Contract Theory: Moral Hazard, Adverse Selection, and Incentive Design 契約理論

Contract theory studies how economic actors construct contractual arrangements in the presence of asymmetric information. The two canonical problems are moral hazard (hidden action — the agent's effort is unobservable) and adverse selection (hidden type — the agent's characteristics are private). The optimal contract balances the principal's desire for risk-sharing against the need to incentivize effort or truthful revelation. Hart and Holmstrom's contributions on incomplete contracts and incentive design form the modern foundation.

• Designing compensation, bonus, or commission structures for employees or contractors
• Evaluating insurance contracts for moral hazard (deductibles, copays) or adverse selection (screening)
• Structuring partnerships, franchise agreements, or procurement contracts with unobservable quality
• Analyzing incomplete contracts where not all states of the world can be specified

• Both parties have symmetric information and trust is established (no incentive problem)
• The relationship is a one-shot anonymous transaction with no contractual enforcement
• Behavioral factors (reciprocity, intrinsic motivation) dominate monetary incentives

IRON LAW: The optimal contract balances risk-sharing against incentive
provision — full insurance destroys incentives, full incentives impose
unbearable risk. There is no contract that achieves first-best when
information is asymmetric.

• The principal is risk-neutral; the agent is risk-averse (standard setup)
• The agent's action (effort) or type is private information
• Output is a noisy signal of the agent's effort: x = f(e) + epsilon
• Both parties are rational and can commit to the contract terms
• Courts can verify output but not effort (contractibility constraint)

Step 1 — Classify the Information Problem Determine whether the core issue is moral hazard (hidden action after contracting), adverse selection (hidden type before contracting), or both. Identify who is the principal and who is the agent.

Step 2 — Specify Constraints Write down: (1) the Incentive Compatibility constraint (IC) — the agent prefers the intended action/type revelation; (2) the Participation Constraint (PC/IR) — the agent accepts the contract over the outside option; (3) Limited Liability (LL) if applicable — payments cannot go below zero.

Step 3 — Solve the Optimal Contract For moral hazard: maximize principal's expected profit subject to IC and PC. The optimal wage schedule w(x) satisfies the Holmstrom informativeness principle — pay should depend on output only insofar as it is informative about effort. For adverse selection: design a menu of contracts that induces self-selection (screening). Expect distortion at the bottom (inefficient allocation for low types) and efficiency at the top.

Step 4 — Assess Completeness and Renegotiation Check whether the contract is complete (covers all verifiable contingencies) or incomplete (residual rights matter). If incomplete, apply Hart's property rights approach: allocate residual control rights to the party whose investment is most important. Consider renegotiation-proofness.

• Holmstrom, B. (1979). "Moral Hazard and Observability." Bell Journal of Economics.
• Hart, O. & Moore, J. (1990). "Property Rights and the Nature of the Firm." Journal of Political Economy.
• Laffont, J.-J. & Martimort, D. (2002). The Theory of Incentives: The Principal-Agent Model.
• Bolton, P. & Dewatripont, M. (2005). Contract Theory. MIT Press.