The history of the ancient family has focused principally on two central questions: how can we classify the structure and formation of the ancient family within a historical framework, and what were the obligations of its members towards one another within that structure? For the first of these questions the issue is one of continuity, or otherwise, from family forms in pre-modern populations through to modern, western European populations; this is an issue of precisely when and where what we now refer to as the nuclear family came into existence. The appearance of the nuclear family in European history is tied to the theory of a European demographic transition: a widespread fertility decline associated with industrialisation, which was in large part a consequence of the tendencies of married couples to delay marriage and/or to consciously limit their family size through means of birth control. This theory, though widely criticised for its inability to explain the socio-economic and cultural aspects of human reproductive behaviour, is one that remains central to many studies of family formation. Yet research has suggested that a number of historical populations for which demographic data are abundant deviated significantly from the patterns this theory predicts.