We propose a model of voter decision-making in proportional representation systems: ultra-rational strategic voters construct expectations of coalitions and policy outcomes based on expected seat distributions and vote to maximize their expected utility from the implemented policy. We examine the predictions of our model using data from the Netherlands and successfully predict the voting behavior of signicant numbers of voters. Nevertheless, other factors matter more than our strategic prediction. Three main take-aways follow: (1) At least to some extent, voters seem to take complex coalition considerations into account. (2) There is a need for large-scale quantitative studies about voter decision-making in proportional representation systems. (3) Narrowly defined strategic voting might matter less in proportional representation systems than in plurality systems.
Strategic voting has been an important topic in psephology as researchers try to understand the ways in which voters decide how to cast their votes. Do they simply vote for their "preferred" party? Or do they target electoral outcomes and derive their vote in a more "strategic" manner? There is no shortage of research documenting that strategic voting figures prominently across electoral systems. Examining 32 elections in 32 countries, Hobolt and Karp (2010) find that, on average, sincere voting can at best account for 85 percent of votes cast, leaving ample room for strategic considerations to play an important role.
Ultimately, however, what we as researchers want to understand about strategic voting is: How does it work? How does a strategic voter decide whom to cast his vote for? That is, we want to get inside the black box and understand the decision-making process. This is a much easier undertaking in a plurality system than it is in a proportional representation system (henceforth PR system). As early as 1869, Henry Droop, an English proponent of proportional representation and the inventor of the Droop quota, described the decisionmaking process in pointing out the susceptibility of plurality systems to strategic voting:
"Each elector has practically only a choice between two candidates or sets of candidates. As success depends upon obtaining a majority of the aggregate votes of all the electors, an election is usually reduced to a contest between the two most popular candidates or sets of candidates. Even if other candidates go to the poll, the electors usually find out that their votes will be thrown away, unless given in favour of one or other of the parties between whom the election really lies" (quoted in Riker 1982: 756).
Here, we have the strategic voter's decision-making process in a plurality system in a nutshell: he votes for that one of the two leading candidates whose policy position he likes better.