Definition
Adverse selection is a market failure arising from asymmetric information in which the parties most likely to buy (or keep) insurance are those with the highest expected costs, because they know more about their own risk than the insurer does. As high-risk individuals disproportionately enroll, average claims rise, premiums increase, lower-risk individuals drop out, and the risk pool deteriorates — potentially unraveling the market (a "death spiral"). It is the information-economics counterpart to moral hazard: adverse selection concerns who buys coverage given hidden risk type, while moral hazard concerns how behavior changes once covered.
Key Ideas
- Hidden information (type), not hidden action: adverse selection stems from private information about one's risk type (health, mortality risk), in contrast to moral hazard's hidden action.
- Pooling and unraveling: when insurers cannot price by risk, a single pooled premium overcharges low risks and undercharges high risks; if low risks exit, premiums rise further — Akerlof's "lemons" logic applied to insurance.
- Policy responses: mandates, automatic enrollment, community rating with subsidies, and risk adjustment are designed to keep low-risk individuals in the pool and counter unraveling. The ACA individual mandate is a canonical example.
- Selection can be advantageous: when the traits driving demand for insurance are negatively correlated with risk (e.g., the risk-averse and cautious buy more coverage and are healthier), selection runs the other way ("advantageous selection") — so its sign is an empirical question.
- Distinguishing selection from frictions: low take-up of subsidized coverage need not reflect rational selection; it can stem from information frictions and inertia. Goldin, Lurie, and McCubbin (2021) find that IRS outreach to mandate-penalty payers raised coverage and reduced mortality, identifying behavioral frictions — not rational adverse selection against coverage — as the driver of non-enrollment. See Tax Salience.
How It Works
Insurers face a demand curve in which willingness to pay rises with hidden risk. At any pooled premium, the marginal buyer is healthier (lower-cost) than the average buyer already enrolled, so raising price worsens the average cost of those who remain; equilibrium can feature incomplete coverage or, in the extreme, no trade. Empirically, the slope of the realized cost curve in price (the Einav–Finkelstein "cost curve" approach) tests for and signs the selection.
Why It Matters
- Rationale for social and mandated insurance: adverse selection is a core efficiency argument for compulsory or heavily subsidized social insurance (Social Security, Medicare, the ACA), where voluntary markets would unravel. See Optimal Social Insurance.
- Design of public programs: risk adjustment, open-enrollment rules, and mandates in disability insurance, health exchanges, and annuity markets are all responses to selection.
- Measurement caution: because selection and behavioral frictions both depress take-up, distinguishing them is essential for predicting how coverage expansions affect costs and health outcomes. See Medicaid and Mortality.
Open Questions
- In which insurance markets does advantageous selection dominate, and how should that change subsidy design?
- How much of low take-up in subsidized programs is selection versus frictions, and which policy levers address each?
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