Why Is the World's Population Changing?

In November 2022, a baby was born somewhere in the world and pushed the global population past eight billion people. That is eight thousand million human beings — more than at any point in history. Yet in some countries, such as Japan and Germany, the population is actually shrinking. How can the world population be growing so fast while individual countries are losing people? That puzzle is exactly what this lesson is about.

A population changes because of three processes: births, deaths, and migration. This lesson focuses on the natural side — births and deaths. The birth rate is the number of live births per 1,000 people in a country each year. The death rate is the number of deaths per 1,000 people per year. When you subtract the death rate from the birth rate, you get the natural increase (or natural decrease if the number is negative). A positive natural increase means the population grows; a negative one means it shrinks — even without anyone moving in or out. Why do birth rates differ between countries? In many low-income countries, families tend to be larger because children help with farming and family income, access to family planning is limited, and women have fewer opportunities outside the home. In high-income countries, birth rates are generally lower because people choose to have fewer children, the cost of raising children is high, and women participate more widely in education and careers. Death rates have fallen dramatically worldwide over the last century, mainly because of better medicine, vaccines, clean water, and improved food supply. When death rates fall faster than birth rates, the population grows quickly. This is called a population explosion. Demographers — scientists who study human populations — use the Demographic Transition Model (DTM) to describe how countries move through stages of population change. In the early stages, both birth and death rates are high so the population stays small. As a country develops, death rates fall first, causing rapid growth. Later, birth rates also fall, slowing growth. In the final stage, birth and death rates are both low; some countries even reach natural decrease. Nigeria, for example, has a birth rate of about 35 per 1,000 and a death rate of about 9 per 1,000, giving a natural increase of roughly 26 per 1,000 — one of the fastest growing populations on Earth. Japan, by contrast, has a birth rate of about 7 per 1,000 and a death rate of about 12 per 1,000, meaning a natural decrease of −5 per 1,000. Its population is shrinking.