Coalitional stability of multi-party systems: Evidence from Poland

The electoral law and voter preferences are modeled as a partition-function form game that represents payoffs of electoral coalitions in a multi-party system. Three empirical hypotheses are formulated under the assumption tha t seat-maximizing parties modify coalitional structures to extract gains fr om coalescing. The hypotheses say that party systems under stable voter pre ferences and electoral law tend to stability with respect to splits and coa lescing. The partition function is estimated for various coalitional struct ures for the Polish party system with simulation based on district-level el ection results and survey data. The estimates show that the Polish party sy stem is split stable. While the opportunities for gains from coalitions on the Right were exhausted between the years of 1993 and 1997, some opportuni ties for such gains emerged on the Left.