Population Growth Through Natural Increase, 1775–2000

Source: Population Reference Bureau.

Teachers Guide: Discussion questions

Question and Answer: When could world population stop growing?

Future Growth

Population change affects all our lives in a much more immediate way today than it has throughout most of human history. For the first one-half million years of human existence, the population growth rate was about zero. The population stayed about the same size from year to year. It was not until the 1700s that the modern era of population growth began. Between 1850 and 1900, the annual growth rate reached 0.5 percent. The rate surged to 2.0 percent by the mid-1960s, dropped to 1.7 percent by the mid-1980s, and declined to about 1.2 percent by 2005.

Why has world population grown at such different rates throughout history? Population change results from the interaction of three variables: births, deaths, and migration. This relationship is summarized by a formula known as the balancing equation. The difference between births and deaths in a population produces the natural increase (or decrease) of a population. Net migration is the difference between the number of persons entering a geographic area (immigrants) and those leaving (emigrants). Natural increase usually accounts for the greatest amount of growth in a population, especially within a short period of time. For the world, growth occurs only when there are more births than deaths; for individual countries, migration is also a factor.

The Mortality Revolution

Human population grew rapidly during the Industrial Revolution, not because the birth rate increased, but because the death rate began to fall. This mortality revolution began in the 1700s in Europe and spread to North America by the mid-1800s. Death rates fell as new farming and transportation technology expanded the food supply and lessened the danger of famine. New technologies and increasing industrialization improved public health and living standards. Late in the 19th century, birth rates also began to fall in Europe and North America, slowing the population growth that had resulted from continued moderately higher birth rates than death rates.

Since 1900, both birth and death rates in the more developed countries have continued to fall in tandem, with a few interruptions. A worldwide influenza pandemic in 1918 caused the death of between 20 million and 40 million people and produced a temporary increase in the death rate. A slight increase in birth rates occurred after World Wars I and II. Since the 1950s, birth rates have continued their decline, while death rates declined into the 1960s but have been slowly increasing since. In some European countries, declining birth rates and an increase in death rates are contributing to declining population size. The total fertility rate (TFR) in many more-developed countries is well below replacement levels of two children per couple.

The Demographic Transition

Demographers have attempted to explain the experience of these more developed countries as a demographic transition from high birth rates and death rates to the current low levels. This process tends to occur in three stages. First, birth and death rates are both high, so little growth occurs. Second, death rates fall due to improved living conditions, while birth rates remain high. During this period population grows rapidly. The third stage of the transition is reached when fertility falls and closes the gap between birth and death rates, resulting again in a slower pace of population growth. The figure "Population Growth Through Natural Increase" is a crude representation of this transition. All the more developed countries have entered this third stage of the demographic transition, and some have gone on to a fourth stage in which death rates are higher than birth rates, and the population declines.

Components of Population Change

In contrast to the more developed countries, the less developed countries—in Asia, Africa, and Latin America—had both higher birth and death rates in the 1900s than Europe and North America had in the 1700s, and these higher rates have continued throughout the 20th century. In most less developed countries, the mortality revolution did not begin in earnest until after World War II, and it followed a different pattern than that in European countries. Birth and death rates were higher at the start of the demographic transition than they had been in Europe or North America. Death rates fell rapidly in less developed countries through the introduction of medical and public health technology; antibiotics and immunization reduced deaths from infectious diseases; and insecticides helped control malaria.

In the second stage of the demographic transition of these regions, mortality declines led to continued population growth. With declining mortality and high fertility rates, the population growth of the less developed countries achieved an unparalleled 2.5 percent per year in the late 1960s. Overall, mortality rates in the less developed countries fell much faster than during the demographic transition in the more developed countries. As a result, there was a large gap in the percentage of growth between these two regions. Since 1970, birth rates have fallen in less developed countries, but the death rate has fallen faster. The population growth rate is still high, about 1.4 percent in 2005. While the patterns of fertility decline have varied dramatically throughout the less developed world, many countries are well into the transition process. Even in sub-Saharan Africa, where birth rates remained high through much of the 1980s and 1990s, fertility rates in most countries are declining.

Projections of World Population

lNo one really knows how large the world's population will be in the future. But we can make educated guesses by looking at past and present trends in two of the components of population growth: births and deaths. The third component, migration, can affect the growth of individual countries, but not world population.

The figure "Future of World Population Growth" illustrates three scenarios for population change, depending on levels of fertility. World population is projected to increase to 8 billion by 2025 and to reach 9.2 billion by 2050 according to the medium scenario where fertility reaches 2.0 children per women.

Future of World Population Growth: Three Scenarios, 2000 to 2050

Source: United Nations, World Population Prospects, The 2006 Revision.

Most of the world's population growth is likely to continue to be in less developed countries. Asia will continue to hold the majority of the world's people, and Africa will gain a larger share than it has at present. The population of these regions would almost double by 2050 according to moderate projections. In 2050, close to 90 percent of world population could live in countries currently considered less developed, compared with about 80 percent today.


Birth rate (or crude birth rate): The annual number of births per 1,000 total population.

Death rate (or crude death rate): The annual number of deaths per 1,000 total population.

Demographic transition: The historical shift of birth and death rates from high to low levels in a population. The decline of mortality usually precedes the decline in fertility, thus producing rapid population growth during the transition period.

Emigration: The process of leaving one country to take up permanent or semipermanent residence in another.

Growth rate: The number of persons added to (or subtracted from) a population in a year due to natural increase and net migration; expressed as a percentage of the population at the beginning of the time period.

Immigration: The process of entering one country from another to take up permanent or semipermanent residence.

Less developed countries: Less developed countries include all countries in Africa, Asia (excluding Japan), and Latin America and the Caribbean, and the regions of Melanesia, Micronesia, and Polynesia.

More developed countries: More developed countries include all countries in Europe, North America, Australia, New Zealand, and Japan.

Mortality: Deaths as a component of population change.

Rate of natural increase: The birth rate minus the death rate, implying the annual rate of population growth without regard for migration. Expressed as a percentage.

Total fertility rate (TFR): The average number of children a women would have assuming that current age-specific birth rates remain constant throughout her childbearing years (usually considered to be ages 15 to 49).