Seats and Votes: The Effects and Determinants of Electoral Systems

The book argues that electoral systems can be analyzed through formal, quantitative laws rather than descriptive typologies. Taagepera and Shugart treat votes as the basic currency of political science, comparable to money in economics, and build a cumulative theory linking parties, votes, seats, and the rules specifying their relations . They identify district magnitude as the central factor determining whether an electoral system produces proportional or disproportional outcomes . The approach is explicitly scientific and deductive, in the style of physics, testing general formulas against empirical data across many countries .

The book develops several formal results. The most famous is the Cube Root Law of assembly sizes: the size of a legislature rises with the cube root of the active citizen population, derived from communication theory and tested empirically . Taagepera and Shugart also propose a 'law of conservation of disproportionality' (D), showing that the two Duverger effects are inversely related: as an electoral system becomes fairer (lower D), more parties offer themselves, creating new discrepancies . They also formalize the effective number of parties, building on Laakso and Taagepera's earlier index [citation:2][citation:17].

Taagepera and Shugart argue that district magnitude (M) is 'the central factor' in determining how votes translate into seats . Its effect depends on the allocation rule: in plurality systems, larger M makes the legislature less proportional; in proportional systems, larger M makes it more proportional . M also affects the number of parties, the largest party's seat share, and the overall size structure of the party system [citation:11]. The book's formal models begin with M and assembly size (S) as the basic inputs from which other outcomes can be estimated [citation:11].

A key criticism is that the book treats electoral systems as given rather than examining how they are designed. Taagepera and Shugart acknowledge that electoral systems are usually designed by elites with vested interests, but they do not formalize this process, focusing instead on the effects of systems once established . The book also devotes little attention to how countrywide campaigns affect party electoral fortunes, and it does not explore intraparty effects on electoral results . Critics note that the formal theory, while elegant, may underestimate the role of factors other than electoral rules in shaping party systems.

The formal equations have been tested against individual countries. A study of Japan found that Taagepera's theoretical equation, which predicts seats from votes using precise district magnitude values, 'passes these tests admirably' [citation:18]. The Japanese case was considered a tough test because its single nontransferable vote system was unusual, but the theory fit well . Reviewers noted that Taagepera and Shugart were candid in acknowledging cases where their theory was not consistent with the data, which strengthened the credibility of their approach .

Seats and Votes is the foundational work in a research program that Taagepera and Shugart continued for decades. Their 2017 book, Votes from Seats: Logical Models of Electoral Systems, represents the mature development of the same project, extending the seat product model and incorporating presidential elections, intraparty dimensions, and more complex electoral systems [citation:8]. The earlier book established the basic formal approach and identified district magnitude as the central variable; the later work refines and generalizes the models. Seats and Votes is now recognized as a classic in the field, with reviewers predicting it would become 'a classic' shortly after publication .