Assembly Size and Electoral Distortion in an SMP System

There is a vast literature in political science concerning the strengths and weaknesses of single member plurality (SMP) electoral systems. Some argue that PR systems are superior because they ensure better representativeness by reducing the distortion between votes received by a party and its seat share. Others say that the benefits of SMP in terms of accountability make the price of electoral distortion bearable. But what if there would be incremental institutional changes that could maintain the benefits derived from SMP elections and still reduce the distortion it causes? In this paper, we make use of an innovative research design to measure the impact of assembly size on seat disproportionality as measured by the Gallagher Index. We make use of Canada as an ideal case. In this country, federal and provincial elections occur at regular intervals, and the numbers of seats at play vary substantially between levels of government within a province. We find that increasing assembly size is associated with reduced disproportionality in a negative logarithmic fashion, making it an especially useful institutional tool to reduce distortion in smaller assemblies. We argue this research brings a new light on an ongoing debate about SMP systems.