The Demographic Transition Theory (DTT) offers a useful framework for explaining why fertility and mortality rates vary so widely across the world. It suggests that as societies modernize, they move through distinct stages of population change.

Stage 1 (High Stationary), typical of pre-industrial societies, had both high birth and death rates. Frequent famines, diseases, and high infant mortality kept populations small and growth nearly stagnant. Few places remain in this stage today.

In Stage 2 (Early Expanding), mortality rates fall sharply thanks to better sanitation, nutrition, and healthcare, while birth rates remain high. This results in rapid population growth. Many Sub-Saharan African countries like Niger and Uganda illustrate this stage, with booming populations as death rates drop faster than fertility.

Stage 3 (Late Expanding) marks a decline in fertility as urbanization, education (especially of women), access to contraception, and shifting values reduce family size preferences. Population growth slows, as seen in countries like India and Brazil.

Stage 4 (Low Stationary) brings both low fertility and mortality, stabilizing population growth at very low rates. Developed nations such as Japan, Germany, and Italy fit this pattern, but they now face challenges of ageing populations and shrinking workforces.

Some demographers even suggest a Stage 5 (Declining), where birth rates fall below death rates, leading to long-term population decline.

By tracing these stages, DTT helps explain why some regions experience youth bulges and rapid growth, while others grapple with ageing populations and potential decline.