Price discrimination requires separation and limited resale

Different elasticities allow different markups.

Price discrimination charges different prices not explained by cost. It requires market power, a way to identify or sort buyers, and limited resale. A firm charges a higher markup to a group with less elastic demand. Student discounts, advance-purchase fares, and quantity pricing can fit the idea when cost differences do not explain the price.

Cost-based price differences are not discrimination in the economic definition. Charging more to deliver a heavy package farther may reflect added cost. Charging two otherwise similar buyers different markups because one group has fewer alternatives is discrimination. The question normally states that cost differences do not explain the prices.

Market separation prevents low-price buyers from reselling to high-price buyers. Identification can use age, purchase timing, location, coupons, or self-selection through product versions. Without separation, arbitrage undermines the price difference by moving units from the low-price group to the high-price group.

For third-degree discrimination, the firm allocates output so marginal revenue is equal across groups and equals common marginal cost. The less elastic group receives the higher markup. This is the inverse-elasticity logic: buyers with fewer alternatives reduce quantity less in response to price.

Perfect first-degree discrimination charges each unit at willingness to pay. Output expands until demand meets MC, eliminating deadweight loss in the ideal model while transferring consumer surplus to the seller. Imperfect discrimination can increase or decrease output relative to single pricing. Do not assume every discrimination scheme is efficient.

Under perfect discrimination, each buyer pays a unit-specific reservation price. There is no single price that must be lowered on earlier units, so the seller captures the area under demand above MC. Output reaches Q_e, where the next buyer’s willingness to pay equals marginal cost. Efficiency rises while distribution shifts strongly toward the seller.

Why resale matters

An airline charges business travelers more than vacation travelers for the same seat when cost is equal. Advance-purchase and change restrictions help separate the groups. If vacation travelers could freely resell tickets to business travelers, the low fare would undermine the high-fare market.

Price discrimination can expand service to low-value buyers by using revenue from high-markup groups, but it can also reallocate output or reduce some groups’ consumption. Without information about total output and cost, do not claim it always raises or lowers welfare.

The economics exam often tests conditions rather than welfare: market power, ability to separate, different elasticities, and limited resale. An answer missing any one can be incomplete.

Self-selection can separate buyers without asking their identity. Refund restrictions, versions, coupons, and quantity blocks induce people with different values or flexibility to choose different offers. The price difference is discriminatory only to the extent it exceeds relevant cost differences.

If arbitrage is easy, low-price buyers resell to the high-price group and collapse the scheme. This is why services, personalized tickets, and time-sensitive products are common examples. Market power is also essential. A price taker cannot maintain group-specific markups against identical rivals.