There is a demographic transition theory to understand the multiple population changes worldwide, that outlines changes in mortality, fertility and growth rates according to each stage or shifts in the population over time.
There are four stages in the demographic transition model:
Stage 1: Pre-transition; Stage 2: Early transition; Stage 3: Late transition; Stage 4: Post-transition;
and during the 2 & 3 transition stages, one can identify two big breaks:
- The 1st big break is the sudden drop in the death rate, which is caused by technological innovation; for example, the development of X-rays, Penicillin, and Insecticides.
- And the 2nd big break is the sudden drop in the birth rate, caused by an improvement in education, and the change in social customs, for example, when people decide to have fewer children by themselves .