Understanding Excess Supply (The Non-Algebraic Version)

I have just taught the theory of the supply curve at the principles level for the umpteenth time, and my conscience is in open rebellion. This business with the horizontal demand curve and setting supply equal to marginal costs is simply rubbish; it denies some of the most important, and obvious, facts about how capitalism actually works.

At the level of an individual firm, the theory obscures what ought to be the starting point for analysis—that in a capitalist economy the normal state of affairs is that firms set production at a level that requires them to chase consumers any way they can, and that the usual result is that some offerings go unsold. Across the entire economy the level of activity is nearly always demand-constrained, not supply-constrained.

Consider the relationship between buyers and sellers at the level of individual enterprises. From observers like Alec Nove and Janos Kornai, we have come to recognize that the prevalence of buyers’ markets is what distinguishes capitalism; in the state-managed systems of pre-1989 socialism, the seller was king. This suggests that excess supply is the most likely state of affairs, excess demand the least likely. An exact equality between demand and supply at the market-determined price is essentially impossible.

Of course, demand is not determinant. It is best represented as a subjective frequency distribution, to which sellers would adjust their production plans. Suppose, to keep things simple, this is a normal distribution. (I don’t think this assumption changes anything important.) Suppose also that marginal costs are symmetric around the mean of expected demand—that the increase of MC for one unit more is equal to its decrease for one unit less. This equalizes the direct financial cost of over- and underproduction, another convenience for our analysis. (If MC increases at an increasing rate, incidentally, this would be a reason for a bias toward underproduction, and therefore excess demand, ceteris paribus; so it’s not the answer we’re looking for.) Now let the seller maximize profit by making an ex ante decision about how much to supply for this uncertain demand.

What we would expect to see is an equal incidence of ex post excess demand and excess supply. But this is not how it is.

Let’s add a couple of new elements: first, suppose that consumers are not utility maximizers, gathering all information about product quality, prices and suppliers costlessly and then making the optimal purchase, but satisficers. They have benchmark price and quality points, and they select the first seller who meets them. Second, consider a sequential model where, in each period, consumers begin their search with the seller they transacted with in the previous period, so, if the seller continues to meet the buyers’ price and quality points, the customer is still theirs. This fits with the literature in marketing, which stresses that a sale should always be seen as the opening to future sales and therefore worth a much greater investment than it would justify from a myopic perspective.

This new element has the effect of biasing the seller’s choice of a production level: it is more expensive to lose out on a sale than to produce or stock an extra item for which there is no ex post demand. Producers in general set their output to the right of the mean of expected demand, and excess supply is the norm.

This is not quite eureka. It works for the special case of constant variable costs, but there is more work to do to incorporate upward-sloping MC and the effect that raising the selling price (due to being further out on the MC curve) has on the proportion of consumers (whose price benchmarks are also stochastic) who will continue to satisfice. What we get, in the end, is a case for generalized excess supply that depends on the relationship between the parameters governing the two forms of uncertainty—the size of the market (the number of desired purchases) and the market share for any single firm (the proportion of buyers who regard a particular selling price as acceptable)—as well as the seller’s marginal cost structure. For further complication one can also relax the assumption that buyers always return if their satisficing conditions are met; this probability could also be governed by a parameter. (Note that one useful result of this model is that it relaxes the so-called law of one price in a way that is consistent with real-world data.)

Such a model would provide microfoundations for demand-constrained macroeconomics. Working this out is of less interest to me, but it should be clear that there is a certain amount of equilibrium slack in such a system. What would the dynamics look like, however? Would chronic excess-suppliers of this sort respond differently to aggregate demand shocks, compared to the Walrasian firms that populate existing general equilibrium models?

During the next two years economists (at least those in the US) will be freed from having to think about policy and can shift their attention to the theoretical foundations of their discipline.