Since its original formulation in the 1940s (Davis 1945; Notestein 1945), the phenomenon known as 'the global demographic transition' has been used to understand the trend of structural population ageing, and with it, the slowing and ultimately the ending of population growth – now anticipated globally around the end of the present century (Lutz, Sanderson & Sherbov 2004). However as originally conceptualised, the theory pertained to 'closed' populations, in which the only dynamics were births

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... nd deaths. Falling death rates cause populations to first become younger and to grow in size, while falling birth rates eventually cause them to become older, and growth to slow – the increased numbers of survivors at older ages becoming an increased proportion of the population (Coale 1972; Chesnais 1990).