In 1996 New Zealand changed its electoral system from single-member-district plurality to a form of proportional representation known as MMP. One of the most persistent theories in political science posits that plurality systems produce two dominant parties, which in turn leads to stable single party government. By contrast, it has been hypothesised that systems of PR multiply the number of parties, thereby encouraging governments which are less stable because more than one party is required to share in government. This thesis uses New Zealand as a test case for this