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 law of demand. For any 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 (being more in demand at lower income) 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.

Background

Evidence for the existence of Giffen goods is limited, but microeconomic mathematical models explain how such a thing could exist. 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.