Abundant research provides evidence that electoral systems have an impact on party system fragmentation. Taking up these findings, and adopting a dynamic approach, this article explores the effect of electoral refoms on electoral disproportionality. Specifically, it demonstrates that permissive changes in the electoral system improve the overall correspondence between vote-shares and seat-shares of parties. The explanation is that underrepresented parties in the parliament obtain more seats the more inclusive the electoral rules become. Likewise, disproportionality is higher after a restrictive electoral reform. The article employs my own data on electoral reforms from 59 established and new democracies between 1945 and 2010. Evidence is found not only that electoral reform has an effect on electoral disproportionality as measured with the Gallagher’s least squares index, but also that this impact is in turn conditioned by the size of the change in the rules and the level of democratic experience.