Seat enlargements in mixed-member proportional systems: evidence from the German Länder

Abstract

The German mixed-member proportional (MMP) system is considered a role model worldwide. Nevertheless, it has a neglected side-effect: it may produce greatly enlarged parliaments, like the 2017 Bundestag with 111 additional seats. Thus, it is highly relevant to know under which conditions MMP systems lead to such seat enlargements. The article explores this question for the German Länder that have used various MMP versions and seen occasional parliamentary inflations. The analysis demonstrates that a two-stage model of party-system features and institutional factors explain enlargements under MMP systems in 156 Länder elections from 1947 to 2019. Concerning the party system features, enlargements are driven by high seat concentrations in single-member districts and low list-vote shares of the largest party. Institutionally, high ratios of SMD seats and full levelling of surplus seats affect parliamentary oversize. These results have important implications for MMP systems in Germany and other countries.