On face value, it might be expected that a period of cohabitation would give couples unique insight into their compatibility, thereby helping them to make informed decisions about whether or not to marry. However, most of the research into this issue in Australia and other western countries suggest quite the opposite trend: indirect marriages tend to have a shorter life than direct marriages. Various explanations have been proposed for this phenomenon. It is the purpose of this paper to present a theoretical framework, based on systems theory, for organising the literature on this issue and on relationship formation and stability in general.