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    Human population growth and the demographic transition

    The world and most regions and countries are experiencing unprecedentedly rapid demographic change. The most obvious example of this change is the huge expansion of human numbers: four billion have been added since 1950. Projections for the next half ...

    Philos Trans R Soc Lond B Biol Sci. 2009 Oct 27; 364(1532): 2985–2990.

    doi: 10.1098/rstb.2009.0137

    PMCID: PMC2781829 PMID: 19770150

    Human population growth and the demographic transition

    John Bongaarts*

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    This article has been cited by other articles in PMC.

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    The world and most regions and countries are experiencing unprecedentedly rapid demographic change. The most obvious example of this change is the huge expansion of human numbers: four billion have been added since 1950. Projections for the next half century expect a highly divergent world, with stagnation or potential decline in parts of the developed world and continued rapid growth in the least developed regions. Other demographic processes are also undergoing extraordinary change: women's fertility has dropped rapidly and life expectancy has risen to new highs. Past trends in fertility and mortality have led to very young populations in high fertility countries in the developing world and to increasingly older populations in the developed world. Contemporary societies are now at very different stages of their demographic transitions. This paper summarizes key trends in population size, fertility and mortality, and age structures during these transitions. The focus is on the century from 1950 to 2050, which covers the period of most rapid global demographic transformation.

    Keywords: population growth, demographic transition, fertility, mortality, age structure

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    After centuries of very slow and uneven growth, the world population reached one billion in 1800. The modern expansion of human numbers started then, rising at a slow but more steady pace over the next 150 years to 2.5 billion in 1950. During the second half of the twentieth century, however, growth rates accelerated to historically unprecedented levels. As a result, world population more than doubled to 6.5 billion in 2005 (United Nations 1962, 1973, 2007). This population expansion is expected to continue for several more decades before peaking near 10 billion later in the twenty-first century. Around 2070, the world's population will be 10 times larger than in 1800.

    The recent period of very rapid demographic change in most countries around the world is characteristic of the central phases of a secular process called the demographic transition. Over the course of this transition, declines in birth rates followed by declines in death rates bring about an era of rapid population growth. This transition usually accompanies the development process that transforms an agricultural society into an industrial one. Before the transition's onset, population growth (which equals the difference between the birth and death rate in the absence of migration) is near zero as high death rates more or less offset the high birth rates typical of agrarian societies before the industrial revolution. Population growth is again near zero after the completion of the transition as birth and death rates both reach low levels in the most developed societies. During the intervening transition period, rapid demographic change occurs, characterized by two distinct phases. During the first phase, the population growth rate rises as the death rate declines while the birth rate remains high. In the second phase, the growth rate declines (but remains positive) due to a decline in the birth rate. The entire transition typically takes more than a century to complete and ends with a much larger population size.

    The plot of world population size over time in figure 1 (top solid line) shows the typical S-shaped pattern of estimated and projected population size over the course of the transition. Population growth accelerated for most of the twentieth century reaching the transition's midpoint in the 1980s and has recently begun to decelerate slightly. Today, we are still on the steepest part of this growth curve with additions to world population exceeding 75 million per year between 1971 and 2016.

    Figure 1.

    Population size estimates, 1900–2005 and projections 2005–2050. High, medium and low variants.

    Contemporary societies are at very different stages of their demographic transitions. Key trends in population size, fertility and mortality during these transitions are summarized below. The focus is on the century from 1950 to 2050, covering the period of most rapid global demographic change. The main source of data is the United Nation's 2006 world population assessment, which provides estimates for 1950–2005 and projections from 2005 to 2050 (United Nations 2007).

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    The projected rise in world population to 9.2 billion in 2050 represents an increase of 2.7 billion over the 2005 population of 6.5 billion. Nearly all of this future growth will occur in the ‘South’—i.e. Africa, Asia (excluding Japan, Australia and New Zealand), and Latin America—where population size is projected to increase from 5.3 to 7.9 billion between 2005 and 2050 (table 1). In contrast, in the ‘North’ (Europe, Northern America, Japan and Australia/New Zealand), population size is forecast to remain virtually stable, growing slightly from 1.22 to 1.25 billion between 2005 and 2050. The difference in trends between these two world regions reflects the later stage of the transition in the North compared with the South.

    Source : www.ncbi.nlm.nih.gov

    Lesson Plans on Human Population and Demographic Studies

    Lesson plans for questions about demography and population. Teacher’s guides with discussion questions and web resources included.

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    Lesson Plans on Human Population and Demographic Studies



    Has the world’s population distribution changed much over time? When could world population stop growing?

    Find out the answers to these questions and more.

    The sections listed below explore eight elements of population dynamics. Charts and graphs supplement each topic with one full-sized chart (in PDF) suitable for class distribution. Along with each topic are a frequently asked question and glossary terms. Teacher’s guides with discussion questions and web resources are also included in each section. For further investigation see also the most recent World Population Data Sheet.

    Grade level: middle to high school

    Time required: one week

    Subjects: social studies, geography, and world history



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

    Teachers Guide: Discussion Questions

    What percentage of the population of the Democratic Republic of Congo, the United States, and Germany are 0–4 years old?

    Which of the three countries has the greatest proportion of people ages 65 and older?


    How can the age-sex structure of a population help determine the needs of that population?

    What does it mean to have a “young” or “old” population?

    How can migration affect the shape of a pyramid?

    What is “zero population growth”? Which pyramid represents this concept?


    The dependency ratio is a measure used to indicate the ratio of people in the “dependent” ages (under 15 and ages 65 and older) per 100 people in the “economically productive” ages (15–64 years of age). The formula for the dependency ratio is:

    The age dependency ratio for the United States is shown below at 49.

    This means that there were 49 people in the dependent ages for every 100 persons in the working ages.

    Calculate the dependency ratios for Kenya, Germany, Brazil, and Japan. Compare the components of each of them.


    Discuss the implications of high or low dependency ratios for economic resources and development.

    Question and Answer: Why Does It Take So Long to Slow or Stop Population Growth?

    Growth through natural increase occurs when the birth rate exceeds the death rate. For example, the U.S. birth rate in 2005 was 14 births per 1,000 people and the death rate was 8, yielding a net increase of six persons for every 1,000 persons in the United States, or approximately 1.7 million additional persons for that year. This rate of natural increase occurred in spite of a very small average family size measured by the total fertility rate—an estimate of the number of births to women during their lifetimes.

    The rate of natural increase of a population depends on birth and death rates, which are strongly influenced by the population age structure. Births occur primarily to people in the younger-adult age groups. If there are comparatively more young adults than older adults where mortality is highest, then even at replacement fertility levels (when each woman has about an average of two children) there will be more births than deaths.

    Hence, a relatively large number of couples each having one or two children can still produce a large excess of births. This phenomenon is known as population momentum.

    In the United States, birth rates are higher than death rates at present, partly due to the relatively young age structure of the U.S. population. Immigrants, who are younger on average than the U.S.-born population, play a significant role in keeping the United States younger than most other developed countries. For example, among U.S. Hispanics, 40 percent of whom are foreign-born, there are approximately 10 births for every death.

    The momentum of population growth in less developed countries will only be slowed when the large number of young adults resulting from previous high fertility have passed out of the childbearing years and a succeeding smaller generation reproduces at replacement level fertility. This momentum is very pronounced in China, where women have about two children, but the number of women having children is now much larger than in the previous generation. Thus, even though it has reached replacement level fertility, China’s population continues to grow.


    The composition of a population as determined by the number or proportion of males and females in each age category. The age-sex structure of a population is the cumulative result of past trends in fertility, mortality, and migration. Information on age-sex composition is essential for the description and analysis of many other types of demographic data.

    A dramatic increase in fertility rates and in the absolute number of births. In the United States this occurred during the period following World War II (1946 to 1964).

    Source : www.prb.org

    Demographic Transition Model

    The demographic transition model seeks to explain the transformation of countries from having high birth and death rates to low birth and death rates.

    Humanities › Geography

    Demographic Transition

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    Charmed88 / Wikimedia Commons / Public Domain

    By Matt Rosenberg

    Updated on October 02, 2019

    The demographic transition model seeks to explain the transformation of countries from having high birth and death rates to low birth and death rates. In developed countries, this transition began in the 18th century and continues today. Less developed countries began the transition later and are still in the midst of earlier stages of the model.

    CBR & CDR

    The model is based on the change in crude birth rate (CBR) and crude death rate (CDR) over time. Each is expressed per thousand population. The CBR is determined by taking the number of births in one year in a country, dividing it by the country's population, and multiplying the number by 1,000. In 1998, the CBR in the United States is 14 per 1,000 (14 births per 1,000 people) while in Kenya it is 32 per 1,000. The crude death rate is similarly determined. The number of deaths in one year is divided by the population and that figure is multiplied by 1,000. This yields a CDR of 9 in the U.S. and 14 in Kenya.

    Stage I

    Prior to the Industrial Revolution, countries in Western Europe had high CBR and CDR. Births were high because more children meant more workers on the farm and with the high death rate, families needed more children to ensure the survival of the family. Death rates were high due to disease and a lack of hygiene. The high CBR and CDR were somewhat stable and meant the slow growth of a population. Occasional epidemics would dramatically increase the CDR for a few years (represented by the "waves" in Stage I of the model.

    Stage II

    In the mid-18th century, the death rate in Western European countries dropped due to improvement in sanitation and medicine. Out of tradition and practice, the birth rate remained high. This dropping death rate but the stable birth rate at the beginning of Stage II contributed to skyrocketing population growth rates. Over time, children became an added expense and were less able to contribute to the wealth of a family. For this reason, along with advances in birth control, the CBR was reduced through the 20th century in developed countries. Populations still grew rapidly but this growth began to slow down.

    Many less developed countries are currently in Stage II of the model. For example, Kenya's high CBR of 32 per 1,000 but low CDR of 14 per 1,000 contribute to a high rate of growth (as in mid-Stage II).

    Stage III

    In the late 20th century, the CBR and CDR in developed countries both leveled off at a low rate. In some cases, the CBR is slightly higher than the CDR (as in the U.S. 14 versus 9) while in other countries the CBR is less than the CDR (as in Germany, 9 versus 11). (You can obtain current CBR and CDR data for all countries through the Census Bureau's International Data Base). Immigration from less developed countries now accounts for much of the population growth in developed countries that are in Stage III of the transition. Countries like China, South Korea, Singapore, and Cuba are rapidly approaching Stage III.

    The Model

    As with all models, the demographic transition model has its problems. The model does not provide "guidelines" as to how long it takes a country to get from Stage I to III. Western European countries took centuries through some rapidly developing countries like the Economic Tigers are transforming in mere decades. The model also does not predict that all countries will reach Stage III and have stable low birth and death rates. There are factors such as religion that keep some countries' birth rate from dropping.

    Though this version of the demographic transition is composed of three stages, you'll find similar models in texts as well as ones that include four or even five stages. The shape of the graph is consistent but the divisions in time are the only modification.

    An understanding of this model, in any of its forms, will help you to better understand population policies and changes in developed and less developed countries around the world.

    Source : www.thoughtco.com

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