2.3.1 Trajectory and Growth of the Population in the Context of the Demographic Transition

Facts/messages: Populations are in constant change as people enter (by birth or in-migration) and exit (by death or out-migration). The number of persons in a specific place can decrease, remain constant, or increase as a consequence of the conjunction of these processes. The study of population dynamics allows an understanding of the relationships among these processes and an assessment of their levels and impact on the characteristics of the population. Even slight changes in fertility and mortality trends can produce large changes in the size of the population. The processes inherent to the demographic transition can have the greatest impact on population growth.

Fertility is defined as a population’s reproductive performance, generally referring to the average number of births or children per woman. Four factors account for the most of the differences across fertility levels: nuptiality, contraceptive use, induced abortion, and duration of breastfeeding. Since the 1970s fertility levels fell worldwide at unprecedented rates and to unprecedented levels. For the first time, since the beginning of the new millennium, the United Nations Population Division projected that future fertility levels in most developing countries will probably fall below 2.1 children per woman the level needed to ensure the long-term replacement of the population in low mortality populations, at some point in the 21st century. The stage in which below-replacement fertility is attained and maintained, considered as the “second demographic transition”, is associated with several other important changes in family-building and social behaviour. Transition to very low fertility levels has major implications, not only for the overall population size, which is expected to decrease, but with respect to population ageing, which accelerates the longer fertility remains at very low levels. Globally, according to current projections, the number of older persons (60 years or over) will nearly quadruple by 2050.

In most countries, mortality is the main exit component of demographic change. There are important mortality differences across age, sex, social classes, cultural groups, countries and regions observed not only in overall mortality, but also in its composition by causes of death.

Spatial or geographic mobility refers to the quantitative aspects of movements made by individuals in geographic space. The study of migration focuses on a change in usual place of residence across an administrative boundary. The reasons underlying these movements can be economic, educational, political, social and/or recreational. However, not all population movements between geographical areas can be defined as migration.

Methodology: Show the advances of the national demographic transition and compare it with other countries in the region, showing their particular features, disparities by population strata (poor and non-poor) and territorial units, as well as their links with structural economic and social changes. Display population trends in absolute terms, population multiplier, and growth rates. The trajectories may be compared with past population projections or with scenarios of constant fertility and mortality starting from 1950 or 1975. A useful instrument for showing the impacts of the various components of population growth (fertility, divided into desired and undesired, mortality, and migration, plus demographic inertia) on the country’s population in 2050 is the population decomposition model recently developed by John Bongaarts of the Population Council, for UNFPA.

Primary Sources:

  • National censuses;
  • National estimates and projections from Central Statistical Offices (CSOs). Be aware of the fact that censuses usually contain a certain percentage of undercount and that earlier population estimates often have to be readjusted in the light of more recent censuses. In some countries in Eastern Europe and Asia, it is possible to estimate population trends based on population registers.

Secondary Sources:

30  Bongaarts, John (1978). “A framework for analyzing the proximate determinants of fertility”. Population and Development Review 4 (1).
31  United Nations Population Division (2010). World Fertility Report 2007.