19. Adverse Selection and Signaling

When information is asymmetric, how markets unravel — and how they save themselves: the lemons market, education signaling, and screening menus.

• Akerlof's Lemons Market — Good cars vanish from a used-car market. Only the seller knows the car's quality. The buyer sets the price by the average. At an average price, good-car owners refuse to sell. Only bad cars remain, so the price falls further. As the price unravels like this, trade itself disappears. For example, there are 10 used 🚗 cars. A good one is worth 10 million won. A bad one is worth 1 million won. The buyer cannot tell them apart and offers the average, 5.5 million. Then every good-car owner walks away. Seeing only bad cars left, the buyer lowers the price again.
• Education Signaling — When skill cannot be shown directly, a diploma speaks instead. Studying is easier for the able worker. So only the able can afford the costly degree. For the less able, copying is just too painful. Firms read the diploma and guess the skill. Yet the education itself builds no skill. So the signaling cost below is pure waste. For example, a fast 🧑 runner easily wins a race medal. A slow runner can hardly win one at all. So a medal tells everyone who is fast. But winning the medal makes nobody faster.
• Screening Menus — A seller lays out a menu, and buyers pick for themselves. The seller cannot tell big spenders from small ones. So the menu offers two sizes. The big option is made in full. The small option is shrunk on purpose. Shrinking it stops big spenders from lying. In return, big spenders pocket a bonus called information rent. The formulas below set the two sizes. For example, a 🏪 shop sells 🍬 candy in a big 📦 box and a small box. Big spenders choose the big box. The small box is made tiny on purpose. Pretending to be a small buyer would only hurt. So big spenders honestly pick the big box.