Electoral Studies 20 (2001) 509–527 www.elsevier.com/locate/electstud Dynamic approaches to government formation and the generic instability of decisive structures in Japan 1 Michael Laver a,* , Junko Kato b a Department of Political Science, Trinity College, Dublin 2, Ireland b University of Tokyo, Tokyo, Japan Abstract This paper relaxes the “party as unitary actor” assumption that characterises much theoretical work on party competition and government formation. It first sketches some of the theoretical implications of assuming that legislators are free to defect from parties of which they are members, and to join other parties that might be willing to accept them. This leads to a dynamic legislative party system that is quite distinct from the type of party system assumed by most models, which remains essentially static between elections. Working from “office-seeking” assumptions about the motivations of legislators that seem very plausible in the Japanese case, it shows that such a dynamic party system is likely to be unstable, since there are generic gains to be derived from the fission and fusion of parties. The paper then moves on to explore the Japanese case in these terms. The Japanese party system is self-evidently dynamic and characterised by a high degree of flux, with legislative parties regularly splitting and fusing between elections. The evidence suggests that large parties that pass a certain size threshold can be attractors in the dynamic system, both offering benefits to and receiving them from defectors from other parties. In this way, a “near-majority” party can pass the majority thres- hold between elections, as happened with the Japanese LDP.  2001 Elsevier Science Ltd. All rights reserved. Keywords: Japan; Party factions; Party splits; Dynamic models; Bargaining power * Corresponding author. Tel.: +353-1-608-2036. E-mail address: mlaver@tcd.ie (M. Laver). 1 A version of this paper was presented at the fourth meeting of ECPR/JPSA joint research group on Party System Change, Kumamoto, November 1998. 0261-3794/01/$ - see front matter  2001 Elsevier Science Ltd. All rights reserved. PII:S0261-3794(00)00047-0