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'A law is valuable not because it is law, but because there is right in it' -- Henry Ward Beecher.
'Laws are like cobwebs, which may catch small flies but which let wasps and hornets break through' -- Jonathan Swift.
Duverger's Law
Among students of electoral systems, there is no better-known, more investigated, nor widely cited proposition than the relationship between plurality electoral laws and two-party systems known as Duverger's Law. Since its publication more than a half-century ago in Political Parties (1951), hundreds of articles, books, and papers have been written to elaborate the workings of Duverger's propositions. This growing literature has produced numerous empirical studies to explain how electoral systems and changes in electoral rules influence the number of political parties which compete for and win office. A parallel effort among formal theorists has led to many insightful works attempting to derive these empirical regularities from first principles, and otherwise to explore further the relationship between electoral rules and the number of political parties. The literature on formal and comparative electoral systems research is far too vast to review here, and indeed has been assessed thoroughly elsewhere (e.g. Shugart, 2005). Rather, the goal of this essay is rather to examine the legacy bequeathed by Duverger to the field of electoral studies in terms of the influence of Duverger's basic ideas, to trace where research into these ideas has led, and briefly to argue where future research in these areas ought to be concentrated.
Duverger's comparative survey of party systems investigated the sources of dualism , or the concentration of political party activity in two main parties. National factors explain a great deal, concluded Duverger, but two-party systems are invariably associated with a particular type of institutional arrangement: the single-member district, plurality electoral system. 'Dualist countries use the simple-majority vote and simple-majority vote countries are dualist' (1959, 217). Elevating this claim to nomological status, Duverger set out his 'law' in a passage that has been cited countless times in the decades that followed:The simple-majority single-ballot system favours the two-party system . Of all the hypotheses that have been defined in this book, this approaches the most nearly perhaps to a true sociological law (Duverger, 1959, 217, emphasis in original).
As a corollary, Duverger...