The dynamical systems theory of groups claims that interpersonal political environment and party identification are dynamically interrelated to provide heuristics under uncertainty. 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. Respondents 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. Stability of party identification predicted voting consistency in both countries, whereas stability of interpersonal political environment made an independent contribution to voting consistency in Japan only. There were cultural differences in levels of interpersonal political environment stability, but the amount of political discussion and ideological stability did not make independent contributions to any of the three main variables. Results provided support for the dynamical systems theory of groups.