Empirical proposition and institutional tendency that single‑member district plurality (first‑past‑the‑post) electoral rules favor the emergence of two dominant political parties, while proportional representation and larger district magnitudes tend to sustain multiparty systems; framed as a tendency conditioned by mechanical and psychological effects rather than a deterministic universal law.

Principle

Two mechanisms produce the tendency: the mechanical effect (how votes are translated into seats under a rule) and the psychological effect (voter and elite strategic behavior, such as strategic voting and party entry/exit) that together make small parties less viable under plurality single‑member rules.

Demonstration

Situation: In a plurality single‑member district, three parties compete. Recognition: Voters recognize that supporting a small third party risks wasting their vote. Action: Some voters shift to one of the two larger parties; potential candidates avoid running third‑party campaigns. Consequence: Over successive elections, electoral incentives consolidate support around two major parties.

Misapplication

Treating Duverger’s law as a deterministic prediction that every plurality system will produce exactly two parties irrespective of social cleavages, regional concentrations, thresholds, or institutional variations. The error ignores conditional factors that produce exceptions.

Consequence

Electoral rule design shapes strategic incentives for voters and politicians and thereby influences party system size, incentives to form coalitions, and the nature of political competition; rule changes can shift these incentives and alter the party landscape over time.

Reversal

Exceptions arise when social fragmentation is regionally concentrated (allowing regional parties to survive), when electoral rules include runoff elections or effective thresholds that modify mechanical effects, or when strong non‑electoral organizations sustain smaller parties despite plurality rules.

Boundary

Clearly within: single‑member plurality rules at district level with national aggregation where party entry and vote‑to‑seat conversion produce mechanical and psychological pressures. Boundary case: mixed‑member or majoritarian systems with compensatory elements where Duvergerian pressures are attenuated. Clearly outside: pure proportional representation with high district magnitudes that structurally permit viable small parties.

Semantic Tension

Stability ↔ Pluralism: the tendency toward fewer parties under plurality rules increases governability and simplicity but reduces representational pluralism, producing a tradeoff that designers of electoral institutions must balance.

Synthesis

Duverger’s Law links electoral formula to party system through vote‑to‑seat mechanics and strategic behavior: it is a conditional generalization about tendencies, useful for predicting incentives rather than providing deterministic counts of parties.