Duverger and the territory: explaining deviations from the two-party-competition-law

According to Duverger’s Law, plurality voting systems lead to two-party competition. However, many results of national parliamentary elections deviate from this rule. In contrast to previous research, which argues that in countries with territorial splits, regional two-party systems aggregate to national multi-party systems, this article shows that this explanation accounts for only a small proportion of the empirical exceptions to Duverger’s Law. Instead, this article distinguishes three explanations for Non-Duvergerian outcomes at the national level. The three mechanisms relate to the level of electoral constituencies, to the aggregation of constituency results, and to the interaction between constituency- and aggregation effects. This article assesses the explanatory power of each of the three explanations on an extensive sample of election results from plurality vote systems, linking national to constituency-level results.