Giffen Good Definition

A Giffen good is a rare type of inferior good characterized by an increase in demand as the price rises, and vice-versa. This behavior contradicts the standard law of demand.

Income and Substitution Effects

The demand for any good is influenced by two effects when its price changes:

• Substitution Effect: When a good's price falls, it becomes relatively cheaper compared to other goods. Consumers tend to substitute towards this cheaper good, increasing the quantity demanded.
• Income Effect: A price change affects the consumer's real purchasing power. For inferior goods, a fall in price increases real income, leading consumers to buy less of the inferior good and more of superior alternatives. Conversely, a price rise decreases real income, leading to increased demand for the inferior good.

Giffen Good Condition

For a Giffen good, the unique situation arises where the demand curve slopes upwards.

This occurs specifically when:

The income effect is stronger than the substitution effect.

In mathematical terms, if $P$ is the price and $Q$ is the quantity demanded:

• A price decrease ($\Delta P < 0$) leads to a substitution effect that increases quantity demanded ($\Delta Q_{sub} > 0$).
• However, for a Giffen good (a strongly inferior good), the associated increase in real income results in a larger decrease in quantity demanded via the income effect ($\Delta Q_{inc} < 0$).
• The overall change in quantity demanded is negative ($\Delta Q_{total} < 0$) even though the price decreased ($\Delta P < 0$), meaning demand increases as price increases. This requires $|\Delta Q_{inc}| > |\Delta Q_{sub}|$.

Therefore, the defining characteristic of a Giffen good is that the income effect dominates the substitution effect.