In the sovereignty of the elected House, in the electoral system, and in the procedure for arriving at decisions in Parliament can be found the rationale of institutionalised political behaviour in New Zealand. Pluralities in single-member constituencies elect representatives. A majority of representatives is necessary for parliamentary decisions. Because Parliament is supreme, governments have to be chosen for their ability to control decision making in Parliament. They must therefore be able to command the support of a majority. In conjunction with electoral requirements and, possibly, social factors, this need has been favourable to the growth of a two-party system. The growth of practices and conventions relating to the composition and responsibility of governments can largely be explained in the same terms.