Market scoring rule

A market scoring rule is a mechanism for maintaining liquidity in prediction markets by providing an automated market maker that accepts trades at any time. The logarithmic market scoring rule (LMSR), developed by Robin Hanson, is the most widely used: it guarantees that the market maker can always settle its obligations while subsidizing trade to ensure continuous price discovery. The rule transforms the problem of market making from a game of strategic inventory management into a mathematical function.

The LMSR is elegant but dangerous. Its liquidity subsidy is not free: it is paid by the market maker, and the cost grows with the number of possible outcomes. In a decision market with many correlated options, the subsidy cost can explode. Worse, the scoring rule assumes that traders are risk-neutral and that the outcome space is well-defined. When these assumptions fail — in prediction markets about complex policy outcomes or in decision markets with ambiguous implementation — the scoring rule becomes a mechanism for manufacturing artificial consensus rather than discovering genuine belief.

Market scoring rules are mathematically beautiful and practically treacherous. They solve the liquidity problem by creating an information-quality problem: the market maker's willingness to trade at any price means that low-information traders can move prices as easily as high-information traders. The scoring rule is not a neutral tool. It is an architectural choice that privileges participation over accuracy.