Electoral System Analysis

The mechanical effect is the direct arithmetic distortion the seat-allocation rule imposes on a fixed distribution of votes: in single-member plurality, for example, small parties win far fewer seats than their vote share, regardless of anyone's behavior. The psychological effect is the behavioral response that anticipates this: voters, donors, and elites, foreseeing that supporting a non-viable party is wasted, defect to viable contenders, shrinking the field before votes are counted. The mechanical effect operates on given votes; the psychological effect changes the votes themselves, and the two reinforce each other.

Duverger's law is the proposition that the single-member plurality (first-past-the-post) electoral rule tends to produce a two-party system, while the related hypothesis holds that proportional representation favors multipartism. It is best understood as a strong tendency rather than a deterministic law: it identifies a real and robust mechanism through the mechanical and psychological effects, but admits exceptions — durable third or regional parties can persist under plurality when support is geographically concentrated — so it constrains rather than dictates the party-system outcome.

District magnitude — the number of seats elected per district — sets how restrictive the system is, as Taagepera and Shugart emphasize. When magnitude is one, only the single largest party in a district wins, producing severe disproportionality and squeezing out smaller parties; as magnitude rises, more parties can win seats, disproportionality falls, and multipartism becomes viable. Because it largely determines the effective threshold a party must clear, district magnitude organizes the relationship between votes and seats more powerfully than the choice of formula alone.