Economic Equilibrium

Economic equilibrium is a state in which the forces acting on a market or an economy balance, so that, given the conditions, no buyer, seller or other participant has a reason to change what they are doing. In the simplest case, a single market is in equilibrium at the price where the quantity buyers want equals the quantity sellers offer. The idea is the backbone of most economic models.

Equilibrium is a reference point, not a claim that real markets are always at rest. Economists use it to predict the direction of change after a shock: a new tax, a harvest failure or a rise in interest rates. The concept appears in microeconomics (single markets), macroeconomics (whole economies) and game theory (strategic choices).

Picture a seesaw with buyers on one side and sellers on the other. If the price is too high, sellers pile on more goods than anyone wants, and the seesaw tips: shops end up with full shelves and start offering discounts. If the price is too low, buyers crowd in, shelves empty, and sellers raise prices. The point where the seesaw is level is the equilibrium.

Nobody has to calculate this point. It emerges because each side reacts to what it sees: unsold stock or empty shelves. When something changes, such as a new fashion or a factory closing, the level point moves, and the market drifts toward the new one.

In a single market, equilibrium is where demand equals supply:

• $Q_d$ is the quantity demanded, which falls as price rises.
• $Q_s$ is the quantity supplied, which rises as price rises.
• $P^*$ is the equilibrium price at which the two are equal.

Worked example: suppose demand is $Q_d = 100 - 2P$ and supply is $Q_s = 20 + 2P$. Setting them equal gives $100 - 2P = 20 + 2P$, so $4P = 80$ and $P^* = 20$. Substituting back, $Q^* = 100 - 40 = 60$ units. At a price of 25, demand would be 50 and supply 70, a surplus of 20 that pushes the price down; at 15, demand would be 70 and supply 50, a shortage that pushes it up.

When demand or supply shifts, the equilibrium moves. A rise in demand raises both price and quantity; a supply shock that cuts supply raises price but lowers quantity. How far price and quantity move depends on the elasticity of each curve. A tax or subsidy drives a wedge between the price buyers pay and the price sellers receive, and the new equilibrium shows who bears it.

Partial equilibrium analysis, associated with Alfred Marshall, studies one market and holds everything else fixed. It is practical and widely used, but it ignores knock-on effects: a rise in the price of steel affects car prices, wages and demand for aluminum.

General equilibrium analysis looks at all markets at once. The French economist Léon Walras set out a system of simultaneous equations for an entire economy in his Elements of Pure Economics (1874). In 1954 Kenneth Arrow and Gérard Debreu proved that, under specific assumptions, a set of prices exists at which every market clears simultaneously. Arrow shared the 1972 Nobel Memorial Prize in Economic Sciences with John Hicks, and Debreu received the prize in 1983. The related welfare theorems link competitive equilibrium to Pareto efficiency under perfect competition.

• Nash equilibrium. In strategic settings, a Nash equilibrium is a set of choices in which no player can do better by changing strategy alone. It is the standard tool for analyzing oligopoly and cartels.
• Macroeconomic equilibrium. Output and the price level settle where aggregate demand meets aggregate supply. Modern central-bank models describe a dynamic equilibrium that evolves over time.
• Labor market equilibrium. The wage at which the number of workers employers want equals the number willing to work. Keynesian economics argues an economy can settle at an equilibrium with persistent involuntary unemployment.
• Financial market equilibrium. Asset pricing theories assume prices adjust until expected returns compensate for risk, and that arbitrage removes simple mispricings.

An equilibrium is stable if the economy returns to it after a disturbance and unstable if a small push sends it further away. Some models have several equilibria, such as a bank that is sound if depositors stay calm but fails if they all withdraw, or an economy stuck in a liquidity trap.

Critics raise several points. The Austrian school stresses that markets are a continuous process of discovery and are rarely, if ever, at rest. Behavioral economists question whether people optimize as the models assume. Others note that frictions such as sticky prices and long contracts mean adjustment can take years. Defenders reply that equilibrium is a benchmark for comparing outcomes, not a literal description of any day's trading.

• OpenStax Principles of Economics: Demand, Supply, and Equilibrium
• The Nobel Prize: Economic Sciences 1983 (Gérard Debreu)