Hamid Sabourian

Herding and Contrarian Behavior in Financial Markets

Rational herd behavior and informationally efficient security prices have long been considered to be mutually exclusive but for exceptional cases. In this paper we describe the conditions on the underlying information structure that are necessary and sufficient for informational herding and contrarianism. In a standard sequential security trading model, subject to sufficient noise trading, people herd if and only if, loosely, their information is sufficiently dispersed so that they consider extreme outcomes more likely than moderate ones. Likewise, people act as contrarians if and only if their information leads them to concentrate on middle values. Both herding and contrarianism generate more volatile prices, and they lower liquidity. They are also resilient phenomena, although by themselves herding trades are self enforcing whereas contrarian trades are self-defeating. We complete the characterization by providing conditions for the absence of herding and contrarianism.

Anonymous repeated games with a large number of players and random outcomes

Abstract Green ( J. Econ. Theory 22 (1980), 155–182) demonstrates that in anonymous repeated games with random outcomes, any play which can be sustained as an equilibrium by a trigger strategy is approximately a Nash equilibrium of the stage game if there is a large, but finite, number of players and if the stage game satisfies a certain continuity assumption. This paper shows that Greens result holds when no restrictions are imposed on the strategies of the players, thus demonstrating the continuity of Nash correspondence at the nonatomic limit for these games. Moreover, the result shows that, in general, to obtain convergence, Greens continuity assumption cannot be relaxed.

Bargaining and markets: complexity and the competitive outcome

Abstract Rubinstein and Wolinsky (Rev. Econ. Stud. 57 (1990) 63–78) consider a simple decentralised market game in which agents meet randomly or voluntarily and bargain over the terms of trade. They show that any individually rational price can be sustained as a sequential equilibrium even though the model has a unique competitive outcome. Here, I consider Rubinstein and Wolinskys model and show that if complexity costs of implementing strategies enter players’ preferences, together with the standard payoff in the game, then every equilibrium is stationary/Markov and induces the unique competitive price. Thus, I demonstrate that aversion to complexity may provide a justification for the competitive outcome.

Cooperation and Effective Computability

A common interest game is a game in which there exists a unique pair of payoffs which strictly Pareto dominates all other payoffs. The authors consider the undiscounted repeated game obtained by the infinite repetition of such a two-player stage game. They show that, if supergame strategies are restricted to be computable within Churchs thesis, the only pair of payoffs that survives any computable tremble with sufficiently large support is the Pareto-efficient pair. The result is driven by the ability of the players to use the early stages of the game to communicate their intention to play cooperatively in the future. Copyright 1995 by The Econometric Society.

Repeated Games with One-Memory

We study the extent to which equilibrium payoffs of discounted repeated games can be obtained by 1-memory strategies. We establish the following in games with perfect (rich) action spaces: First, when the players are sufficiently patient, the subgame perfect Folk Theorem holds with 1-memory. Second, for arbitrary level of discounting, all strictly enforceable subgame perfect equilibrium payoffs can be approximately supported with 1-memory if the number of players exceeds two. Furthermore, in this case all subgame perfect equilibrium payoffs can be approximately supported by an [epsilon]-equilibrium with 1-memory. In two-player games, the same set of results hold if an additional restriction is assumed: Players must have common punishments. Finally, to illustrate the role of our assumptions, we present robust examples of games in which there is a subgame perfect equilibrium payoff profile that cannot be obtained with 1-memory. Thus, our results are the best that can be hoped for.

Multi-stage voting, sequential elimination and Condorcet consistency

A class of voting procedures based on repeated ballots and elimination of one candidate in each round is shown to always induce an outcome in the top cycle and is thus Condorcet consistent, when voters behave strategically. This is an important class as it covers multi-stage, sequential elimination extensions of all standard one-shot voting rules (with the exception of negative voting), the same one-shot rules that would fail Condorcet consistency. The necessity of repeated ballots and sequential elimination are demonstrated by further showing that Condorcet consistency would fail in all standard voting rules that violate one or both of these conditions.

Efficient Repeated Implementation

This paper examines repeated implementation of a social choice function (SCF) with infinitely-lived agents whose preferences are determined randomly in each period. An SCF is repeated-implementable in (Bayesian) Nash equilibrium if there exists a sequence of (possibly history-dependent) mechanisms such that (i) its equilibrium set is non-empty and (ii) every equilibrium outcome corresponds to the desired social choice at every possible history of past play and realizations of uncer- tainty. We first show, with minor qualifications, that in the complete information environment an SCF is repeated-implementable if and only if it is effcient. We then extend this result to the incomplete information setup. In particular, it is shown that in this case efficiency is sufficient to ensure the characterization part of repeated implementation. For the existence part, incentive compatibility is sufficient but not necessary. In the case of interdependent values, existence can also be established with an intuitive condition stipulating that deviations can be detected by at least one agent other than the deviator. Our incomplete information analysis can be extended to incorporate the notion of ex post equilibrium.

Coase theorem, complexity and transaction costs

This paper, by introducing complexity considerations, provides a dynamic foundation for the Coase theorem and highlights the role of transaction costs in generating inefficient bargaining/negotiation outcomes. We show, when the players have a preference for less complex strategies, the Coase theorem holds in negotiation models with repeated surplus and endogenous disagreement payoffs if and only if there are no transaction costs. Specifically, complexity considerations select only efficient equilibria in these models without transaction costs while every equilibrium induces perpetual disagreement and inefficiency with transaction costs. We also show the latter is true in the Rubinstein bargaining model with transaction costs.

Markov equilibria in dynamic matching and bargaining games

Rubinstein and Wolinsky (1990) show that a simple homogeneous market with exogenous matching has a continuum of (non-competitive) perfect equilibria; however, the unique Markov perfect equilibrium is competitive. By contrast, in the more general case of heterogeneous markets, we show there exists a continuum of (non-competitive) Markov perfect equilibria. However, a refinement of the Markov property, which we call monotonicity, does suffice to guarantee perfectly competitive behavior: we show that a Markov perfect equilibrium is competitive if and only if it is monotonic. The monotonicity property is closely related to the concept of Nash equilibrium with complexity costs.

Distributing awards efficiently: More on King Solomon's problem

We consider a multi-award generalisation of King Solomon’s problem: k identical and indivisible awards should be distributed among agents, k 1) prizes efficiently in sub-game perfect equilibria without any monetary transfers in equilibrium. Finally, in the multi-awards case we relax the complete information assumption and achieve implementation of efficient allocation by iterative elimination of weakly dominated strategies, using generalisation of Olszewski’s (2003) mechanism.