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In economics, economic equilibrium is a situation in which economic forces such as supply and demand are balanced and in the absence of external influences the (equilibrium) values of economic variables will not change. For example, in the standard text perfect competition, equilibrium occurs at the point at which quantity demanded and quantity supplied are equal.[1] Market equilibrium in this case is a condition where a market price is established through competition such that the amount of goods or services sought by buyers is equal to the amount of goods or services produced by sellers. This price is often called the competitive price or market clearing price and will tend not to change unless demand or supply changes, and quantity is called the "competitive quantity" or market clearing quantity. But the concept of equilibrium in economics also applies to imperfectly competitive markets, where it takes the form of a Nash equilibrium.

In a competitive equilibrium, supply equals demand. Property P1 is satisfied, because at the equilibrium price the amount supplied is equal to the amount demanded. Property P2 is also satisfied. Demand is chosen to maximize utility given the market price: no one on the demand side has any incentive to demand more or less at the prevailing price. Likewise supply is determined by firms maximizing their profits at the market price: no firm will want to supply any more or less at the equilibrium price. Hence, agents on neither the demand side nor the supply side will have any incentive to alter their actions.

To see whether Property P3 is satisfied, consider what happens when the price is above the equilibrium. In this case there is an excess supply, with the quantity supplied exceeding that demanded. This will tend to put downward pressure on the price to make it

Equilibrium property P1: The behavior of agents is consistent.

Equilibrium property P2: No agent has an incentive to change its behavior.

Equilibrium property P3: Equilibrium is the outcome of some dynamic process (stability).

In a competitive equilibrium, supply equals demand. Property P1 is satisfied, because at the equilibrium price the amount supplied is equal to the amount demanded. Property P2 is also satisfied. Demand is chosen to maximize utility given the market price: no one on the demand side has any incentive to demand more or less at the prevailing price. Likewise supply is determined by firms maximizing their profits at the market price: no firm will want to supply any more or less at the equilibrium price. Hence, agents on neither the demand side nor the supply side will have any incentive to alter their actions.

To see whether Property P3 is satisfied, consider what happens when the price is above the equilibrium. In this case there is an excess supply, with the quantity supplied exceeding that demanded. This will tend to put downward pressure on the price to make it return to equilibrium. Likewise where the price is below the equilibrium point there is a shortage in supply leading to an increase in prices back to equilibrium. Not all equilibria are "stable" in the sense of equilibrium property P3. It is possible to have competitive equilibria that are unstable. However, if an equilibrium is unstable, it raises the question of reaching it. Even if it satisfies properties P1 and P2, the absence of P3 means that the market can only be in the unstable equilibrium if it starts off there.

In most simple microeconomic stories of supply and demand a static equilibrium is observed in a market; however, economic equilibrium can be also dynamic. Equilibrium may also be economy-wide or general, as opposed to the partial equilibrium of a single market. Equilibrium can change if there is a change in demand or supply conditions. For example, an increase in supply will disrupt the equilibrium, leading to lower prices. Eventually, a new equilibrium will be attained in most markets. T

To see whether Property P3 is satisfied, consider what happens when the price is above the equilibrium. In this case there is an excess supply, with the quantity supplied exceeding that demanded. This will tend to put downward pressure on the price to make it return to equilibrium. Likewise where the price is below the equilibrium point there is a shortage in supply leading to an increase in prices back to equilibrium. Not all equilibria are "stable" in the sense of equilibrium property P3. It is possible to have competitive equilibria that are unstable. However, if an equilibrium is unstable, it raises the question of reaching it. Even if it satisfies properties P1 and P2, the absence of P3 means that the market can only be in the unstable equilibrium if it starts off there.

In most simple microeconomic stories of supply and demand a static equilibrium is observed in a market; however, economic equilibrium can be also dynamic. Equilibrium may also be economy-wide or general, as opposed to the partial equilibrium of a single market. Equilibrium can change if there is a change in demand or supply conditions. For example, an increase in supply will disrupt the equilibrium, leading to lower prices. Eventually, a new equilibrium will be attained in most markets. Then, there will be no change in price or the amount of output bought and sold — until there is an exogenous shift in supply or demand (such as changes in technology or tastes). That is, there are no endogenous forces leading to the price or the quantity.

The Nash equilibrium is widely used in economics as the main alternative to competitive equilibrium. It is used whenever there is a strategic element to the behavior of agents and the "price taking" assumption of competitive equilibrium is inappropriate. The first use of the Nash equilibrium was in the Cournot duopoly as developed by Antoine Augustin Cournot in his 1838 book.[3] Both firms produce a homogenous product: given the total amount supplied by the two firms, the (single) industry price is determined using the demand curve. This determines the revenues of each firm (the industry price times the quantity supplied by the firm). The profit of each firm is then this revenue minus the cost of producing the output. Clearly, there is a strategic interdependence between the two firms. If one firm varies its output, this will in turn affect the market price and so the revenue and profits of the other firm. We can define the payoff function which gives the profit of each firm as a function of the two outputs chosen by the firms. Cournot assumed that each firm chooses its own output to maximize its profits given the output of the other firm. The Nash equilibrium occurs when both firms are producing the outputs which maximize their own profit given the output of the other firm.

In terms of the equilibrium properties, we can see that P2 is satisfied: in a Nash equilibrium, neither firm has an incentive to deviate from the Nash equilibrium given the output of the other firm. P1 is satisfied since the payoff function ensures that the market price is consistent with the outputs supplied and that each firms profits equal revenue minus cost at this output.

Is the equilibrium stable as required by P3? Cournot himself argued that it was stable using the stability concept implied by best response dynamics. The reaction function for each firm gives the output which maximizes profits (best response) in terms of output for a firm in terms of a given output of the other firm. In the standard Cournot model this is downward sloping: if the other firm produces a higher output, the best response involves producing less. Best response dynamics involves firms starting from some arbitrary position and then adjusting output to their best-response to the previous output of the other firm. So long as the reaction functions have a slope of less than -1, this will converge to the Nash equilibrium. However, this stability story is open to much criticism. As Dixon argues: "The crucial weakness is that, at each step, the firms behave myopically: they choose their output to maximize their current profits given the output of the other firm, but ignore the fact that the process specifies that the other firm will adjust its output...".[4] There are other concepts of stability that have been put forward for the Nash equilibrium, evolutionary stabilityIn terms of the equilibrium properties, we can see that P2 is satisfied: in a Nash equilibrium, neither firm has an incentive to deviate from the Nash equilibrium given the output of the other firm. P1 is satisfied since the payoff function ensures that the market price is consistent with the outputs supplied and that each firms profits equal revenue minus cost at this output.

Is the equilibrium stable as required by P3? Cournot himself argued that it was stable using the stability concept implied by best response dynamics. The reaction function for each firm gives the output which maximizes profits (best response) in terms of output for a firm in terms of a given output of the other firm. In the standard Cournot model this is downward sloping: if the other firm produces a higher output, the best response involves producing less. Best response dynamics involves firms starting from some arbitrary position and then adjusting output to their best-response to the previous output of the other firm. So long as the reaction functions have a slope of less than -1, this will converge to the Nash equilibrium. However, this stability story is open to much criticism. As Dixon argues: "The crucial weakness is that, at each step, the firms behave myopically: they choose their output to maximize their current profits given the output of the other firm, but ignore the fact that the process specifies that the other firm will adjust its output...".[4] There are other concepts of stability that have been put forward for the Nash equilibrium, evolutionary stability for example.

Most economists, for example Paul Samuelson,[5]:Ch.3,p.52 caution against attaching a normative meaning (value judgement) to the equilibrium price. For example, food markets may be in equilibrium at the same time that people are starving (because they cannot afford to pay the high equilibrium price). Indeed, this occurred during the Great Famine in Ireland in 1845–52, where food was exported though people were starving, due to the greater profits in selling to the English – the equilibrium price of the Irish-British market for potatoes was above the price that Irish farmers could afford, and thus (among other reasons) they starved.[6]

## Interpretations

In most inte

In most interpretations, classical economists such as Adam Smith maintained that the free market would tend towards economic equilibrium through the price mechanism. That is, any excess supply (market surplus or glut) would lead to price cuts, which decrease the quantity supplied (by reducing the incentive to produce and sell the product) and increase the quantity demanded (by offering consumers bargains), automatically abolishing the glut. Similarly, in an unfettered market, any excess demand (or shortage) would lead to price increases, reducing the quantity demanded (as customers are priced out of the market) and increasing in the quantity supplied (as the incentive to produce and sell a product rises). As before, the disequilibrium (here, the shortage) disappears. This automatic abolition of non-market-clearing situations distinguishes markets from central planning schemes, which often have a difficult time getting prices right and suffer from persistent shortages of goods and services.[7]

This view came under attack from at least two viewpoints. Modern mainstream economics points to cases where equilibrium does not correspond to market clearing (but instead to unemployment), as with the This view came under attack from at least two viewpoints. Modern mainstream economics points to cases where equilibrium does not correspond to market clearing (but instead to unemployment), as with the efficiency wage hypothesis in labor economics. In some ways parallel is the phenomenon of credit rationing, in which banks hold interest rates low to create an excess demand for loans, so they can pick and choose whom to lend to. Further, economic equilibrium can correspond with monopoly, where the monopolistic firm maintains an artificial shortage to prop up prices and to maximize profits. Finally, Keynesian macroeconomics points to underemployment equilibrium, where a surplus of labor (i.e., cyclical unemployment) co-exists for a long time with a shortage of aggregate demand.

To find the equilibrium price, one must either plot the supply and demand curves, or solve for the expressions for supply and demand being equal.

An example may be: