Economic equilibrium

• 1. Introduction

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  Economic equilibrium sits at the heart of how economists make sense of markets. It describes the moment when the forces of supply and demand are perfectly balanced, and economic variables stop moving. To a first-time listener, that might sound like a moment of stasis, even stagnation. But what it really describes is something more remarkable: a condition where every buyer is getting what they want at a price they accept, and every seller is producing exactly what they can profitably sell. No single person, acting alone, can improve their situation by doing anything differently.

  This concept was not invented by economists. It was borrowed from the physical sciences, where systems in balance stay that way until an outside force disturbs them. The economists who adopted the idea asked whether markets work the same way. The answer turns out to be complicated. Sometimes they do, often beautifully. Sometimes they do not, with consequences that can include mass unemployment, artificial shortages, and in the starkest historical case, famine.

  What does it actually mean for a price to clear a market? Why does a monopoly reach equilibrium by a completely different path than a competitive market? And what does it mean when two firms are locked in strategic competition, each watching the other's every move? Those questions will carry us through this documentary.

• 2. Three Properties

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  Huw Dixon proposed three fundamental properties that any equilibrium, in any economic setting, must satisfy. The first is consistency: the behavior of all agents must fit together without contradiction. The second is stability of incentives: no agent should have any reason to change what they are doing. The third is stability of process: the equilibrium should be reachable through some dynamic path, not merely a theoretical stopping point that the market can never actually find.

  These three properties, labeled P1, P2, and P3, form a kind of checklist. A competitive market, where many buyers and sellers set a single price, clears all three. At the equilibrium price, the amount supplied exactly equals the amount demanded, so P1 is satisfied. Buyers are already maximizing their utility at that price, and sellers are already maximizing their profits, so P2 holds. And if the price drifts above equilibrium, excess supply pushes it back down; if it falls below, a shortage drives it back up, so P3 holds too.

  But not every equilibrium is stable in Dixon's third sense. An unstable equilibrium can satisfy P1 and P2 while failing P3. In that case, the market can only be in that equilibrium if it started there. Any small disturbance sends it somewhere else entirely.

• 3. The Monopoly Exception

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  A monopoly reaches equilibrium in a way that violates the first two of Dixon's properties. Where a competitive market sets price where supply meets demand, a monopolist sets price where marginal revenue equals marginal cost. Those are fundamentally different rules, and they produce fundamentally different outcomes.

  At the monopolist's equilibrium price, the quantity demanded and the quantity supplied are not equal. Consumers would like more of the product at that price, but the monopolist is deliberately supplying less, because restricting output pushes the price up and maximizes profit. This means P1 fails: the amounts on each side of the market do not match. P2 also fails on the demand side: consumers have every incentive to demand more, because they are being rationed. The monopolist, however, has no incentive to change anything. Their profit is already maximized.

  The stability result under P3 is more surprising. When there is excess supply in a monopoly market, the monopolist recognizes the situation as a departure from the profit-maximizing quantity and applies upward pressure on price to return to equilibrium. The market does stabilize, just not in the way that benefits consumers. This divergence between the monopolist's profit-maximizing quantity and the socially optimal quantity is precisely what makes monopoly equilibrium a focus of concern for economists and regulators.

• 4. Antoine Cournot's Duopoly

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  The Nash equilibrium, now the main alternative to competitive equilibrium in economics, was first put to work in a specific, concrete setting: a duopoly analyzed by the French economist Antoine Augustin Cournot in his 1838 book. Two firms produce an identical product. The total output of both firms together determines the market price through the demand curve. Each firm's profit equals that market price times the firm's own output, minus the cost of producing it.

  The strategic tension is immediate. If one firm increases its output, the market price falls, and the other firm's revenue drops even though it did nothing. Cournot described each firm as choosing its own output to maximize its profits, taking the other firm's output as fixed. The Nash equilibrium is the pair of output levels where each firm is already doing the best it can, given what the other is producing. Neither has any reason to deviate.

  Cournot himself believed this equilibrium was stable, using a process now called best response dynamics. Each firm repeatedly adjusts its output to the best possible response to whatever the other firm just produced. In the standard Cournot model, as one firm raises output, the other's best response is to cut output, producing a downward-sloping reaction function. So long as those reaction functions have a slope less steep than negative one, the adjustment process converges to the Nash equilibrium.

  But Huw Dixon identified the crucial weakness in this stability story. At each step, firms behave myopically: they choose output to maximize current profits given the other firm's current output, but ignore the fact that the process specifies the other firm will adjust in the next round. The firms are not actually thinking ahead. Evolutionary stability is one of the alternative stability concepts that has been put forward to address this gap.

• 5. When Markets Clear and People Starve

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  Paul Samuelson was among the economists who warned against attaching any moral meaning to the equilibrium price. A price can be efficient in the strict economic sense while producing outcomes that are, from a human perspective, catastrophic. The market clears, yet people suffer.

  The starkest historical illustration came during the Great Famine in Ireland, which ran from 1845 to 1852. Food was exported from Ireland throughout the famine, because the profit available from selling to English buyers exceeded the price Irish farmers could afford to pay. The equilibrium price of the Irish-British market for potatoes was above the threshold that Irish farmers could meet. The market, by this accounting, was functioning correctly. And people were dying.

  Samuelson's caution echoes in the modern critiques of equilibrium thinking. Mainstream economics now points to cases where equilibrium does not correspond to market clearing at all. Efficiency wage theory in labor economics describes equilibria where unemployment persists not because wages are stuck, but because firms deliberately keep wages above the market-clearing level to boost worker productivity. Credit rationing works similarly: banks hold interest rates low to create excess demand for loans, so they can select among borrowers rather than raising rates until supply meets demand. Keynesian macroeconomics adds yet another category: underemployment equilibrium, where a surplus of labor co-exists over long stretches with a shortage of aggregate demand. Adam Smith's picture of markets automatically abolishing disequilibrium turns out to be only one chapter in a much longer story.

• 6. Static, Dynamic, and Disequilibrium

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  Most introductory economics courses deal in static equilibrium: a fixed supply curve, a fixed demand curve, and a single price where they cross. But economic equilibrium can also be dynamic, a concept with different implications entirely.

  In a static equilibrium, all quantities are unchanging. In a dynamic equilibrium, by contrast, quantities may all be growing, but growing at the same rate, so their ratios stay constant. The neoclassical growth model provides a worked example. Working population grows at a rate determined outside the model by non-economic forces. In dynamic equilibrium, output and the physical capital stock grow at that same rate. Output per worker and capital per worker stay constant. A permanent increase in the saving rate in this model leads to a new dynamic equilibrium with permanently higher capital per worker and productivity per worker, but the growth rate of output does not change. Economists call the comparison of two such equilibria comparative dynamics, the moving counterpart to the comparative statics used for static cases.

  Disequilibrium, the condition of a market not yet at equilibrium, can be brief or prolonged. Goods markets occupy a middle position: prices in those markets can be slow to adjust because of menu costs, long-term contracts, and other impediments, but they do not stay out of equilibrium indefinitely. Labor markets are the more troubling case. Many economists regard labor markets as persistently in disequilibrium, specifically in excess supply, meaning that more people want work at prevailing wages than can find it. That persistent gap is another name for unemployment, and it is one reason economic equilibrium remains a contested and consequential idea rather than a settled one.