A market failure that occurs when asymmetric information about private qualities or risks, present before a transaction, causes one side to participate with a higher probability of unfavourable characteristics, altering prices, allocations or market participation in ways that can reduce mutually beneficial trade.

Principle

When prices or contract terms pool across private types, agents with characteristics that make the contract relatively more attractive will self-select into participation, shifting the composition of market participants and potentially driving out lower-risk or higher-quality parties.

Demonstration

Situation: An insurer offers a single premium because it cannot observe individual risk. Recognition: High-risk individuals value insurance more at that premium. Action: High-risk agents buy coverage more often; low-risk agents opt out. Consequence: Average risk rises, premiums may increase, and coverage can contract, reducing market efficiency.

Misapplication

Confusing adverse selection with moral hazard (hidden actions after contract) or treating any unequal outcome as adverse selection without establishing pre-transaction hidden information and selection into the market.

Consequence

Can reduce the scope of mutually beneficial transactions, raise equilibrium prices or eliminate certain markets; motivates mechanisms such as screening, signalling, risk-based pricing, mandates, or pooling arrangements to mitigate selection effects.

Reversal

Adverse selection is mitigated when credible screening, signalling, mandatory disclosure, third-party verification, or contract differentiation permits separating or risk-adjusted participation; in some institutional settings pooling with subsidies may be chosen for redistributional reasons rather than efficiency.

Boundary

Clearly within: pre-contract hidden heterogeneity that affects selection into transactions. Boundary case: correlation between observable and unobservable traits that permits partial risk classification. Clearly outside: post-contract hidden actions (moral hazard) and deterministic quality differences fully observable ex ante.

Semantic Tension

Adverse Selection ↔ Moral Hazard — both stem from asymmetric information but differ by timing: adverse selection arises before the contract (selection into the transaction); moral hazard arises after (hidden actions).

Synthesis

Adverse selection describes how private information about quality or risk alters who trades and under what terms; remedies must change incentives or information before selection occurs to restore efficient participation.