Collusion properties of values

Two players may enter the game with a prior proxy or association agreement in order to strengthen their positions. There exist weighted majority voting games where a proxy agreement weakens the two palyers’ collective power: the sum of their Shapley values with the agreement is less than without the agreement. This phenomenon cannot happen in non-trivial one man-one vote majority voting games. However, an association agreement weakens the two players’ collective power in one man-one vote majority voting games with a sufficiently high quorum. In contrast, the sum of the two players’ Banzhaf values turns out to be always immune against manipulation via a proxy or association agreement. Each of these neutrality properties can be used as part of an axiomatic characterization of the Banzhaf value.