Supply and Demand - Criticisms

Criticisms

At least two assumptions are necessary for the validity of the standard model: first, that supply and demand are independent; and second, that supply is "constrained by a fixed resource"; If these conditions do not hold, then the Marshallian model cannot be sustained. Sraffa's critique focused on the inconsistency (except in implausible circumstances) of partial equilibrium analysis and the rationale for the upward slope of the supply curve in a market for a produced consumption good. The notability of Sraffa's critique is also demonstrated by Paul A. Samuelson's comments and engagements with it over many years, for example:

"What a cleaned-up version of Sraffa (1926) establishes is how nearly empty are all of Marshall's partial equilibrium boxes. To a logical purist of Wittgenstein and Sraffa class, the Marshallian partial equilibrium box of constant cost is even more empty than the box of increasing cost.".

Aggregate excess demand in a market is the difference between the quantity demanded and the quantity supplied as a function of price. In the model with an upward-sloping supply curve and downward-sloping demand curve, the aggregate excess demand function only intersects the axis at one point, namely, at the point where the supply and demand curves intersect. The Sonnenschein–Mantel–Debreu theorem shows that the standard model cannot be rigorously derived in general from general equilibrium theory.

The model of prices being determined by supply and demand assumes perfect competition. But:

"economists have no adequate model of how individuals and firms adjust prices in a competitive model. If all participants are price-takers by definition, then the actor who adjusts prices to eliminate excess demand is not specified".

Goodwin, Nelson, Ackerman, and Weissskopf write: "If we mistakenly confuse precision with accuracy, then we might be misled into thinking that an explanation expressed in precise mathematical or graphical terms is somehow more rigorous or useful than one that takes into account particulars of history, institutions or business strategy. This is not the case. Therefore, it is important not to put too much confidence in the apparent precision of supply and demand graphs. Supply and demand analysis is a useful precisely formulated conceptual tool that clever people have devised to help us gain an abstract understanding of a complex world. It does not—nor should it be expected to—give us in addition an accurate and complete description of any particular real world market."