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This article examines party discipline and party cohesion or defection, offering as an illustration the rebellion over postal privatization in 2005 by members of Japan's Liberal Democratic Party (LDP). It explores the importance of party rules – especially the seniority rule and policy specialization for district rewards – as intervening variables between election rules and party defection in a decentralized and diverse party. It is argued that in such cases, party rules like seniority can help prevent defection. When these rules are changed, as in the postal case of 2005, defection is more probable, but it is found that the relationship between defection and seniority is likely to be curvilinear, and also that the curvilinearity is conditional upon each legislator's having different incentives for vote, policy and office.

The diffusion of mixed-member electoral systems over the last three decades has prompted extensive scholarly attention to their consequences. One approach, labeled controlled comparison, emphasizes how the juxtaposition of two different election rules allows scholars to assess the implications of Duverger’s propositions. Another approach, labeled contamination effects, emphasizes how the simultaneous use of two different election rules creates interaction that potentially undermines the implications of Duverger’s propositions. The literature building upon these two approaches is inconclusive; some research finds evidence of contamination effects and other research does not. This chapter strives to reconcile the two approaches and proposes a research agenda that could uncover why scholarship on mixed-member systems has not reached consensus on their consequences.

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