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We study electoral rule choice in a multi-party model with office-motivated parties and electoral outcome uncertainty. We show that when all dominant parties (parties with positive probability of winning the election) have sufficiently good chances of winning, they agree to switch from the PR rule to a more majoritarian one in order to increase their chances of forming a single-party government. We identify the exact degree of disproportionality of the new rule and we prove that it is increasing in the expected vote share of the smaller parties (parties with zero probability of winning otherwise). The necessary and sufficient conditions for such collusion in favor of a majoritarian rule are: (a) the high rents from a single-party government and (b) sufficient uncertainty over the electoral outcome. Our theoretical predictions regarding the degree of the disproportionality of the electoral rule are supported by empirical evidence. Copyright Springer Science+Business Media New York 2015