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In districts where only one seat is contested, the electoral formula (plurality or majority) should be a major determinant of the number of parties that receive votes. Specifically, plurality rule should generate two-party competition while other institutional arrangements should generate electoral fragmentation. Yet tests of these propositions using district-level data have focused on a limited number of cases; they rarely contrast different electoral systems and have reached mixed conclusions. This study analyses district-level data from 6,745 single-member district election contests from 53 democratic countries to test the evidence for Duverger's Law and Hypothesis. Double-ballot majoritarian systems have large numbers of candidates, as predicted, but while the average outcome under plurality rule is generally consistent with two-party competition, it is not perfectly so. The two largest parties typically dominate the districts (generally receiving more than 90 per cent of the vote), and there is very little support for parties finishing fourth or worse. Yet third-place parties do not completely disappear, and ethnic divisions shape party fragmentation levels, even under plurality rule. Finally, institutional rules that generate multiparty systems elsewhere in the country increase electoral fragmentation in single-member plurality districts. [ABSTRACT FROM PUBLISHER]

Abstract: A central tenet of electoral systems'' research is that more parties should get votes in districts with large magnitudes than in districts with smaller magnitudes. This proposition is largely untested at the district level, even though that is the level at which relevant pressures are expected to work. At the aggregate level, research has found that there are systematic deviations from Duverger''s law related to incentives from ethnolinguistic fragmentation, institutions, and strategic voting. This analysis confirms that many of these results hold at the district level, which is the most appropriate level for testing Duverger''s law. District level party-system fragmentation patterns in 44 countries support Duverger''s basic hypotheses. The effect of electoral institutions is contingent, however, upon the presence of social cleavages that generate pressures for additional parties, the establishment of patterns of party-system competition that help voters evaluate contenders'' viability, and the absence of competing incentives generated by districts of varying magnitudes. These effects are robust to different specifications of social heterogeneity. However, we find no evidence that institutional features like federalism or presidentialism reduce the strategic effects of district level factors. [Copyright &y& Elsevier]