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Unit 5: Human Population Dynamics // Section 4: World Population Growth Through History


Human population has grown very slowly for most of its existence on earth. Scientists currently estimate that modern human beings (Homo sapiens) evolved roughly 130,000 to 160,000 years ago. Many threats, from diseases to climate fluctuations, kept life expectancy short and death rates high in pre-industrial society, so it took until 1804 for the human population to reach one billion. From that point forward, however, population growth accelerated very quickly (Table 1).

Table 1. World population milestones. Source: United Nations Secretariat, Department of Economic and Social Affairs, The World At Six Billion (1999), p. 8.
World population reached: Year Time to add 1 billion
1 billion 1804
2 billion 1927 123 years
3 billion 1960 33 years
4 billion 1974 14 years
5 billion 1987 13 years
6 billion 1999 12 years

Through the early decades of the Industrial Revolution, life expectancies were low in western Europe and the United States. Thousands of people died from infectious diseases such as typhoid and cholera, which spread rapidly in the crowded, filthy conditions that were common in early factory towns and major cities, or were weakened by poor nutrition. But from about 1850 through 1950, a cascade of health and safety advances radically improved living conditions in industrialized nations. Major milestones included:

By the mid-20th century, most industrialized nations had passed through the demographic transition. As health technologies were transferred to developing nations, many of these countries entered the mortality transition and their population swelled. The world's population growth rate peaked in the late 1960s at just over 2 percent per year (2.5 percent in developing countries).

Demographers currently project that Earth's population will reach just over nine billion by 2050, with virtually all growth occurring in developing countries (Fig. 8). Future fertility trends will strongly affect the course of population growth. This estimate assumes that fertility will decline from 2.6 children per woman in 2005 to slightly over 2 children per woman in 2050. If the rate falls more sharply, to 1.5 children per woman, world population would be 7.7 billion in 2050, whereas a slower decline to 2.5 children per woman would increase world population to 10.6 billion by 2050.

World population

Figure 8. World population
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Source: 2004. United Nations, World Population Prospects.

Many people interpret forecasts like this to mean that population growth is out of control. In fact, as noted above, world population growth rates peaked in the late 1960s and have declined sharply in the past four decades (Fig. 9). The world's total population is still rising because of population momentum stemming from large increases that occurred in developing countries in the 1950s and early 1960s. But fertility rates are falling as many developing countries pass through the demographic transition, thanks to factors that include lower infant mortality rates; expanding rights, education, and labor market opportunities for women; and increased access to family planning services.

Population growth rate

Figure 9. Population growth rate
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Source: 2004. United Nations, World Population Prospects.

World population growth in the 21st century will be different from previous decades in several important ways. First, humans are living longer and having fewer children, so there will be more older people (age 60 and above) than very young people (age zero to four). Second, nearly all population growth will take place in urban areas. Third, fertility rates will continue to decline (footnote 8).

All of these trends will affect nations' economic development. (On urbanization, see section 6, "Urbanization and Megacities.") Senior citizens can be active and productive members of society, but they have many unique needs in areas ranging from medical care to housing and transportation. Growing elderly populations will strain social services, especially in countries that do not have well-developed social safety nets to guarantee adequate incomes for older citizens. In countries that have "Pay As You Go" social security programs, increasing ratios of older to younger people may create budget imbalances because fewer workers are paying funds into the system to support growing numbers of retirees.

As societies age, demand for younger workers will increase, drawing more people into the labor force and attracting immigrants in search of work. Declining fertility rates allow more women to work outside of the home, which increases the labor supply and may further accelerate the demographic transition (Fig. 10).

Woman supervising 25 employees at the Vegetable Dehydrates Factory, Parwan Province, Afghanistan

Figure 10. Woman supervising 25 employees at the Vegetable Dehydrates Factory, Parwan Province, Afghanistan
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Source: Courtesy Jeremy Foster, United States Agency for International Development.

As fertility rates fall, some countries have already dropped below replacement level—the number of children per woman that keeps population levels constant when births and deaths are considered together over time (assuming no net migration). Replacement-level fertility requires a total fertility rate of about 2.1 to offset the fact that some children will die before they reach adulthood and have their own families (in a society with higher mortality rates, replacement-level fertility would require more births) (footnote 9). Total fertility rates in most European and some Asian and Caribbean countries currently range from about 1.2 to 1.8, well below replacement level.

Some observers argue that declining fertility rates in both industrialized and developing countries will lead to a "birth dearth," with shrinking populations draining national savings and reducing tax revenues. However, societies can transition successfully from high mortality and fertility to low mortality and fertility with sound planning. Promoting good health standards (especially for children), expanding education, carefully opening up to international trade, and supporting older citizens through retirement are all policies that can help to offset the negative impacts on society of an aging population (footnote 10).

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