDP also depends a lot on inflation and interest rates, since it's nominal and a net present value.
Why is AD upward-sloping? Because when people expected higher lifetime incomes (higher NNPVFGDP), they spend more.
Why is AS downward-sloping? There's one ironclad reason and a subtler one I haven't thought through properly yet. The ironclad reason is that if people expect higher lifetime incomes, one of the things they'll consume more of is leisure. They'll work less. That reduces the aggregate quantity supplied. The subtle reason is that businesses that expect higher demand in the future might produce less goods for current consumption and more for future consumption-- you see why I'm unsure whether to include investment in the quantity on the horizontal axis.
Something else to bear in mind. This is irritating but important. When I say that people will "work less," I don't necessarily mean they'll put in fewer hours in the office. Work is not just a way of earning money. People get satisfaction from work in itself. So when NNPVFGDP rises, people may shift from jobs that pay better to jobs they like better. I think this is important, but it would be very hard to tease it out of the data. However, the downward-sloping AS curve captures this in a way. Higher NNPVFGDP might mean people are less productive even though they work the same number of hours or even more. They have faith in the future, so they do more of what they like.
Why is real GDP on the horizontal axis? Two reasons. (a) Real GDP is what we care about so it's nice to have it in the chart. But also, (b) it seems realistic to me that sellers and buyers both make their plans in real terms. What real standard of living do I plan on having? What real production plans does a firm plan to execute? They also have expectations about prices, of course, but they make plans about real behaviors. So the AD schedule captures the real plans that people make as consumers, for the current period, given NNPVFGDP expectations; and AD captures the real plans that people and firms make as workers/suppliers/sellers, given NNPVFGDP expectations. And these plans might be inconsistent.
I think the slopes of these AD and AS curves could be much better defended than those in the traditional AD-AS model. There is much more reason to think that AD is an increasing function of NNPVFGDP than to think that it's a downward-sloping function of the price level. Likewise with AS.
OK, now suppose you're in equilibrium and there's a negative exogenous shock to NNPVFGDP. What happens? You end up here:
The drop in NNPVFGDP puts the economy out of equilibrium, with AS taking a higher value than AD. What this means is that sellers want to sell a lot more than buyers want to buy. That doesn't seem like a sustainable situation, but the graph doesn't tell you how it will get resolved. We canNOT assume that AD and AS stay put and equlibrium is restored by changes in NNPVFGDP and spending/sales/real GDP: as I said before, demand and supply are exogenous in micro because of the ceteris paribus assumption, which can't be applied in a macro model. There are lots of ways the disequilibrium situation might be resolved, such as:
1. Interest rates fall. The future is discounted less, so NNPVFGDP rises, and the economy moves up the AD curve and down the AS curve back to equilibrium.
2. Prices fall due to supply-side competition. If the price drop is expected to be temporary, this pushes AD out and AS down. If permanent, maybe NNPVFGDP rises. Either change might leave current GDP in about the same place.
3. Unemployment results in a destruction of economic capacity. AS moves down and NNPVFGDP falls.
There are a lot of combinations of these, and there are other possibilities. In general, a lot will depend on which prices in the economy are more and less flexible, or perhaps asymmetrically flexible: wages are said to be downwardly rigid for example. This version of AD-AS provides a framework into which a lot of other stories might fit.
It's clear how monetary policy works here. Suppose the Fed lowers interest rates. Then NNPVFGDP rises, since the relevant discount rate for future incomes is reduced. The catch is that the Fed can't necessarily control the relevant interest rate, since what matters for NNPVFGDP is mainly long-term rates, and the Fed directly controls only very short-term rates. The other catch is that the Fed might not be able to push nominal interest rates below zero.
How does fiscal policy come in here? Let's suppose it can't affect AD through the price level. Maybe it gives money to credit-constrained people and they buy more. That would push AD outward for any given level of NNPVFGDP. On the other hand, non-credit-constrained people might expect higher taxes, offsetting, maybe fully offsetting, or even more, the boost to AD. Maybe the government could bring the economy closer to equilibrium with highly distortionary policies that are meant to be temporary and don't affect NNPVFGDP too much. But it would be an equilibrium with lower GDP. This is a possibility suggested by the model; I'm not sure of the interpretation.
The verdict seems to be that monetary policy can work, fiscal policy can't.
OK, now what does the boom look like? Like this:
So in this case, AD is greater than AS: buyers want to consume more now than sellers want to supply them, given projected NNPVFGDP. This situation isn't too hard to resolve: either interest rates rise, or prices rise. That may be why booms don't seem to cause as serious problems as recessions.
This is very preliminary, I'm just brainstorming here, jotting down thoughts. It kind of seems like a better place to start. Maybe if I formalized it a bit, the intuitions would become clearer.
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