Introduction

Demography is the study of populations, with populations commonly defined as: persons living within a geographic boundary who tend to share a common language and culture. Traditionally, demographers have focused on the three sources of change – the ‘population dynamics’ – that influence the size and growth or decline of populations. Change in the components of population dynamics, at least as observed in human populations, inevitably lead to population ageing and have tremendous consequences for the lifecourse of individuals. In this chapter, we describe population dynamics and explain how they lead to population ageing, illustrating the complex and varying means by which populations age. We then discuss the consequences for the individual lifecourse, and we offer suggestions for incorporating demographic reasoning into micro-level lifecourse research.

Population dynamics

Population size is influenced by three factors: births into the population, deaths out of the population, and net migration, where net migration is the sum of migration into the population (immigration) and migration out of the population (emigration). The fundamental ‘balancing equation’ in demography demonstrates how population change occurs:

where Pt is the population size at time t, P﹛t–k﹜ is the population size at time t – k, B﹛t–k,t﹜ is the number of births in the population between t-k and t, D﹛t-k,t﹜ is the number of deaths in the population between t – k and t, and NM﹛t–k,t﹜ is the net migration into the population between t – k and t. If Pt > P﹛t–k﹜, then the population is growing over time; if Pt < P﹛t–k﹜, then the population is declining; if Pt = P﹛t–k﹜, then the population is stable (Preston et al, 2000).

Historically, most populations in the world have grown over time because births have tended to outpace deaths, and this is especially so in societies prior to the 20th century. Eventually, as countries become more developed, via agricultural and industrial revolutions, most countries experience a demographic transition (Thompson, 1929). A demographic transition is marked by declining death rates, followed by declining fertility rates, inevitably leading to both rapid population growth and a change in the population's age structure.