Il problema dei limoni: selezione avversa nel mercato delle auto usate

Adverse selection occurs when products of varying quality are sold at a single price due to asymmetric information, where sellers know more than buyers. In the used car market, all cars sell for $14,000 regardless of quality. High-quality cars (plums) are undervalued, while low-quality cars (lemons) are overvalued, causing plum sellers to exit and lemons to dominate the market.

Sellers of high-quality cars (plums) know their vehicles are worth more than $14,000, but buyers only pay the expected average price. Plum sellers realize they receive less than their car's true value, so many choose to exit rather than accept undervalued offers. This withdrawal of quality products accelerates the adverse selection problem.

As high-quality cars exit the market, buyers become more skeptical about overall quality and lower their willingness to pay. This declining price further discourages plum sellers while encouraging lemon sellers, whose cars are now overvalued. The cycle continues, making lemons increasingly common until the market consists largely of low-quality vehicles.

Asymmetric information means sellers know the actual condition of their cars while buyers do not. Buyers calculate an expected price based on the probability of getting a plum or lemon. Since this expected price doesn't reflect individual car quality, it creates the conditions for adverse selection where quality products are systematically undervalued.

Buyers calculate an expected value by multiplying the probability of a car being a plum by its value, plus the probability of it being a lemon by its value. This expected price becomes the single price buyers offer for all used cars. Since individual car quality is unknown, this uniform price fails to reflect actual vehicle condition.

When adverse selection persists without intervention, the market eventually consists largely of lemons. High-quality products have been driven out by continuous undervaluation and declining prices. This market failure demonstrates how asymmetric information can destroy market efficiency and leave only low-quality goods available to consumers.

Lemon sellers benefit from uniform pricing because their low-quality cars are overvalued at the expected market price. They receive more than their cars' actual worth, creating a strong incentive to sell. This economic advantage encourages lemon sellers to remain active while plum sellers exit, fundamentally shifting market composition.