The rule
Votes of 55:45 yield seats of about 65:35. Kendall and Stuart (1950) tested it on British elections; it fits Duverger's (1954) plurality-two-party link.
Generalisation
Taagepera (1973) tied the exponent to electorate and district numbers; Tufte (1973) estimated swing ratios; Taagepera and Shugart (1989) synthesised. Under PR the exponent nears 1.
Critiques
Geographic concentration and third parties break it. King and Browning (1987) separated bias from responsiveness; Gelman and King (1994) offered probabilistic tools for electoral design.
FAQ
- What is it in simple terms?
- Under first-past-the-post, a small vote lead becomes a large seat lead.
- Does it always hold?
- No; it is a rough tendency that fails with regional or third-party voting.
Sources reviewed: 2026-10-26
References
- Kendall, M. G., & Stuart, A. (1950). The law of the cubic proportion in election results. British Journal of Sociology, 1(3), 183–196. The classic statistical examination of the cube law using British elections.
- Duverger, M. (1954). Political parties: Their organization and activity in the modern state. Methuen. The classic on plurality rule and two-party systems, giving institutional context.
- Taagepera, R. (1973). Seats and votes: A generalization of the cube law of elections. Social Science Research, 2(3), 257–275. Generalises the exponent using electorate and district numbers.
- Tufte, E. R. (1973). The relationship between seats and votes in two-party systems. American Political Science Review, 67(2), 540–554. Estimates seat-vote swing ratios by regression.
- King, G., & Browning, R. X. (1987). Democratic representation and partisan bias in congressional elections. American Political Science Review, 81(4), 1251–1273. Separates responsiveness from partisan bias in US House elections.
- Taagepera, R., & Shugart, M. S. (1989). Seats and votes: The effects and determinants of electoral systems. Yale University Press. A standard text on the effects of electoral systems.
- Gelman, A., & King, G. (1994). A unified method of evaluating electoral systems and redistricting plans. American Journal of Political Science, 38(2), 514–554. A probabilistic method for evaluating seat-vote curves and redistricting.