Week 4: Red Flags & Adverse Selection

Akerlof's market for lemons, and why the worst-behaved participants are often the easiest to find

13 March 2028

Open the slides

Akerlof’s used-car market fails not because sellers of good cars are rare, but because buyers can’t tell good cars from bad ones, so good sellers exit and the average car for sale gets worse. The same mechanism produces the “red flag” folk-wisdom: it isn’t a personality trait, it’s a market prediction — the pool self-selects toward whoever adverse selection didn’t scare off.

This week reframes red-flag-spotting as adverse-selection detection rather than character-reading, which is a meaningfully different — and more teachable — skill.

Outline#

• the market for lemons, worked from the original model
• why “if it seems too good to be true” is adverse selection stated as a proverb
• what information asymmetry a red flag is actually evidence of
• why “still available” can mean two opposite things, and the model can’t tell them apart

One thing this framework won’t explain#

The model reads “still on the market” as evidence of low quality — that’s the whole mechanism. But “still available” has (at least) two causes that look identical from outside the pool: nobody wanted them, or they were holding out for something specific and turned down offers that would have taken them off the market. Adverse selection predicts the same observable pattern either way, and has no term for telling a self-selecting holdout apart from someone the market has already judged.

That’s a third kind of gap this course keeps finding: Week 1’s model couldn’t price the value of not being rushed; Week 3’s couldn’t tell a credible signal from a relevant one; this one can’t tell two different causes of the same pool apart. Week 12 is where all three come back at once.

Further reading#

• Akerlof, G. A. (1970). “The Market for ‘Lemons’: Quality Uncertainty and the Market Mechanism.” Quarterly Journal of Economics, 84(3), 488–500. The paper this week is built on — the used-car example is Akerlof’s own, not an analogy layered on afterward. Shared the 2001 Nobel Memorial Prize in Economic Sciences with Spence (Week 3’s citation) and Stiglitz, for the same broader research program on information asymmetry.