Risk-Based Pricing and the Adverse Selection Spiral
A flat-rate insurer is unstable: a competitor cherry-picks the low risks, raising the remaining pool's average cost, forcing a price rise that drives out the next-safest tier, and so on to insolvency. The spiral is driven by competition, not greed, which makes risk rating load-bearing infrastructure. The reliable alternative is a mandate removing the exit — which is why social insurance can price flat and private insurance cannot.
Insurers charge different prices to different customers because uniform pricing is unstable. Not unfair — *unstable*. A flat-rate insurer cannot survive in a competitive market, and understanding why explains most of how insurance is structured. ## The adverse selection spiral Suppose an insurer charges every house the same price, set at the average expected loss across its pool. A competitor then offers a lower price to the safest houses only — houses whose true expected loss is below average. Those customers leave, because the new offer is cheaper *and* fairly priced for them. The flat-rate insurer now holds a pool whose average risk is higher than before, so its average price no longer covers its claims. It raises prices. The next-safest tier now finds a better offer elsewhere and leaves. The average risk rises again. Repeat. This is the **adverse selection death spiral**, and it terminates with only the worst risks remaining at unaffordable prices. It is a specific case of the dynamic described in Information Asymmetry: When One Side Knows More Than the Other. The key point: **the spiral is driven by competition, not by insurer greed.** Any single insurer that wants to price flat is punished by any competitor willing to segment. Risk-based pricing is a stable equilibrium; flat pricing is not. ## What this implies - **Risk rating is load-bearing infrastructure**, not a profit-maximising add-on. It is what keeps the pool from unravelling. - **Mandates are the alternative.** The one reliable way to get uniform pricing is to remove the exit: compulsory participation, as in mandatory auto insurance or a tax-funded national scheme, means low risks cannot leave. This is why social insurance can price flat and private insurance cannot. See Public Insurance: Where Governments Already Nationalised Risk. - **Uniform pricing is redistribution.** With a mandate it works, but it is a transfer from low risks to high risks — a legitimate political choice, not a free efficiency gain. ## The mirror-image problem Adverse selection is about who buys; **moral hazard** is about what they do afterwards. Insurance that fully removes the consequences of a loss weakens the incentive to prevent one — which is why policies carry deductibles and exclusions, and why underpriced public schemes are criticised for encouraging construction in flood plains. Both are consequences of the insurer knowing less than the insured, and both shape the products that exist.