Jeffrey C. Ely

A Robust Folk Theorem for the Prisoner's Dilemma☆

We prove the folk theorem for the Prisoners dilemma using strategies that are robust to private monitoring. From this follows a limit folk theorem: when players are patient and monitoring is sufficiently accurate, (but private and possibly independent) any feasible individually rational payoff can be obtained in sequential equilibrium. The strategies used can be implemented by finite (randomizing) automata.

When is Reputation Bad

In traditional reputation models, the ability to build a reputation is good for the long-run player. In [Ely, J., Valimaki, J., 2003. Bad reputation. NAJ Econ. 4, 2; http://www.najecon.org/v4.htm. Quart. J. Econ. 118 (2003) 785–814], Ely and Valimaki give an example in which reputation is unambiguously bad. This paper characterizes a class of games in which that insight holds. The key to bad reputation is that participation is optional for the short-run players, and that every action of the long-run player that makes the short-run players want to participate has a chance of being interpreted as a signal that the long-run player is “bad.†We allow a broad set of commitment types, allowing many types, including the “Stackelberg type†used to prove positive results on reputation. Although reputation need not be bad if the probability of the Stackelberg type is too high, the relative probability of the Stackelberg type can be high when all commitment types are unlikely.

Implementation with near-complete information

Many refinements of Nash equilibrium yield solution correspondences that do not have closed graph in the space of payoffs or information. This has significance for implementation theory, especially under complete information. If a planner is concerned that all equilibria of his mechanism yield a desired outcome, and entertains the possibility that players may have even the slightest uncertainty about payoffs, then the planner should insist on a solution concept with closed graph. We show that this requirement entails substantial restrictions on the set of implementable social choice rules. In particular, when preferences are strict (or more generally, hedonic), while almost any social choice function can be implemented in undominated Nash equilibrium, only monotonic social choice functions can be implemented in the closure of the undominated Nash correspondence. Copyright Econometric Society, 2002.

Mechanism design without revenue equivalence

We study mechanism design problems in quasi-linear environments where the envelope theorem and revenue equivalence principle fail due to non-convex and non-differentiable valuations. We obtain a characterization of incentive compatibility based on the Mirrlees representation of the indirect utility and a monotonicity condition on the allocation rule, which pin down the range of possible payoffs as a function of the allocation rule. To illustrate our approach we derive the optimal selling mechanism in a buyer–seller situation where the buyer is loss-averse; we find a budget-balanced, efficient mechanism in a public goods location model; and we consider a principal–agent model with ex post non-contractible actions available to the agent.

Adverse selection and unraveling in common-value labor markets

We investigate a common-value labor setting in which firms interview a worker prior to hiring. When firms have private information about the worker’s value and interview decisions are kept private, many firms may enter the market, interview, and hire with positive probability. When firms’ interview decisions are revealed, severe adverse selection arises. As a result, all firms except for the highest-ranked firm are excluded from the hiring process.