Partisan bias in Japan's single member districts

The delineation of constituency boundaries and variations in vote distribution across districts often favor certain parties at the expense of others. Applying a hitherto under-utilized formula (Brookes, 1959; Johnston et al., 1999), this study investigates whether the mechanism translating votes into seats in Japan's single-member districts results in systematic partisan advantage that may influence election outcomes. Simulations are conducted for the 2003 and 2005 general elections under two scenarios: where the governing coalition and the main opposition party receive equal vote shares, and where their vote shares are reversed from the actual results. Components of electoral bias are then disaggregated into size and distribution effects, and the impact of malapportionment, electorate size, turnout, and the role of third party/ independent candidates on overall electoral bias is examined. Results show that while partisan bias exists, disadvantages toward one party in some components are likely to cancel out benefits derived from others, producing a relatively small net effect. Furthermore, electoral bias in Japan is found to award sectoral rather than partisan seat bonuses.