Japan’s Historic Election

On August 30, Japanese voters decisively chose the Democratic Party of Japan (DPJ) over the ruling Liberal Democratic Party (LDP).  The DPJ took 308 of the 480 seats in Japan’s House of Representatives, the more powerful of the two chambers of its legislature. This was a truly historic election as the LDP has been in power since 1955 (except for about 9 months in 1993 and 1994).  In 2009, after more than fifty years in power, voters overwhelmingly rejected the LDP.

Or did they?  The election is a good example of the effects of electoral rules on election outcomes.  Japan uses a mixed member system to elect the House of Representatives: they elect 300 members from single member districts using First-Past-The-Post (FPTP) and the other 180 from multi-member districts using Proportional Representation (PR).  PR allocates seats according to the share of votes the party wins while FPTP awards the seat to the candidate who gets the most votes.  Thus, the difference between the number of seats a party wins in the PR and the FPTP components gives a good idea of how FPTP distorts the margin of victory.  In the election, the DPJ won just under 50% of the PR seats and about 75% of the FPTP seats.

The FPTP system thus makes the DPJ appear far more popular than the popular vote suggests.  While the DPJ won 42% of the popular vote, due to the distorting effect of FPTP, it controls 64% of the seats in the House of Representatives.  At the same time, the LDP lost close to 60% of the seats they controlled (approximately 70% of their single member seats and 30% of their PR seats), while their popular vote share declined by just over 20%.  Consequently, due to the electoral rules, looking at the number of seats each party controls alone exaggerates substantially popular support for the DPJ and rejection of the LDP.

Renard Sexton at FiveThirtyEight.com and Matt Shugart have more analysis.