Adverse selection occurs when products of varying quality are all sold at the same price. These products are sold at a single price irrespective of their quality because of asymmetric information, where one party knows more than the other.

For example, in the used cars market, the car's actual condition is only known by sellers. As a result, buyers are only willing to pay an expected price given some are high quality (and high relative value) and some are low quality (and low relative value). The expected value of a given used car can be calculated as the product of the probability of a car being a plum and the value of a plum added to the probability of a car being a lemon and the value of a lemon. 

This expected price is the single price buyers are willing to pay for a used car. Since this price does not reflect the actual quality of individual cars, sellers of high-quality cars (plums) receive an amount that is lower than their car’s true value. This undervaluation discourages many plum sellers from participating. Over time, many plum sellers exit the market, unwilling to accept undervalued offers. 

At the same time, the sellers of low-quality cars (lemons) receive an amount higher than their actual value. This overvaluation encourages more lemon sellers to sell. Plum sellers are aware of this fact, that they are receiving less than what their cars are worth.

As high-quality products (plums) continue to leave the market due to lower-than-acceptable prices, the proportion of low-quality products (lemons) increases. With fewer plums available, buyers become more skeptical about the overall quality of goods in the market. This skepticism leads them to lower the price they are willing to pay. The cycle of declining plum availability and decreasing prices continues, making lemons more common in the market. Eventually, the market can consist largely of lemons.

Therefore, understanding and mitigating adverse selection is crucial for maintaining market efficiency.