One of the most noteworthy political regularities in the early twentieth century was the shift away from majoritarian electoral rules in Western Europe. The conventional wisdom suggests that proportional representation (pr ) was introduced by elites who believed that under the existing majoritarian rules (simple plurality, block-vote, two-ballot rules) they would soon lose power to rapidly growing socialist parties. But this does not explain why many electoral reforms were carried out in countries with weak or nonexistent socialist parties. The author shows that increasing the number of parties distorts the seat-vote properties of electoral rules to a larger degree than previously anticipated. Under increasing party competition, electoral regimes display larger partisan biases than those observed in two-party races and crowd out minority parties that have territorially dispersed constituencies in favor of minority parties that have territorially concentrated constituencies. Using a dynamic Bayesian model for seats and votes, the author measures the partisan biases brought about by the expansion of voting rights in the late nineteenth century to explain the drive to reform majoritarian electoral systems.