On the Limits of the Reduction in Population Disparity Between Single-Member Election Districts in Japan

Abstract

Since 1994 the House of Representatives election in Japan has had 300 single-member districts. The maximum population disparity ratio between districts remains 1 to 2.064 in spite of redistricting in 2002. In this paper, we numerically explore the cause of this large population disparity, and show by using a mathematical optimization technique, the disparity ratio can be reduced to only 1 to 1.750 under the present election law. Furthermore, even if the principle of population proportion apportionment is sacrificed, the ratio is reduced only to 1 to 1.722. These results mean that the difference in the weight of one vote in Japan cannot be improved significantly, even if both the apportionment method and the redistricting guidelines are modified.