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Duverger's famous law is clear and straightforward: « le scrutin majoritaire à un seul tour tend au dualisme des partis » (Duverger, 1951, p. 247), or 'the plurality rule leads to a two party system'. I will assume that Duverger had in mind the single-member district plurality (SMP) system, and, like Cox (1997), I take it that the most appropriate test of the law is at the district level. I am interested here in the party system at the electoral level, that is, the distribution of votes among the parties. I thus focus on the psychological impact of electoral laws, that is, their impact on voters' choices. I show that the contemporary evidence tends to disconfirm Duverger's law but I argue that the basic intuition behind the law is valid.
The logic underlying Duverger's law has been laid out beautifully by Cox (1997). Under SMP, the winner is the candidate/party that has the most votes in the district. If there are three candidates running in the election, the supporters of the weakest candidate will rally to support the candidate that they prefer among the top two contenders. As Cox points out, this assumes that both the parties and the voters are short-term utility maximizers and that they have accurate perceptions of the strength of the various parties. Cox (1997, p. 75) adds that there may be non-Duvergerian equilibria, with more than two candidates, if and when the expected support of two or more candidates is too close for anyone to be 'winnowed out from the field of viable candidates'. In short, a two-party (or candidate) system should be the norm, but there may be more candidates in exceptional circumstances.
The basic reason why Duverger predicted only two parties or candidates to receive any significant degree of support is that voters would not want to waste their vote on candidates with no or little chance of winning, which assumes that they are short-term utility maximizers and are well informed about the chances of the various candidates. But the number of 'significant' candidates also depends on the decisions made by the parties and the candidates. The assumption is that potential candidates want to win, that they incur a cost in running a campaign, and that they understand...