Rules in change? Births, migrations, cohort replacement and homeostasis in world population: 1950-2100

Gianpiero Dalla Zuanna, University of Padua
Francesco C. Billari, Università Bocconi

How do contemporary populations evolve through the replacement of successive cohorts, via the dynamics of the components of demographic change? Can we observe important relationships, such as for instance homeostasis, between how these components evolve? In this paper we focus on birth-cohort size, and on the comparison between the sizes of successive cohorts, in particular in the space of a generation. A very simple approach indeed, but it relates fundamentally to the following key question: will people aged e.g. 30 at time t be “replaced” by a larger, equal or smaller number of people aged 30 at time t+30? We take a global approach by analyzing world- and regional-level cohort replacement as affected by the key components of population dynamics, i.e. fertility, survival and migration for 1950-2010, using UN data for the whole world (UNPD 2011). Moreover, we document that during this period, the relationship between the components of demographic change has followed, at least partially, a homeostatic principle. Finally, we analyze the medium variant of UN World Population Prospects (2010 revision) to assess the potential future of cohort replacement and the presence of a homeostatic logic behind the assumptions embedded in projections.