Adverse Selection

02What People Think

“If everyone gets the same terms, the group joining the deal should be a fair cross-section of everybody.”

03What’s Underneath

A shared house advertises one flat contribution for electricity, without asking applicants about their usage. Light users decide it is expensive and look elsewhere. Heavy users find it attractive. If their usage is not visible to the organizer, the eventual group can be much costlier than the average applicant originally imagined.

Adverse selection concerns who enters an arrangement when relevant characteristics are unevenly known. The problem can arise with hidden risk, quality or expected cost. Offered terms influence participation, and participation then changes the mix of people or goods the other side receives.

The participants need not change their habits or deliberately deceive anyone. They can simply know more about their own circumstances. That distinguishes adverse selection from moral hazard, where the arrangement changes later behavior. In the house example, already-heavy electricity users joining is a selection effect; previously careful residents increasing usage afterward would be a different mechanism.

04The Game

Who finds these terms especially attractive, who quietly walks away, and what private information helps explain the difference? Looking only at how many people sign up can miss the changing mix.