ScienceWeek

DEMOGRAPHIC SCIENCES: ON HUMAN POPULATION GROWTH

The following points are made by Joel E. Cohen (Science 2003 302:1172):

1) It is a convenient but potentially dangerous fiction to treat population projections as exogenous inputs to economic, environmental, cultural, and political scenarios, as if population processes were autonomous. Belief in this fiction is encouraged by conventional population projections, which ignore food, water, housing, education, health, physical infrastructure, religion, values, institutions, laws, family structure, domestic and international order, and the physical and biological environment. Other biological species are recognized explicitly only in the recent innovation of quantifying the devastating demographic impacts of HIV and AIDS. The absence from population projection algorithms of influential external variables indicates scientific ignorance of how external variables influence demographic rates rather than any lack of influence (1). Demographic projections stimulate fears of overpopulation in some, fears of demographic decline and cultural extinction in others(2).

2) Earth's population grew about 10-fold from 600 million people in 1700 to 6.3 billion in 2003 (3). These and all demographic statistics are estimates. It took from the beginning of time until about 1927 to put the first 2 billion people on the planet; less than 50 years to add the next 2 billion people (by 1974); and just 25 years to add the next 2 billion (by 1999). The population doubled in the most recent 40 years. Never before the second half of the 20th century had any person lived through a doubling of global population. Now some have lived through a tripling. The human species lacks any prior experience with such rapid growth and large numbers of its own species.

3) From 1750 to 1950, Europe and the New World experienced the most rapid population growth of any region, while the populations of most of Asia and Africa grew very slowly. Since 1950, rapid population growth shifted from Western countries to Africa, the Middle East, and Asia.

4) The most important demographic event in history occurred around 1965-70. The global population growth rate reached its all-time peak of about 2.1% per year (pa). It then gradually fell to 1.2% pa by 2002 (4). The global total fertility rate fell from 5 children per woman per lifetime in 1950-55 to 2.7 children in 2000-05. The absolute annual increase in population peaked around 1990 at 86 million and has fallen to 77 million. Concurrent trends included worldwide efforts to make contraception and reproductive health services available, improvements in the survival of infants and children, widespread economic development and integration, movements of women into the paid labor market, increases in primary and secondary education for boys and girls, and other cultural changes.

5) In 1960, five countries had total fertility rates at or below the level required to replace the population in the long run. By 2000, there were 64 countries such countries, with about 44% of all people (4,5).

6) Worldwide urbanization has taken place for at least two centuries and accelerated greatly in the 20th century. In 1800, roughly 2% of people lived in cities; in 1900, 12%; in 2000, more than 47%, and nearly 10% of those city dwellers lived in cities of 10 million people or larger. Between 1800 and 1900, the number of city dwellers rose more than 11-fold, from 18 million to 200 million; between 1900 and 2000, the number of city dwellers rose another 14-fold or more, from 200 million to 2.9 billion. In 1900, no cities had 10 million people or more. By 1950, one city did: New York. In 2000, 19 cities had 10 million people or more. Of those 19 cities, only four (Tokyo, Osaka, New York, and Los Angeles) were in industrialized countries.

References (abridged):

1. J. E. Cohen, How Many People Can the Earth Support? (W. W. Norton, New York, 1995)

2. L. Shriver, Popul. Dev. Rev. 29 (no. 2), 153 (2003)

3. United States Census Bureau, Historical Estimates of World Population (online). Available at www.census.gov/ipc/www/worldhis.html

4. United Nations Population Division, World Population Prospects: the 2002 Revision, Highlights (online database). ESA/P/WP.180, revised 26 February 2003, p. vi. Available at: http://esa.un.org/unpp/

5. United Nations Population Division, Partnership and Reproductive Behavior in Low-Fertility Countries, ESA/P/WP.177, revised May 2003.

Science http://www.sciencemag.org

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DEMOGRAPHY: INCREASE OF MAXIMUM LIFE-SPAN IN SWEDEN 1861-1999

The term "demography" refers to the numerical and quantitative analysis of populations and their distributions. In 1997, a woman named Jeanne Calment died in France at a documented age of 122.45 years, and the event and its publicity underscored the prime question in human demographics: Is the maximum human life-span fixed or can it be substantially increased? Although this question and a number of subsidiary questions continue to be at the forefront of research in human demography, few studies have been carried out on well-defined human populations over extensive periods.

The following points are made by J.R. Wilmoth et al (Science 2000 289:2366):

1) The authors present a study of several human demographic questions related to life-span, the authors using Swedish national demographic data from 1861 to 1999, which are the longest available series of reliable information on the upper limits of achieved human life span. The authors make the following points:

2) The authors point out that national demographic statistics suggest that the maximum age at death has been rising steadily in industrialized countries for more than 100 years, and that two important questions arise from this observation: 1) Has this upward trend been steady over time, or has it changed pace in recent years? Perhaps the increase has accelerated due to an intensification of efforts to promote the health of the elderly and to prevent or even cure ailments such as coronary heart disease, stroke, and cancer. Or perhaps the trend has decelerated because maximum ages now observed for humans are approaching a hypothetical biological limit. 2) What accounts for the apparent increase in the maximum age at death? There are two competing explanations: a) the increase is due merely to the larger size of contemporary populations, which increases the probability that at least one individual will survive to an extreme old age; or b) the increase reflects improvements in an individual's probability of survival, especially at older ages.

3) The authors report they examined maximum age at death in Sweden, where the maximum age has risen from approximately 101 years during the 1860s to approximately 108 years during the 1990s. The authors report the pace of increase was 0.44 years per decade before 1969, but accelerated to 1.11 years per decade after that date. More than 70 percent of the rise in the maximum age at death from 1861 to 1999 is apparently attributable to reductions in death rates above age 70. The rest are due to increased numbers of survivors to old age (both large birth cohorts and increased survivorship from infancy to age 70). The more rapid rise in the maximum age since 1969 is apparently due to the faster pace of old-age mortality decline during recent decades.

4) The authors conclude: "Our analysis refutes the common assertion that the human life-span is fixed and unchanging over time. Although the maximum has increased much more slowly than the average, the entire distribution of ages at death has been shifting upward for more than a century in Sweden and, presumably, in other countries as well. Reductions in death rates at older ages, which have accelerated in recent decades, seem likely to continue and may gradually extend the limits of achieved human longevity even further."

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