Giffen Commodity Analysis: Price Decline Effects

A Giffen commodity is a rare type of inferior good where demand increases as the price increases, and decreases as the price decreases. This behavior contradicts the standard law of demand. Understanding the effects of a price change is key.

Price Decline Impact

When the price of any good declines, it triggers two effects:

• Substitution Effect: The good becomes relatively cheaper compared to substitutes. This effect always encourages consumers to buy more of the cheaper good. For a price fall, the substitution effect is positive.
• Income Effect: The fall in price increases the consumer's real purchasing power. For inferior goods (including Giffen goods), an increase in real income leads to a decrease in consumption. For a price fall of an inferior good, the income effect is negative.

Giffen Good Specifics

For a Giffen commodity, the unique characteristic is that demand falls when the price falls. This happens specifically when the negative income effect is stronger than the positive substitution effect.

Let $P$ be the price of the Giffen commodity.

Consider a decline in $P$.

• The quantity demanded due to the substitution effect increases (let's denote this change as $ \Delta Q_{sub} > 0 $).
• The quantity demanded due to the income effect decreases (let's denote this change as $ \Delta Q_{inc} < 0 $).
• For a Giffen good, the total change in quantity demanded ($ \Delta Q_{total} $) is negative when the price falls ($ \Delta P < 0 $). This means $ \Delta Q_{total} = \Delta Q_{sub} + \Delta Q_{inc} < 0 $.
• This condition ($ \Delta Q_{sub} + \Delta Q_{inc} < 0 $) implies that the magnitude of the decrease caused by the income effect is greater than the magnitude of the increase caused by the substitution effect.
• In other words, the positive substitution effect is less than the magnitude of the negative income effect.

Conclusion

Therefore, in a situation of decline in the price of a Giffen Commodity, the positive substitution effect is less than the negative income effect.