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.