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 and 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).
Increasingly, migration has made the progression of population ageing much more difficult to study, as the majority of those who move are at young adult/ reproductive age, and their leaving and arriving affects the age structures of the populations from which they left and to which they arrive (Bedford & Pool 2001; World Bank 2009; Dyson 2011). By and large, leavers of youthful ages cause the population to age; arrivals cause it to become younger. But there is also the growing trend of older people moving, typically for retirement and/or to be close to family, services and facilities (Champion 1992; Rogers & Raymer 2003). Older people moving out of an area slow its structural ageing, older people moving in accelerate it. Similarly, the movement of families affects the age structure across the childhood and reproductive ages – leavers cause populations to age, arrivals cause them to become younger. The question, then, is to what extent overall migration is accelerating or ameliorating structural ageing, and with it hastening or slowing the theorized shift to the ending of population growth; and, does the situation unfold differently at national and subnational level? We examine these questions in this Supplementary Issue of Policy Quarterly, with a focus on subnational New Zealand.
This article also introduces and sets the context for each of the seven articles within this Supplementary Issue of Policy Quarterly, and considers these article's demographic and policy significance and relevance to New Zealand today.