Commentary
About the data
Definitions
Data sources
Over the next several decades the populations of low- and middle-income countries will continue to grow. The rates of growth will decline, but the absolute increases will be large—and accompanied by substantial shifts in the age structure. Even when fertility reaches the replacement level of about two children per couple, the number of births will remain high—and population growth will not stop for several decades. This phenomenon, called "population momentum," is a facet of the youthful age structures typical of populations of developing countries. It occurs because large cohorts born in previous years move through the reproductive ages, generating more births than are offset by deaths in the smaller, older cohorts. Here population momentum is measured as the ratio of the population when zero growth has been achieved to the population in 1995, assuming that fertility is at replacement level in 1995 and remains at that level.
Because of population momentum, the full effect of lower fertility on the growth and age structure of a population takes several decades to be felt. As the population pyramids for low-income countries show, before the smaller birth cohorts born recently make their way through the age structures, the larger birth cohorts from the past will mean large increases in the number of women of reproductive age (figure 2.2a). In high-income countries, where fertility rates are well below replacement level and age structures are older, population will increase much less.
A longer-term effect of momentum is the large increase in absolute population size projected for developing regions during the next century. In China, where replacement fertility was reached around 1990, the population is expected to grow by another 400 million people before stabilizing. In India the combination of above-replacement fertility and momentum is projected to double its current population, which will surpass China's in 50 years.
In low- and middle-income economies slightly more than 85 percent of the projected increase between 1995 and 2035 is from population momentum and mortality decline—and the rest, from fertility above replacement level (figure 2.2b). Regional disparities among developing countries remain. Slightly less than half the increase in population growth in Sub-Saharan Africa is from high fertility (figure 2.2c), compared with only 7 percent in Asia (figure 2.2d).
In Europe and Central Asia fertility is below replacement level. In that region the increase in population results from the dynamics of the current age structure, which produces more births than deaths (figure 2.2g).
About the data
Population dynamics indicators, or vital rates, are based on data derived from registration systems, censuses, and sample surveys conducted by national statistical offices. As with the basic demographic data in table 2.1, international comparisons are limited by differences in definitions, data collection, and estimation methods.
Registration systems in many developing countries in Africa, Asia, and Latin America are incomplete because of deficiencies in geographic coverage, in coverage of population groups, or both. For these countries vital rates are estimated by applying various estimation techniques to incomplete vital registration data or to data from demographic surveys.
The crude death, crude birth, and total fertility rates for 1995 are often based on projections from censuses or surveys from earlier years (see Primary data documentation for the most recent census or survey year and registration completion status). Contraceptive prevalence rates are obtained mainly from demographic and health surveys and contraceptive prevalence surveys (see Primary data documentation for the most recent survey year).
• Crude death rate and crude birth rate indicate the number of deaths and the number of live births occurring during the year, per 1,000 midyear population. The difference between the crude death and birth rates is the rate of natural increase.
• Total fertility rate represents the number of children that would be born to a woman if she were to live to the end of her childbearing years and bear children in accordance with prevailing age-specific fertility rates.
• Contraceptive prevalence rate is the percentage of women who are practicing, or whose sexual partners are practicing, any form of contraception and is usually measured for women aged 15-49.
• Population momentum is measured as the ratio of the population when zero growth has been achieved to the population in year t, given the assumption that fertility remains at replacement level from year t onward.
Vital rates estimates are produced by the World Bank's Human Development and International Economics Departments in consultation with World Bank country departments. Important inputs come from the following sources:
Population censuses. Eurostat, Demographic Statistics. United Nations Department of Economic and Social Information and Policy Analysis, World Population Prospects: The 1996 Edition and Population and Vital Statistics Report. Demographic and health surveys from national sources.