Giffen good

In economics and consumer theory, a Giffen good is a product that people consume more of as the price rises and vice versa—violating the basic law of demand in microeconomics. For any other sort of good, as the price of the good rises, the substitution effect makes consumers purchase less of it, and more of substitute goods; for most goods, the income effect (due to the effective decline in available income due to more being spent on existing units of this good) reinforces this decline in demand for the good. But a Giffen good is so strongly an inferior good in the minds of consumers (being more in demand at lower incomes) that this contrary income effect more than offsets the substitution effect, and the net effect of the good's price rise is to increase demand for it.

Giffen goods are named after Scottish economist Sir Robert Giffen, to whom Alfred Marshall attributed this idea in his book Principles of Economics. Giffen first proposed the paradox from his observations of the purchasing habits of the Victorian era poor.

It has been suggested by Etsusuke Masuda and Peter Newman that Simon Gray described "Gray goods" in his 1815 text entitled The Happiness of States: Or An Inquiry Concerning Population, The Modes of Subsisting and Employing It, and the Effects of All on Human Happiness. The chapter entitled A Rise in the Price of Bread Corn, beyond a certain Pitch, tends to increase the Consumption of it, contains a detailed account of what have come to be called Giffen goods, and which might better be called Gray goods.

For all products, the demand curve has negative slope (slope, which is constant in case of a straight demand function, must not be confused with price elasticity of demand, that varies depending on the point of the curve on which it is calculated and is not therefore constant even along a straight demand curve; see the mathematical definition for more), and meets upwards sloping supply curve at a price point that defines the market for any given good or service at a particular moment in time.

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