Balancing district and party seats: The arithmetic of mixed-member proportional electoral systems

Mixed-member proportional electoral systems (MMP) are widely praised because they combine the direct, personal election of MPs from single-seat constituencies with a proportional seat allocation. However, the size of the proportional tier matters for the question of whether the system's overall proportionality is preserved. Hence, a key challenge for constitution-makers and scholars is finding the right balance between district and proportional seats, so as to maximise district representation and guarantee proportional representation.This paper develops the first theoretical model that helps to locate this sweet spot for district and party seats. The novel solution builds on Taagepera's “logical models” about party sizes. The model is tested on 58 national parliamentary elections under MMP rules.