Slide 29 of 37
Notes:
Knowledge of the existing age pyramid within a country is essential for informed demographic predictions. However the actual population will depend on the mortality rate for each cohort, and on the fertility rate. The mortality rate tends to decrease slowly as medical services improve with economic development, though in some countries it is increasing as a result of AIDS. These trends are relative stable and predictable, except for perturbations induced by war and epidemics. The fertility rate is subject to more rapid and unpredictable changes. In addition migration across national boundaries (legal and illegal) can be substantial, accounting for most of the current population growth in the U.S. From a world perspective, however, it cancels out. Thus long term prediction of population changes is accordingly less than an exact science.