The demographic transition model describes the shift from high birth and death rates to low birth and death rates that accompanies economic development. First outlined in 1945 by demographer Frank Notestein, it remains the central framework for understanding population change.

Stage 1 — pre-industrial: both birth and death rates are high and roughly balanced, so population grows slowly. This described all of human history until roughly the 18th century.

Stage 2 — early industrial: death rates fall sharply due to better food, sanitation, and medicine, but birth rates stay high. The gap causes rapid population growth. Much of sub-Saharan Africa sits here today.

Stage 3 — late industrial: birth rates begin to fall as urbanization, female education, and contraception spread. Growth slows but remains positive. India, Egypt, and much of South America are in this stage.

Stage 4 — post-industrial: both rates are low and close together, so growth is near zero. Most of Europe, Japan, and North America are here. Some countries have entered a "Stage 5" where births fall below deaths and the population shrinks — Japan, Italy, and South Korea among them.