Reverse Contamination: Burning and Building Bridges in Mixed-Member Systems

Why would a candidate in a mixed-member electoral system willingly forego the chance to be dual listed in the party list tier along with the single-member district tier? Mixed-member systems create a "reverse contamination effect" through which list rankings provide important information to voters and thus influence behavior in the nominal tier. Rankings signal importance of the candidate within the party and also constitute information about the likelihood that the candidate will be elected off the list tier. Mixed-member majoritarian (MMM) and mixed-member proportional (MMP) systems create different incentives for parties and candidates to send voters different signals. Candidates in Japan's MMM "burned their bridges" successfully and gained more votes. In New Zealand's MMP system, parties instead built "bridges" between the proportional representation and nominal tiers by sending different signals to voters through list rankings.