Demography is the study of a human population, a definable group of people, and of additions to and subtractions from its number. A population is increased by births and immigration and decreased by deaths and emigration. In a "closed population," there is no migration and attention is paid only to reproduction and mortality.

demography, statistical study of human populations, especially with reference to size and density, distribution, and vital statistics (births, marriages, deaths, etc.). Contemporary demographic concerns include the “population explosion,” the interplay between population and economic development, the effects of birth control, urban.

Afghanistan is currently in this stage. Stage 3 occurs once a society is thoroughly industrialized; birth rates decline, while life expectancy continues to increase. Death rates continue to decrease. Mexico’s population is at this stage. In the final phase, Stage 4, we see the postindustrial era of a society.