Subgame perfect implementation of the deserving winner of a competition with natural mechanisms

A jury has to decide the winner of a competition among a group of contestants. All members of the jury know who the deserving winner is, but this contestant is unknown to the planner. The social optimum is that the jury select the deserving winner. Each individual juror may be biased in favor (friend) or against (enemy) some contestant, and therefore her goal does not necessarily coincide with the social objective. We analyze the problem of designing extensive form mechanisms that give the jurors the right incentives to always choose the deserving winner when the solution concept is subgame perfect equilibrium. We restrict the class of mechanisms considered to those which satisfy two conditions: (1) the jurors take turns to announce the contestant they think should win the competition, and (2) telling the truth is always part of a profile equilibrium strategies. A necessary condition for these mechanisms to exist is that, for each possible pair of contestants, there is at least one juror who is impartial with respect to them. This condition, however, is not sufficient. In addition, the planner must know the friend or the enemy of at least one juror.