"Repeated Games with Private Monitoring: Two Players"

We investigate two-player infinitely repeated games where the discount factor is less than but close to unity. Monitoring is private and players cannot communicate. We require no condition concerning the accuracy of players' monitoring technology. We show the folk theorem for the prisoners' dilemma with conditional independence. We also investigate more general games where players' private signals are correlated only through an unobservable macro shock. We show that efficiency is sustainable for generic private signal structures when the size of the set of private signals is sufficiently large. Finally, we show that cartel collusion is sustainable in price-setting duopoly.

Effects of Reputation in Bubbles and Crashes ( Revised in April 2008 )

We analyze the stock market by modeling it as a timing game among arbitrageurs for beating the gun. We assume that (1) arbitrageurs are behavioral with a small probability, (2) the bubble soft-lands, and (3) the postcrash price increases as the X-day is postponed. Due to these assumptions, the effect of reputation assumes importance because any rational arbitrageur is willing to build a reputation in order to ride the bubble. It is demonstrated that the bubble persists for a long period as an outcome of a unique symmetric Nash equilibrium, even if all arbitrageurs are almost certainly rational.

Financing Harmful Bubbles

We model the stock market as a timing game, in which arbitrageurs who are not expected to be certainly rational compete over profit by bursting the bubble caused by investors? euphoria. The manager raises money by issuing shares and the arbitrageurs use leverage. If leverage is weakly regulated, it is the unique Nash equilibrium that the bubble persists for a long time. This holds even if the euphoria is negligible and all arbitrageurs are expected to be almost certainly rational. This bubble causes serious harm to the society, because the manager uses the money raised for his personal benefit.

"Implementation and Mind Control"

This paper incorporates social psychology into implementation theory, where an uninformed principal manipulates a dynamic decision-making process without employing any tailored contractual device. We demonstrate the principal's mind-control method through which he can effectively utilize social psychology tactics to incentivize informed agents to announce their information in keeping with his wishes. We show that with incentive compatibility, the principal can implement any alternative that he wishes as the unique Nash equilibrium outcome, even if the psychological cost of each agent from disobeying the principal's wishes is small as compared to his total material benefits.

"Detail-Free Mechanism Design in Twice Iterative Dominance: Large Economies"

This paper investigates unique implementation in large economies with incomplete information and interdependent values; we degenerate the common knowledge assumptions and assume that a central planner is unaware of the specifications of an environment. With a minor restriction on the class of environments, we demonstrate that there exists a detail-free mechanism that virtually implements competitive allocations with complete information in twice iterative dominance, irrespective of how the environment is specified.

"Honesty-Proof Implementation"

We investigate implementation of social choice functions that map from states to lotteries and may depend on factors other than agents' preferences. We assume that agents are not only purely self-interested but also honesty-oriented in a lexicographical way. We define iterative honesty-proofness by iteratively removing messages dominated by more honest messages. We show that in the complete information environments with small fines, every social choice function is implementable in iterative honesty-proofness. This is in contrast with the standard implementation model, because any 'normative' social choice function depending on non-preference factors is never implementable when agents are not influenced by factors other than pure self-interest. We extend this result to the incomplete information environments with quasi-linearity and with correlated private signals. Next, we assume that it is costly for each agent to report dishonestly and this cost may be close to zero. We show that in the incomplete information environments, every incentive compatible social choice function can be implemented by the mechanism that is universal in the sense that it does not depend on the private signal structure.

"Role of Honesty in Full Implementation"

This paper introduces a new concept of full implementation that takes into account agents' preferences for understanding how the process works. We assume that the agents have intrinsic preferences for honesty in the sense that they dislike the idea of lying when it does not influence their welfare but instead goes against the intention of the central planner. We show that the presence of such preferences functions effectively in eliminating unwanted equilibria from the practical perspective, even if the degree of the preference for honesty is small. The mechanisms designed are detail-free and involve only small fines.

Tit-For-Tat Equilibria in Discounted Repeated Games with Private Monitoring

We investigate infinitely repeated games with imperfect private monitoring. We focus on a class of games where the payoff functions are additively separable and the signal for monitoring a player's action does not depend on the other player's action. Tit-for-tat strategies function very well in this class, according to which each player's action in each period depends only on the signal for the opponent's action one period before. With almost perfect monitoring, we show that even if the discount factors are fixed low, efficiency is approximated by a tit-for-tat Nash equilibrium payoff vector.

Role of Honesty in Full Implementation ( Revised version of CARF-F-062(2006); The further revision was subsequently published in "Journal of Economic Theory" as a short article. )

This paper introduces a new concept of full implementation that takes into account agents? preferences for understanding how the process works. We assume that the agents have intrinsic preferences for honesty in the sense that they dislike the idea of lying when it does not influence their welfare but instead goes against the intention of the central planner. We show that the presence of such preferences functions effectively in eliminating unwanted equilibria from the practical perspective, even if the degree of the preference for honesty is small. The mechanisms designed are detail-free and involve only small fines.

"Role of Honesty in Full Implementation"

This paper introduces a new concept for full implementation that takes into account agents' preferences for understanding how the "process" works. We assume that the agents have an intrinsic preference for honesty in the sense that they dislike the idea of lying when it does not influence their welfare but instead goes against the intention of the principal. We show that the presence of such preferences functions very effectively in eliminating unwanted equilibria from the practical perspectives, even if the degree of preference for honesty is small. The mechanisms designed are detail-free and involve only small fines.