Supply and demand

The law of supply and demand is a fundamental law of economics. It can be stated as follows:

• the quantity of a commodity that consumers are prepared to buy, (normally) rises when the price of that commodity falls,and vice versa;
• the quantity of a commodity that suppliers are prepared to sell, (normally) rises when the price of that commodity rises, and vice versa; so that,
• the price at which transactions eventually take place is that at which the quantity that consumers are prepared to buy is the same as the quantity that suppliers are prepared to sell.

(The word "normally" is inserted in recognition of the exceptions noted in the concluding paragraph of this article)

The eventual price at which the hypothetical bargaining settles down is referred to as the market clearing or equilibrium price. Higher prices result in unsold surpluses, prompting suppliers to reduce their prices, and vice versa.

This is among the most familiar of all economic laws; so much so that it might be considered to be a statement of the obvious, and has often been referred to as a tautology, but its dissemination by Alfred Marshal in 1891 introduced a major departure in economic theory. Previous economists, from Adam Smith to Karl Marx, had taught that prices were determined by the cost of production. (See history of economic thought).

The operation of the law of supply and demand in particular markets may be quantified by the use of the following concepts :

• Elasticity of demand denotes the change in demand in response to changes of price, income or the prices of substitutes.
• The income effect denotes the fact that the demand for most products increases as consumers' income increases.
• The substitution effect denotes the fact that the demand for a commodity is influenced by the price of close substitutes.
• Consumer's surplus denotes the amount by which consumers value a commodity over above what they have to pay for it.

These concepts are among the basic tools of microeconomics and some of their applications are described in the article on that subject. They are applicable to all market activity and do not depend upon any assumptions concerning the nature of the commodities concerned. That remains true notwithstanding the fact that they can assume zero or even negative values. The concept of elasticity of demand, for example, is not invalidated by the existence of commodities that are so essential that the demand for them is insensitive to price, nor by the existence of goods that are prized simply because they are expensive.