The Mechanical Effect of Electoral Systems on Disproportionality

Abstract

  This article defines the mechanical effect of electoral systems according to the literature on causal inference. An electoral system is decomposed into six parts: assembly size, district magnitude, malapportionment, geographical vote distribution, interaction between these, and electoral formula. The article proposes a new index of disproportionality, which is derived from the log likelihood ratio of the multinomial distribution. The mechanical effect of electoral systems on disproportionality is illustrated using simulation as well as election data from both Houses of Japanese Diet from 1890 to 2007.