抄録
Democracy is an ideal political system that is completely or almost completely responsive to the preferences of all its citizens, considered as political equals (Dahl 1971). For a modern large-scale democratic political system to approximate this ideal, its electoral system must ensure that all full citizens have equal opportunities to participate and have equal influence over electoral outcomes.
In the election of members of the House of Representatives, legislative seats are apportioned to jurisdictions—such as states or prefectures—in proportion to their population. Then, districts are drawn within these jurisdictions. One of the best approaches to create legislative districts is: (1) to use a true proportional representation formula for apportionment; and (2) to design legislative districts with, as nearly as practicable, equal populations to ensure equal representation within each jurisdiction.
The United States, the United Kingdom, and Canada adopt single-member constituency systems for lower house elections. Each country employs the above approach while considering local circumstances. Thus, it can be said that this has become the global standard. In contrast, Japan has not followed this method—either in the single-member constituency system established for the first House of Representatives in 1889, or in the current single-member constituency—proportional representation system introduced in 1994. As a result, Japan falls short of fully implementing “one man, one vote” principle.
Under the 1994 system, one seat is allocated to each prefecture, and the remaining seats are apportioned proportionally among prefectures. This method has overrepresented less populous prefectures relative to more populous ones. Furthermore, the redistricting criterion specifies that the nationwide ratio of the largest to the smallest constituency population should not substantially exceed 2:1. Consequently, the vote of the most overrepresented constituent is worth more than twice that of the most underrepresented constituent.
In its 2011 decision, the Supreme Court ruled that this apportionment method was a major factor contributing to constituency population disparity and deemed it unconstitutional. In response, the Diet enacted a revised law mandating the use of the Adams method for reapportionment and setting a nationwide constituency population ratio ceiling of 2:1. However, even minor population shifts have since resulted in disparities exceeding this threshold.
This paper proposes reforms to Japan's constituency redistricting method. The apportionment of seats to prefectures using the Adams method has achieved a population-per-seat ratio of less than 2:1 at the prefectural level. However, under the current redistricting criterion—based solely on a nationwide constituency population ratio of less than 2:1—unacceptably wide disparities among constituencies would still be permitted.
Therefore:
1. The current redistricting criterion should be replaced with a new standard requiring that each constituency's population fall within ±10% of the average population per seat in its prefecture;
2. As this standard is likely to keep the nationwide constituency population ratio below 2:1, mid-decade redistricting will no longer be necessary; and
3. Constituency boundaries can therefore remain unchanged for the full 10-year period.
