Archive for the 'supply-demand analysis' Category

The Rises and Falls of Keynesianism and Monetarism

The following is extracted from a paper on the history of macroeconomics that I’m now writing. I don’t know yet where or when it will be published and there may or may not be further installments, but I would be interested in any comments or suggestions that readers might have. Regular readers, if there are any, will probably recognize some familiar themes that I’ve been writing about in a number of my posts over the past several months. So despite the diminished frequency of my posting, I haven’t been entirely idle.

Recognizing the cognitive dissonance between the vision of the optimal equilibrium of a competitive market economy described by Marshallian economic theory and the massive unemployment of the Great Depression, Keynes offered an alternative, and, in his view, more general, theory, the optimal neoclassical equilibrium being a special case.[1] The explanatory barrier that Keynes struggled, not quite successfully, to overcome in the dire circumstances of the 1930s, was why market-price adjustments do not have the equilibrating tendencies attributed to them by Marshallian theory. The power of Keynes’s analysis, enhanced by his rhetorical gifts, enabled him to persuade much of the economics profession, especially many of the most gifted younger economists at the time, that he was right. But his argument, failing to expose the key weakness in the neoclassical orthodoxy, was incomplete.

The full title of Keynes’s book, The General Theory of Employment, Interest and Money identifies the key elements of his revision of neoclassical theory. First, contrary to a simplistic application of Marshallian theory, the mass unemployment of the Great Depression would not be substantially reduced by cutting wages to “clear” the labor market. The reason, according to Keynes, is that the levels of output and unemployment depend not on money wages, but on planned total spending (aggregate demand). Mass unemployment is the result of too little spending not excessive wages. Reducing wages would simply cause a corresponding decline in total spending, without increasing output or employment.

If wage cuts do not increase output and employment, the ensuing high unemployment, Keynes argued, is involuntary, not the outcome of optimizing choices made by workers and employers. Ever since, the notion that unemployment can be involuntary has remained a contested issue between Keynesians and neoclassicists, a contest requiring resolution in favor of one or the other theory or some reconciliation of the two.

Besides rejecting the neoclassical theory of employment, Keynes also famously disputed the neoclassical theory of interest by arguing that the rate of interest is not, as in the neoclassical theory, a reward for saving, but a reward for sacrificing liquidity. In Keynes’s view, rather than equilibrate savings and investment, interest equilibrates the demand to hold the money issued by the monetary authority with the amount issued by the monetary authority. Under the neoclassical theory, it is the price level that adjusts to equilibrate the demand for money with the quantity issued.

Had Keynes been more attuned to the Walrasian paradigm, he might have recast his argument that cutting wages would not eliminate unemployment by noting the inapplicability of a Marshallian supply-demand analysis of the labor market (accounting for over 50 percent of national income), because wage cuts would shift demand and supply curves in almost every other input and output market, grossly violating the ceteris-paribus assumption underlying Marshallian supply-demand paradigm. When every change in the wage shifts supply and demand curves in all markets for good and services, which in turn causes the labor-demand and labor-supply curves to shift, a supply-demand analysis of aggregate unemployment becomes a futile exercise.

Keynes’s work had two immediate effects on economics and economists. First, it immediately opened up a new field of research – macroeconomics – based on his theory that total output and employment are determined by aggregate demand. Representing only one element of Keynes’s argument, the simplified Keynesian model, on which macroeconomic theory was founded, seemed disconnected from either the Marshallian or Walrasian versions of neoclassical theory.

Second, the apparent disconnect between the simple Keynesian macro-model and neoclassical theory provoked an ongoing debate about the extent to which Keynesian theory could be deduced, or even reconciled, with the premises of neoclassical theory. Initial steps toward a reconciliation were provided when a model incorporating the quantity of money and the interest rate into the Keynesian analysis was introduced, soon becoming the canonical macroeconomic model of undergraduate and graduate textbooks.

Critics of Keynesian theory, usually those opposed to its support for deficit spending as a tool of aggregate demand management, its supposed inflationary bias, and its encouragement or toleration of government intervention in the free-market economy, tried to debunk Keynesianism by pointing out its inconsistencies with the neoclassical doctrine of a self-regulating market economy. But proponents of Keynesian precepts were also trying to reconcile Keynesian analysis with neoclassical theory. Future Nobel Prize winners like J. R. Hicks, J. E. Meade, Paul Samuelson, Franco Modigliani, James Tobin, and Lawrence Klein all derived various Keynesian propositions from neoclassical assumptions, usually by resorting to the un-Keynesian assumption of rigid or sticky prices and wages.

What both Keynesian and neoclassical economists failed to see is that, notwithstanding the optimality of an economy with equilibrium market prices, in either the Walrasian or the Marshallian versions, cannot explain either how that set of equilibrium prices is, or can be, found, or how it results automatically from the routine operation of free markets.

The assumption made implicitly by both Keynesians and neoclassicals was that, in an ideal perfectly competitive free-market economy, prices would adjust, if not instantaneously, at least eventually, to their equilibrium, market-clearing, levels so that the economy would achieve an equilibrium state. Not all Keynesians, of course, agreed that a perfectly competitive economy would reach that outcome, even in the long-run. But, according to neoclassical theory, equilibrium is the state toward which a competitive economy is drawn.

Keynesian policy could therefore be rationalized as an instrument for reversing departures from equilibrium and ensuring that such departures are relatively small and transitory. Notwithstanding Keynes’s explicit argument that wage cuts cannot eliminate involuntary unemployment, the sticky-prices-and-wages story was too convenient not to be adopted as a rationalization of Keynesian policy while also reconciling that policy with the neoclassical orthodoxy associated with the postwar ascendancy of the Walrasian paradigm.

The Walrasian ascendancy in neoclassical theory was the culmination of a silent revolution beginning in the late 1920s when the work of Walras and his successors was taken up by a younger generation of mathematically trained economists. The revolution proceeded along many fronts, of which the most important was proving the existence of a solution of the system of equations describing a general equilibrium for a competitive economy — a proof that Walras himself had not provided. The sophisticated mathematics used to describe the relevant general-equilibrium models and derive mathematically rigorous proofs encouraged the process of rapid development, adoption and application of mathematical techniques by subsequent generations of economists.

Despite the early success of the Walrasian paradigm, Kenneth Arrow, perhaps the most important Walrasian theorist of the second half of the twentieth century, drew attention to the explanatory gap within the paradigm: how the adjustment of disequilibrium prices is possible in a model of perfect competition in which every transactor takes market price as given. The Walrasian theory shows that a competitive equilibrium ensuring the consistency of agents’ plans to buy and sell results from an equilibrium set of prices for all goods and services. But the theory is silent about how those equilibrium prices are found and communicated to the agents of the model, the Walrasian tâtonnement process being an empirically empty heuristic artifact.

In fact, the explanatory gap identified by Arrow was even wider than he had suggested or realized, for another aspect of the Walrasian revolution of the late 1920s and 1930s was the extension of the equilibrium concept from a single-period equilibrium to an intertemporal equilibrium. Although earlier works by Irving Fisher and Frank Knight laid a foundation for this extension, the explicit articulation of intertemporal-equilibrium analysis was the nearly simultaneous contribution of three young economists, two Swedes (Myrdal and Lindahl) and an Austrian (Hayek) whose significance, despite being partially incorporated into the canonical Arrow-Debreu-McKenzie version of the Walrasian model, remains insufficiently recognized.

These three economists transformed the concept of equilibrium from an unchanging static economic system at rest to a dynamic system changing from period to period. While Walras and Marshall had conceived of a single-period equilibrium with no tendency to change barring an exogenous change in underlying conditions, Myrdal, Lindahl and Hayek conceived of an equilibrium unfolding through time, defined by the mutual consistency of the optimal plans of disparate agents to buy and sell in the present and in the future.

In formulating optimal plans that extend through time, agents consider both the current prices at which they can buy and sell, and the prices at which they will (or expect to) be able to buy and sell in the future. Although it may sometimes be possible to buy or sell forward at a currently quoted price for future delivery, agents planning to buy and sell goods or services rely, for the most part, on their expectations of future prices. Those expectations, of course, need not always turn out to have been accurate.

The dynamic equilibrium described by Myrdal, Lindahl and Hayek is a contingent event in which all agents have correctly anticipated the future prices on which they have based their plans. In the event that some, if not all, agents have incorrectly anticipated future prices, those agents whose plans were based on incorrect expectations may have to revise their plans or be unable to execute them. But unless all agents share the same expectations of future prices, their expectations cannot all be correct, and some of those plans may not be realized.

The impossibility of an intertemporal equilibrium of optimal plans if agents do not share the same expectations of future prices implies that the adjustment of perfectly flexible market prices is not sufficient an optimal equilibrium to be achieved. I shall have more to say about this point below, but for now I want to note that the growing interest in the quiet Walrasian revolution in neoclassical theory that occurred almost simultaneously with the Keynesian revolution made it inevitable that Keynesian models would be recast in explicitly Walrasian terms.

What emerged from the Walrasian reformulation of Keynesian analysis was the neoclassical synthesis that became the textbook version of macroeconomics in the 1960s and 1970s. But the seemingly anomalous conjunction of both inflation and unemployment during the 1970s led to a reconsideration and widespread rejection of the Keynesian proposition that output and employment are directly related to aggregate demand.

Indeed, supporters of the Monetarist views of Milton Friedman argued that the high inflation and unemployment of the 1970s amounted to an empirical refutation of the Keynesian system. But Friedman’s political conservatism, free-market ideology, and his acerbic criticism of Keynesian policies obscured the extent to which his largely atheoretical monetary thinking was influenced by Keynesian and Marshallian concepts that rendered his version of Monetarism an unattractive alternative for younger monetary theorists, schooled in the Walrasian version of neoclassicism, who were seeking a clear theoretical contrast with the Keynesian macro model.

The brief Monetarist ascendancy following 1970s inflation conveniently collapsed in the early 1980s, after Friedman’s Monetarist policy advice for controlling the quantity of money proved unworkable, when central banks, foolishly trying to implement the advice, prolonged a needlessly deep recession while central banks consistently overshot their monetary targets, thereby provoking a long series of embarrassing warnings from Friedman about the imminent return of double-digit inflation.


[1] Hayek, both a friend and a foe of Keynes, would chide Keynes decades after Keynes’s death for calling his theory a general theory when, in Hayek’s view, it was a special theory relevant only in periods of substantially less than full employment when increasing aggregate demand could increase total output. But in making this criticism, Hayek, himself, implicitly assumed that which he had himself admitted in his theory of intertemporal equilibrium that there is no automatic equilibration mechanism that ensures that general equilibrium obtains.

General Equilibrium, Partial Equilibrium and Costs

Neoclassical economics is now bifurcated between Marshallian partial-equilibrium and Walrasian general-equilibrium analyses. With the apparent inability of neoclassical theory to explain the coordination failure of the Great Depression, J. M. Keynes proposed an alternative paradigm to explain the involuntary unemployment of the 1930s. But within two decades, Keynes’s contribution was subsumed under what became known as the neoclassical synthesis of the Keynesian and Walrasian theories (about which I have written frequently, e.g., here and here). Lacking microfoundations that could be reconciled with the assumptions of Walrasian general-equilibrium theory, the neoclassical synthesis collapsed, owing to the supposedly inadequate microfoundations of Keynesian theory.

But Walrasian general-equilibrium theory provides no plausible, much less axiomatic, account of how general equilibrium is, or could be, achieved. Even the imaginary tatonnement process lacks an algorithm that guarantees that a general-equilibrium solution, if it exists, would be found. Whatever plausibility is attributed to the assumption that price flexibility leads to equilibrium derives from Marshallian partial-equilibrium analysis, with market prices adjusting to equilibrate supply and demand.

Yet modern macroeconomics, despite its explicit Walrasian assumptions, implicitly relies on the Marshallian intuition that the fundamentals of general-equilibrium, prices and costs are known to agents who, except for random disturbances, continuously form rational expectations of market-clearing equilibrium prices in all markets.

I’ve written many earlier posts (e.g., here and here) contesting, in one way or another, the notion that all macroeconomic theories must be founded on first principles (i.e., microeconomic axioms about optimizing individuals). Any macroeconomic theory not appropriately founded on the axioms of individual optimization by consumers and producers is now dismissed as scientifically defective and unworthy of attention by serious scientific practitioners of macroeconomics.

When contesting the presumed necessity for macroeconomics to be microeconomically founded, I’ve often used Marshall’s partial-equilibrium method as a point of reference. Though derived from underlying preference functions that are independent of prices, the demand curves of partial-equilibrium analysis presume that all product prices, except the price of the product under analysis, are held constant. Similarly, the supply curves are derived from individual firm marginal-cost curves whose geometric position or algebraic description depends critically on the prices of raw materials and factors of production used in the production process. But neither the prices of alternative products to be purchased by consumers nor the prices of raw materials and factors of production are given independently of the general-equilibrium solution of the whole system.

Thus, partial-equilibrium analysis, to be analytically defensible, requires a ceteris-paribus proviso. But to be analytically tenable, that proviso must posit an initial position of general equilibrium. Unless the analysis starts from a state of general equilibrium, the assumption that all prices but one remain constant can’t be maintained, the constancy of disequilibrium prices being a nonsensical assumption.

The ceteris-paribus proviso also entails an assumption about the market under analysis; either the market itself, or the disturbance to which it’s subject, must be so small that any change in the equilibrium price of the product in question has de minimus repercussions on the prices of every other product and of every input and factor of production used in producing that product. Thus, the validity of partial-equilibrium analysis depends on the presumption that the unique and locally stable general-equilibrium is approximately undisturbed by whatever changes result from by the posited change in the single market being analyzed. But that presumption is not so self-evidently plausible that our reliance on it to make empirical predictions is always, or even usually, justified.

Perhaps the best argument for taking partial-equilibrium analysis seriously is that the analysis identifies certain deep structural tendencies that, at least under “normal” conditions of moderate macroeconomic stability (i.e., moderate unemployment and reasonable price stability), will usually be observable despite the disturbing influences that are subsumed under the ceteris-paribus proviso. That assumption — an assumption of relative ignorance about the nature of the disturbances that are assumed to be constant — posits that those disturbances are more or less random, and as likely to cause errors in one direction as another. Consequently, the predictions of partial-equilibrium analysis can be assumed to be statistically, though not invariably, correct.

Of course, the more interconnected a given market is with other markets in the economy, and the greater its size relative to the total economy, the less confidence we can have that the implications of partial-equilibrium analysis will be corroborated by empirical investigation.

Despite its frequent unsuitability, economists and commentators are often willing to deploy partial-equilibrium analysis in offering policy advice even when the necessary ceteris-paribus proviso of partial-equilibrium analysis cannot be plausibly upheld. For example, two of the leading theories of the determination of the rate of interest are the loanable-funds doctrine and the Keynesian liquidity-preference theory. Both these theories of the rate of interest suppose that the rate of interest is determined in a single market — either for loanable funds or for cash balances — and that the rate of interest adjusts to equilibrate one or the other of those two markets. But the rate of interest is an economy-wide price whose determination is an intertemporal-general-equilibrium phenomenon that cannot be reduced, as the loanable-funds and liquidity preference theories try to do, to the analysis of a single market.

Similarly partial-equilibrium analysis of the supply of, and the demand for, labor has been used of late to predict changes in wages from immigration and to advocate for changes in immigration policy, while, in an earlier era, it was used to recommend wage reductions as a remedy for persistently high aggregate unemployment. In the General Theory, Keynes correctly criticized those using a naïve version of the partial-equilibrium method to recommend curing high unemployment by cutting wage rates, correctly observing that the conditions for full employment required the satisfaction of certain macroeconomic conditions for equilibrium that would not necessarily be satisfied by cutting wages.

However, in the very same volume, Keynes argued that the rate of interest is determined exclusively by the relationship between the quantity of money and the demand to hold money, ignoring that the rate of interest is an intertemporal relationship between current and expected future prices, an insight earlier explained by Irving Fisher that Keynes himself had expertly deployed in his Tract on Monetary Reform and elsewhere (Chapter 17) in the General Theory itself.

Evidently, the allure of supply-demand analysis can sometimes be too powerful for well-trained economists to resist even when they actually know better themselves that it ought to be resisted.

A further point also requires attention: the conditions necessary for partial-equilibrium analysis to be valid are never really satisfied; firms don’t know the costs that determine the optimal rate of production when they actually must settle on a plan of how much to produce, how much raw materials to buy, and how much labor and other factors of production to employ. Marshall, the originator of partial-equilibrium analysis, analogized supply and demand to the blades of a scissor acting jointly to achieve a intended result.

But Marshall erred in thinking that supply (i.e., cost) is an independent determinant of price, because the equality of costs and prices is a characteristic of general equilibrium. It can be applied to partial-equilibrium analysis only under the ceteris-paribus proviso that situates partial-equilibrium analysis in a pre-existing general equilibrium of the entire economy. It is only in general-equilibrium state, that the cost incurred by a firm in producing its output represents the value of the foregone output that could have been produced had the firm’s output been reduced. Only if the analyzed market is so small that changes in how much firms in that market produce do not affect the prices of the inputs used in to produce that output can definite marginal-cost curves be drawn or algebraically specified.

Unless general equilibrium obtains, prices need not equal costs, as measured by the quantities and prices of inputs used by firms to produce any product. Partial equilibrium analysis is possible only if carried out in the context of general equilibrium. Cost cannot be an independent determinant of prices, because cost is itself determined simultaneously along with all other prices.

But even aside from the reasons why partial-equilibrium analysis presumes that all prices, but the price in the single market being analyzed, are general-equilibrium prices, there’s another, even more problematic, assumption underlying partial-equilibrium analysis: that producers actually know the prices that they will pay for the inputs and resources to be used in producing their outputs. The cost curves of the standard economic analysis of the firm from which the supply curves of partial-equilibrium analysis are derived, presume that the prices of all inputs and factors of production correspond to those that are consistent with general equilibrium. But general-equilibrium prices are never known by anyone except the hypothetical agents in a general-equilibrium model with complete markets, or by agents endowed with perfect foresight (aka rational expectations in the strict sense of that misunderstood term).

At bottom, Marshallian partial-equilibrium analysis is comparative statics: a comparison of two alternative (hypothetical) equilibria distinguished by some difference in the parameters characterizing the two equilibria. By comparing the equilibria corresponding to the different parameter values, the analyst can infer the effect (at least directionally) of a parameter change.

But comparative-statics analysis is subject to a serious limitation: comparing two alternative hypothetical equilibria is very different from making empirical predictions about the effects of an actual parameter change in real time.

Comparing two alternative equilibria corresponding to different values of a parameter may be suggestive of what could happen after a policy decision to change that parameter, but there are many reasons why the change implied by the comparative-statics exercise might not match or even approximate the actual change.

First, the initial state was almost certainly not an equilibrium state, so systemic changes will be difficult, if not impossible, to disentangle from the effect of parameter change implied by the comparative-statics exercise.

Second, even if the initial state was an equilibrium, the transition to a new equilibrium is never instantaneous. The transitional period therefore leads to changes that in turn induce further systemic changes that cause the new equilibrium toward which the system gravitates to differ from the final equilibrium of the comparative-statics exercise.

Third, each successive change in the final equilibrium toward which the system is gravitating leads to further changes that in turn keep changing the final equilibrium. There is no reason why the successive changes lead to convergence on any final equilibrium end state. Nor is there any theoretical proof that the adjustment path leading from one equilibrium to another ever reaches an equilibrium end state. The gap between the comparative-statics exercise and the theory of adjustment in real time remains unbridged and may, even in principle, be unbridgeable.

Finally, without a complete system of forward and state-contingent markets, equilibrium requires not just that current prices converge to equilibrium prices; it requires that expectations of all agents about future prices converge to equilibrium expectations of future prices. Unless, agents’ expectations of future prices converge to their equilibrium values, an equilibrium many not even exist, let alone be approached or attained.

So the Marshallian assumption that producers know their costs of production and make production and pricing decisions based on that knowledge is both factually wrong and logically untenable. Nor do producers know what the demand curves for their products really looks like, except in the extreme case in which suppliers take market prices to be parametrically determined. But even then, they make decisions not on known prices, but on expected prices. Their expectations are constantly being tested against market information about actual prices, information that causes decision makers to affirm or revise their expectations in light of the constant flow of new information about prices and market conditions.

I don’t reject partial-equilibrium analysis, but I do call attention to its limitations, and to its unsuitability as a supposedly essential foundation for macroeconomic analysis, especially inasmuch as microeconomic analysis, AKA partial-equilibrium analysis, is utterly dependent on the uneasy macrofoundation of general-equilibrium theory. The intuition of Marshallian partial equilibrium cannot fil the gap, long ago noted by Kenneth Arrow, in the neoclassical theory of equilibrium price adjustment.

What’s so Great about Supply-Demand Analysis?

Just about the first thing taught to economics students is that there are demand curves for goods and services and supply curves of goods and services. Demand curves show how much customers wish to buy of a particular good or service within a period of time at various prices that might be charged for that good or service. The supply curve shows how much suppliers of a good or service would offer to sell at those prices.

Economists assume, and given certain more basic assumptions can (almost) prove, that customers will seek to buy less at higher prices for a good or service than at lower prices. Similarly, they assume that suppliers of the good or service offer to sell more at higher prices than at lower prices. Reflecting those assumptions, demand curves are downward-sloping and supply curve are upward-sloping. An upward-sloping supply curve is likely to intersect a downward-sloping demand curve at a single point, which corresponds to an equilibrium that allows customers to buy as much as they want to and suppliers to sell as much as they want to in the relevant time period.

This analysis is the bread and butter of economics. It leads to the conclusion that, when customers can’t buy as much as they would like, the price goes up, and, when suppliers can’t sell as much as they would like, the price goes down. So the natural tendency in any market is for the price to rise if it’s less than the equilibrium price, and to fall if it’s greater than the equilibrium price. This is the logic behind letting the market determine prices.

It can also be shown, if some further assumptions are made, that the intersection of the supply and demand curves represents an optimal allocation of resources in the sense that the total value of output is maximized. The necessary assumptions are, first, that the demand curve measures the marginal value placed on additional units of output, and, second, that the supply curve measures the marginal cost of producing additional units of output. The intersection of the supply and the demand curves corresponds to the maximization of the total value of output, because the marginal cost represents the value of output that could have been produced if the resources devoted to producing the good in question had been shifted to more valuable uses. When the supply curve rises above the demand curve it means that the resources would produce a greater value if devoted to producing something else than the value of the additional output of the good in question.

There is much to be said for the analysis, and it would be wrong to dismiss it. But it’s also important to understand its limitations, and, especially, the implicit assumptions on which it relies. In a sense, supply-demand analysis is foundational, the workhorse model that is the first resort of economists. But its role as a workhorse model does not automatically render analyses untethered to supply and demand illegitimate.

Supply-demand analysis has three key functions. First, it focuses attention on the idea of an equilibrium price at which all buyers can buy as much as they would like, and all sellers can sell as much as they would like. In a typical case, with an upward sloping supply curve and a downward-sloping demand curve, there is one, and only one, price with that property.

Second, as explained above, there is a sense in which that equilibrium price, aside from enabling the mutual compatibility of buyers’ and sellers’ plans to buy or to sell, has optimal properties.

Third, it’s a tool for predicting how changes in market conditions, like imposing a sales or excise tax, affect customers and suppliers. It compares two equilibrium positions on the assumption that only one parameter changes and predicts the effect of the parameter change by comparing the new and old equilibria. It’s the prototype for the comparative-statics method.

The chief problem with supply-demand analysis is that it requires a strict ceteris-paribus assumption, so that everything but the price and the quantity of the good under analysis remains constant. For many reasons, that assumption can’t literally be true. If the price of the good rises (falls), the real income of consumers decreases (increases). And if the price rises (falls), suppliers likely pay more (less) for their inputs. Changes in the price of one good also affect the prices of other goods, which, in turn, may affect the demand for the good under analysis. Each of those consequences would cause the supply and demand curves to shift from their initial positions. How much the ceteris-paribus assumption matters depends on how much of their incomes consumers spend on the good under analysis. The more they spend, the less plausible the ceteris paribus assumption.

But another implicit assumption underlies supply-demand analysis: that the economic system starts from a state of general equilibrium. Why must this assumption be made? The answer is that it‘s implied by the ceteris-paribus assumption that all other prices remain constant. Unless other markets are in equilibrium, it can’t be assumed that all other prices and incomes remain constant; if they aren’t, then prices for other goods, and for inputs used to produce the product under analysis, will change, violating the ceteris-paribus assumption. Unless the prices (and wages) of the inputs used to produce the good under analysis remain constant, the supply curve of the product can’t be assumed to remain unchanged.

On top of that, Walras’s Law implies that if one market is in disequilibrium, then at least one other market must also be in disequilibrium. So an internal contradiction lies at the heart of supply-demand analysis. The contradiction can be avoided, but not resolved, only by assuming that the market being analyzed is so minute relative to the rest of the economy, or so isolated from all other markets, that a disturbance in that market that changes its equilibrium position either wouldn’t disrupt the existing equilibrium in all other markets, or that the disturbances to the equilibria in all the other markets are so small that they can be safely ignored.

But we’re not done yet. The underlying general equilibrium on which the partial equilibrium (supply-demand) analysis is based, exists only conceptually, not in reality. Although it’s possible to prove the existence of such an equilibrium under more or less mathematically plausible assumptions about convexity and the continuity of the relevant functions, it is less straightforward to prove that the equilibrium is unique, or at least locally stable. If it is not unique or locally stable, there is no guarantee that comparative statics is possible, because a displacement from an unstable equilibrium may cause an unpredictable adjustment violates the ceteris-paribus assumption.

Finally, and perhaps most problematic, comparative statics is merely a comparison of two alternative equilibria, neither of which can be regarded as the outcome of a theoretically explicable, much less practical, process leading from initial conditions to the notional equilibrium state. Accordingly, neither is there any process whereby a disturbance to – a parameter change in — an initial equilibrium would lead from the initial equilibrium to a new equilibrium. That is what comparative statics means: the comparison of two alternative and disconnected equilibria. There is no transition from one to the other merely a comparison of the difference between them attributable to the change in a particular parameter in the initial conditions underlying the equilibria.

Given all the assumptions that must be satisfied for the basic implications of conventional supply-demand analysis to be unambiguously valid, that analysis obviously cannot provide demonstrably true predictions. As just explained, the comparative-statics method in general and supply-demand analysis in particular provide no actual predictions; they are merely conjectural comparisons of alternative notional equilibria.

The ceteris paribus assumption is often dismissed as making any theory tautological and untestable. If an ad hoc assumption introduced when observations don’t match the predictions derived from a given theory is independently testable, it adds to the empirical content of the theory, as demonstrated by the ad hoc assumption of an eighth planet (Neptune) in our solar system when predictions about the orbits of the seven known planets did not accord with their observed orbits.

Friedman’s famous methodological argument that only predictions, not assumptions, matter is clearly wrong. Economists have to be willing to modify assumptions and infer the implications that follow from modified or supplementary assumptions rather than take for granted that assumptions cannot meaningfully and productively affect the implications of a general analytical approach. It would be a travesty if physicists maintained the no-friction assumption, because it’s just a simplifying assumption to make the analysis tractable. That approach is a prescription for scientific stagnation.

The art of economics is to identify the key assumptions that ought to be modified to make a general analytical approach relevant and fruitful. When they are empirically testable, ad hoc assumptions that modify the ceteris paribus restriction constitute scientific advance.

But it’s important to understand how tenuous the connection is between the formalism of supply-demand analysis and of the comparative-statics method and the predictive power of that analysis and that method. The formalism stops far short of being able to generate clear and unambiguous conditions. The relationship between the formalism and the real world is tenuous and the apparent logical rigor of the formalism must be supplemented by notable and sometimes embarrassing doses of hand-waving or question-begging.

And it is also worth remembering the degree to which the supposed rigor of neoclassical microeconomic supply-demand formalism depends on the macroeconomic foundation of the existence (and at least approximate reality) of a unique or locally stable general equilibrium.


About Me

David Glasner
Washington, DC

I am an economist in the Washington DC area. My research and writing has been mostly on monetary economics and policy and the history of economics. In my book Free Banking and Monetary Reform, I argued for a non-Monetarist non-Keynesian approach to monetary policy, based on a theory of a competitive supply of money. Over the years, I have become increasingly impressed by the similarities between my approach and that of R. G. Hawtrey and hope to bring Hawtrey’s unduly neglected contributions to the attention of a wider audience.

My new book Studies in the History of Monetary Theory: Controversies and Clarifications has been published by Palgrave Macmillan

Follow me on Twitter @david_glasner

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