The Cube Rule (sometimes called the "cube law") is an empirical generalization in electoral studies stating that in a two-party, single-member-district plurality system, the ratio of seats won by the two parties tends to equal the cube of the ratio of votes they receive. Formally, if parties A and B win votes in the ratio V_A : V_B, their seats will approximate the ratio V_A³ : V_B³.
The rule was first noted by British observers in the early 20th century and given systematic expression by M.G. Kendall and A. Stuart in their 1950 article "The Law of the Cubic Proportion in Election Results" (British Journal of Sociology). It was applied most famously to UK general elections and to US House elections, where it appeared to fit results reasonably well for much of the mid-20th century.
The practical implication is a strong seat bonus for the winning party: a modest lead in votes translates into a much larger lead in seats. For example, a 55:45 vote split implies roughly a 65:35 seat split under the rule. This amplification helps explain why first-past-the-post (FPTP) systems often produce single-party majority governments even when the popular vote is close.
The cube rule is descriptive, not a law of nature. Its fit depends on the geographic distribution of partisan support, the number of competitive districts, and the presence of third parties. As UK and US politics fragmented in the late 20th century — with the rise of the Liberal Democrats, regional parties like the SNP, and increased partisan sorting in the US — empirical fits weakened. Political scientists such as Rein Taagepera generalized it into the "seats-votes equation" or generalized cube rule, where the exponent varies with district magnitude and assembly size rather than being fixed at 3.
The rule remains a useful heuristic for understanding disproportionality in plurality systems and is frequently invoked in debates about electoral reform, gerrymandering, and the fairness of FPTP versus proportional representation.
Example
In the 1950 UK general election, observers noted that Labour and Conservative seat shares roughly tracked the cube of their vote shares, prompting Kendall and Stuart's formalization of the rule that year.