The law of Demand

Demand is a key concept in both macroeconomics and microeconomics. In the former, consumption is mainly a function of income; whereas in the latter, consumption or demand is primarily, but not exclusively, a function of price. This analysis of demand relates to microeconomic theory.

The theory of demand was mostly implicit in the writing of classical economists before the late nineteenth century. Current theory rests on the foundation laid by Marshall (1890), Edgeworth (1881), and Pareto (1896). Marshall viewed demand in a cardinal context, in which utility could be quantified. Most contemporary economists hold the approach taken by Edgeworth and Pareto, in which demand has only ordinal characteristics and in which indifference or preference become central to the analysis.

Much economic analysis focuses on the relation between prices and quantities demanded, the other variables being provisionally held constant. At the various prices that could prevail in a market during some period of time, different quantities of a good or service would be bought. Demand, then, is considered as a list of prices and quantities, with one quantity for each possible price. With price on the vertical axis and quantity on the horizontal axis, the demand curve slopes downward from left to right, signifying that smaller quantities are bought at higher prices and larger quantities are bought at lower prices. The inverse relation between price and quantity is usually called the law of demand. The law rests on two foundations. One is the theory of the consumer, the logic of which shows that the consumer responds to lower prices by buying more. The other foundation is empirical, with innumerable studies of demand in actual markets having demonstrated the existence of downward-sloping demand curves.

Exceptions to the law of demand are the curiosa of theorist. The best-know exception is the Given effect – a consumer buys more, not less of a commodity at higher prices when a negative income effect dominates over the substitution effect. Another is the Vehien effect – some commodities are theoretically wanted solely for their higher prices. The higher these prices are, the more the use of such commodities fulfills the requirements of conspicuous consumption, and thus the stronger the demand for them.

Change in Consumer Demand.

Increases or decreases in demand are changes in the quantities that would be bought at any of the possible array of prices. Change in demand are shifts or movements of the entire demand curve. A shift to the right means an increase in demand. It can come from any one or a combination of the following: a change in consumer desire or taste, sometimes augmented by volumes of advertising; a rise in consumers income; a rise in the prices of substitutes; or a fall in the prices of complements. Of course, opposite change in these factor cause a decrease in demand, i.e.. a leftward shift of the entire demand curve. An exception here applies to inferior goods, which are defined as those goods and services bought in smaller amounts as consumer incomes rise. In an uncertain economy, especially an inflationary one, price expectation can affect demand. For storable commodities, when consumers believe that expected future prices will be higher, buyers tend to increase their current demand and thus tend to make their expectations self-fulfilling. The demands for durable goods can fluctuate widely over time, as consumers’ incomes vary. A durable good has both a stock demand and a flow demand. The stock demand is for the amount that consumers want to hold over a period of years. The flow demand in a given year consists of replacement demand, i.e., for purchases to maintain the stock at some level, and of expansion demand, i.e., for purchases to increase the stock.