Dynamics of Interpersonal Political Environment and Party Identification: Longitudinal Studies of Voting in Japan and New Zealand Ken’ichi Ikeda The University of Tokyo James H. Liu Victoria University of Wellington Masahiko Aida University of Michigan Marc Wilson Victoria University of Wellington The dynamical systems theory of groups claims that interpersonal political environment and party identification are dynamically interrelated to provide heuristics under uncer- tainty. Panel data over the course of a year examined the longitudinal dynamics between social networks, social identifications, and voting behavior among a national sample of registered voters in Japan and a regional sample in Wellington, New Zealand. Respon- dents with more stable party identification had greater stability in the political preferences of their interpersonal network in both countries; moreover, stability in party identification was predicted by interpersonal political environment and older age in both countries. Sta- bility of party identification predicted voting consistency in both countries, whereas sta- bility of interpersonal political environment made an independent contribution to voting consistency in Japan only. There were cultural differences in levels of interpersonal polit- ical environment stability, but the amount of political discussion and ideological stability did not make independent contributions to any of the three main variables. Results pro- vided support for the dynamical systems theory of groups. KEY WORDS: dynamics of voting, interpersonal environment, party identification, Japan, New Zealand Political Psychology, Vol. 26, No. 4, 2005 0162-895X © 2005 International Society of Political Psychology Published by Blackwell Publishing. Inc., 350 Main Street, Malden, MA 02148, USA, 9600 Garsington Road, Oxford, OX4 2DQ 517