Voting for two different parties in mixed-member systems  

By Pedro Riera (University Carlos III, Madrid), and Damien Bol (King’s College London).

What is the story?

In some countries, voters have two votes: (1) a ‘candidate vote’ usually in a single-member district like in the UK, and (2) a ‘party-list vote’ in a multi-member district like in Spain. These are called mixed-member systems.

A remarkable increasing number of democracies around the world have adopted a mixed-member system in the last twenty-five years. They represent a good compromise between majoritarian and proportional representation systems. Emblematic examples include Germany, New Zealand, and Japan.

A substantial portion of voters use these two votes to support different parties. This is typically referred to as a split-ticket. In a recently published paper, we look at this intuitively-odd behavior, and show that even a very subtle difference in the electoral system can have important consequences for vote choice.

Hypothesis

There are two types of mixed-member system. In the first type, called MMP for mixed-member proportional, there is a linkage between the allocation of seats across tiers. The system works in two steps. First, the single-member votes are counted and transformed into seats. Then, some extra seats are allocated to parties, so that the final seat share of each party corresponds to its proportional representation vote share. In other words, proportional representation votes are used to compensate single-member district votes.

In the second type, called MMM for mixed-member majoritarian, there is no linkage between the tiers. Everything works just as if there were two elections organized in parallel: one in single-member districts, and the other under proportional representation.

Our argument is that the type of mixed-system should influence the propensity to cast a split-ticket. In MMP, voters should not care about which party is elected in single-member districts. Regardless of which party is elected, the proportional representation votes will compensate. In other words, single-member district votes have no effect on the final seat share of the parties.

Voters should thus feel to vote for another party than their favorite one in the single-member districts of MMP systems. This vote will not influence the overall success of their favorite party. But this is not the case in MMM systems where both votes count. Thus, split-tickets votes should be more likely in MMP than in MMM.

Results

To test this hypothesis, we gathered survey data across the four available waves of the Comparative Study of Electoral System Project. We also added all the survey data we could find for countries using mixed-member systems. In total, our data is covering 18 mixed-member elections in 7 countries/regions.

In each of these surveys, respondents were asked about their two votes for the latest election.
We consider that they are split-ticket voters if the single-member district vote is different from the proportional representation vote. Table 1 shows the proportion of split-ticket votes in our data. We observe that there are many more split-ticket votes in MMP elections than in MMM elections. The difference is about 15 percentage points.

In the paper, we estimate a series of regressions in which we show that this difference is robust to the inclusion of many controls variables (socio-demographics, incentives to cast a strategic vote, democratic background of the country…) and many specifications (including the exclusion of some countries from the dataset).

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Conclusion

With our paper, we would like to argue that it is important to study the intricacies of electoral systems to examine their impact on voting behavior and elections. More importantly, it is crucial to consider even the smallest detail of electoral systems. A difference that many might see as anecdotal, such as the existence of seat-linkage in mixed-member systems, can have important consequences for vote choice

For more details, see Pedro Riera and Damien Bol. 2017. >Ticket-splitting in Mixed-member Systems: On the Importance of Seat Linkage Between Electoral Tiers. West European Politics 40(3): 584-597.

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