Population, term referring to the total human inhabitants of a specified area, such as a city, country, or continent, at a given time. Population study as a discipline is known as demography. It is concerned with the size, composition, and distribution of populations; their patterns of change over time through births, deaths, and migration; and the determinants and consequences of such changes. Population studies yield knowledge important for planning, particularly by governments, in fields such as health, education, housing, social security, employment, and environmental preservation. Such studies also provide information needed to formulate government population policies, which seek to modify demographic trends in order to achieve economic and social objectives.
The Field of Demography Demography is an interdisciplinary field involving mathematics and statistics, biology, medicine, sociology, economics, history, geography, and anthropology. The field of demography has a relatively brief history. Its beginning often is dated from the publication in 1798 of An Essay on the Principle of Population by the British economist Thomas Robert Malthus. In this work Malthus warned of the constant tendency for human population growth to outstrip food production and classified the various ways that such growth would, in consequence, be slowed.
He distinguished between "positive checks" to population growth (such as war, famine, and disease) and "preventive checks" (celibacy and contraception). The development of demography has been tied closely to the gradually increasing availability of data on births and deaths from parish and civil registers, and on population size and composition from the censuses that became common in the 19 th century (see Census). The growth of behavioral sciences in the 20 th century and advances in the fields of statistics and computer sciences further stimulated demographic research Subfields of mathematical, economic, and social demography have grown rapidly in recent decades. Demographic Data and Measurements Modern national governments and international organizations place a high priority on the accurate determination of national and worldwide populations. Describing the present population and predicting those of the future with reasonable accuracy requires reliable data. Methods of Research National censuses, civil registration, and, since the 1960 s, national sample surveys are the major sources of demographic data.
They provide the raw materials for investigating the causes and consequences of population changes. The most common source is the population census, a count of all persons by age and with specified social and economic characteristics within a given area at a particular time. A register is a continuous record of births, deaths, migrations, marriages, and divorces, often maintained by a local government; reliability varies with the scrupulousness of citizens in reporting these data. In the sample survey, a statistically selected portion is used to represent the total population. In the U. S.
, decennial censuses have been taken since 1790. Since the 1950 s the U. S. Bureau of the Census has conducted an annual Current Population Survey, a highly detailed sample survey of many aspects of demographic behavior and related socioeconomic factors. International population data are compiled in systematic form by the United Nations Statistical Office, which prepares an annual Demographic Yearbook; by the United Nations Demographic Division, which issues biennial assessments and projections of world population; and by the International Bank for Reconstruction and Development.
Measures of Population The numbers of births, deaths, immigrants, and emigrants over a specified time interval determine the change in population size. For comparative purposes, these components of change are expressed as proportions of the total population, to yield the birth rate, death rate, migration rates, and the population growth rate. (Birth and death rates typically are stated as numbers per 1000 population per year. ) These rates are affected by the age-composition of the population; for example, a very healthy population, which, as a result, has a relatively large proportion of old people, might have a death rate similar to that of a poor population made up of predominantly younger members. Demographers, therefore, often use measures that are free of this age-distribution influence.
Two such widely used measures are the total fertility rate (TFR) and the life expectancy at birth. The total fertility rate is the number of children a woman would have during her reproductive life if she experienced the prevailing rates of fertility at each age. High-fertility countries may have birth rates of 40 or even 50 per 1000 population (per year); corresponding levels of the TFR would be 5 to 7 children per woman. Low-fertility countries have birth rates of 15 to 20 per 1000 and Tfr of about 2.
"Replacement level" fertility (the level at which each person on average has a single successor in the next generation) corresponds to a TFR of about 2. 1 under low-mortality conditions. The life expectancy at birth is the average length of life that would be observed in a population in which the currently prevailing mortality risks at each age continued indefinitely. Preindustrial populations were characterized by large fluctuations in mortality; long-run averages, however, would probably have shown death rates of 30 to 40 per 1000 and life expectancies of 25 to 35 years. Under modern health conditions, death rates below 10 per 1000 and life expectancies above 70 years are common.
Another important mortality measure is the infant mortality rate. This is the probability of death in the first year of life, usually stated as a number per 1000 births. Many less-developed countries have infant mortality rates above 100 per 1000-that is, more than 10 percent of the children die in their first year. In countries with effective health and educational systems, infant mortality rates are about 15 per 1000, or even lower. World Population Growth and Distribution The United Nations, an accepted authority on population levels and trends, estimates that the world population reached 5. 3 billion in 1990, and is increasing annually by more than 90 million persons.
The rate of increase, 1. 7 percent per year, has fallen below the peak rate of 2 percent per year attained by 1970. However, absolute yearly increments are not expected to start declining until after the year 2000. Past and Present Growth Estimates of world population before 1900 are based on fragmentary data, but scholars agree that, for most of human existence, long-run average population growth approached approximately 0. 002 percent per year, or 20 per million inhabitants.
Growth was not steady but was marked by oscillations dictated by climate, food supply, disease, and war. Starting in the 17 th century, great advances in scientific knowledge, agriculture, industry, medicine, and social organization made possible substantial increases in population. Inanimate energy gradually replaced human and animal labor. People slowly acquired the knowledge and means to control disease.
All continents shared in a fivefold population increase over a 300-year period-from about 500 million in 1650 to 2. 5 billion in 1950-but increases were most striking in regions where new technologies were devised and applied. Beginning about 1950, a new phase of population growth was ushered in when famine and disease could be controlled even in areas that had not yet attained a high degree of literacy or a technologically developed industrial society. This happened as a result of the modest cost of importing the vaccines, antibiotics, insecticides, and high-yielding varieties of seeds produced since the 1950 s. With improvements in water supplies, sewage-disposal facilities, and transportation networks, agricultural yields increased, and deaths from infectious and parasitic diseases greatly declined.
Life expectancy at birth in most developing countries increased from about 35-40 years in 1950 to 61 years by 1990. The rapid decline in deaths among people who maintained generally high fertility rates led to annual population growth that exceeded 3. 1 percent in many developing nations-a rate that doubles population size in 23 years. Regional Distribution As of 1990, 1. 2 billion people lived in the developed nations of the world, and 4. 1 billion people lived in the less-developed countries.
By region, over half the world's population was in East and South Asia; China, with more than 1. 2 billion inhabitants, and India, with some 880 million, were the dominant contributors. Europe and the countries of the former USSR contained 15 percent, North and South America made up 14 percent, and Africa had 12 percent of world population. Differences in regional growth rates are altering these percentages over time. Africa's share of the world population is expected to more than double by the year 2025, while the population of South Asia and Latin America remains nearly constant and the other regions, including East Asia, decline appreciably in relative size.
The share of the present developed nations in world population-23 percent in 1990-is expected to fall to 17 percent by 2025. Nine out of every ten persons who are now being added to the world's population are living in the less-developed countries. Urban Concentration As a country develops from primarily an agricultural to an industrial economy, large-scale migration of rural residents to towns and cities takes place. During this process, the growth rate of urban areas is typically double the pace of overall population increase.
Some 29 percent of the world population was living in urban areas in 1950; this figure was 43 percent in 1990, and is projected to rise to about 50 percent by the year 2000. Urbanization eventually leads to a severe decline in the number of people living in the countryside, with negative population growth rates in rural areas. Rapid growth of overall population has deferred this event in most less-developed countries, but it is projected to occur in the early decades of the 21 st century. Most migrants to the cities can be assumed to have bettered themselves in comparison to their former standard of living, despite the serious problems of overcrowding, substandard housing, and inadequate municipal services that characterize life for many arrivals to urban centers. Dealing with these conditions, especially in very large cities, presents massive difficulties for the governments of less-developed countries. Population Projections Most of the potential parents of the next two decades have already been born.
Population projections over this interval can, therefore, be made with reasonable confidence, barring catastrophic changes. Beyond two decades, however, uncertainties about demographic magnitudes and other characteristics of human societies build up rapidly, making any projections somewhat speculative. The UN medium projections issued in 1990 show the world population increasing from 5. 3 billion in 1990 to 6. 2 billion in 2000, and 8. 5 billion in 2025.
"High" and "low" projections for 2025 are 9. 1 billion and 7. 9 billion respectively. The average world birth rate is projected to decline from the 1990 level of 26 per 1000 to 22 per 1000 at the end of the century and to 17 per 1000 in 2025 (corresponding to a fall in TFR from 3. 3 in 1990 to 2. 3 in 2025).
Because of the expanding share of the population at high-mortality ages, the average world death rate is expected to decline only slightly; from 9 (per 1000) in 1990 to 8 in 2025. Average world life expectancy, however, is projected to rise from 65 years in 1990 to 73 years in 2025. Wide variations in population growth will undoubtedly persist. In the developed world, population growth will continue to be very low and in some nations will even decline. Western Europe as a whole is projected to have a declining population after 2000. U.
S. Census Bureau projections, assuming middle fertility and mortality levels and net immigration averaging 880, 000 per year, show U. S. population increasing from 249 million in 1990 to 334 million in 2025 and 383 million in 2050. Thereafter, growth would be virtually zero.
The UN expects the less-developed countries to have steadily falling rates of population growth. For the less-developed world as a whole, the 1990 growth rate of 2. 0 percent per year is projected to be cut in half by 2025. Africa will remain the region with the highest growth rate. In 1990 this rate was 3. 1 percent; in 2025 it is projected to be about 2.
2 percent. Africa's population would almost triple, from 682 million in 1990 to 1. 58 billion in 2025, and then continue growing at a rate that would almost double the population size in another 35 years. Population Policies Government population policies seek to contribute to national development and welfare goals through measures that, directly or indirectly, aim to influence demographic processes-in particular, fertility and migration. Examples include statutory minimum ages for marriage, programs to promote the use of contraception, and controls on immigration. (When such policies are adopted for other than demographic reasons, they can be termed implicit policies.
) Population Policy in the U. S. The early immigrants to North America found a vast continent with a relatively small indigenous population. Overcrowding was incomprehensible because of the expanse of land to the west. In the mid-20 th century, as the rest of the world awakened to the potential crisis brought on by unchecked population growth, the U. S.
government examined the possible impact of overpopulation in the nation. The President's Commission on Population Growth and the American Future began a two-year study in 1970. Submitted to President Richard M. Nixon in 1972, it welcomed the prospect of zero population growth in the U.
S. , but did not propose that the government take strong measures to attain it. The commission did, however, advocate education on family planning and widely available access to contraception and abortion services. Primarily because of this, the president rejected the commission's recommendations. Since then, U. S.
fertility has fallen below replacement level. This is due in part to the implicit policies that, taken together, make bearing and raising children very costly to parents. Future policy concerns may reflect worry over population aging and the demographic aspects of funding social security. In addition, the conflicting interests involved in determining numbers and characteristics of migrants is likely to keep immigration policy on the political agenda. Population Policies in Developed Nations European countries did not address the issue of a national population policy until the 20 th century. Subsidies were granted to expanding families by such disparate nations as Great Britain, Sweden, and the USSR.
The Italian Fascists in the 1920 s and the National Socialists (Nazis) in Germany during the 1930 s made population growth an essential part of their doctrines. Japan, with an economy comparable to those of the European nations, was the first developed country in modern times to initiate a birth-control program. In 1948 the Japanese government formally instituted a policy using both contraception and abortion to limit family size. European pronatalist policies were conspicuously unsuccessful in the 1930 s, and their milder variations over the past few decades (in, for example, France and many Eastern European nations) have apparently done little to slow a continuing fertility decline.
Government control of migration is more straightforward. Short-term migration tied to labor demands (guest workers) has been a common practice in Western Europe, allowing the various nations the flexibility to curtail migration during economic recessions. Population Policies in the Third World In 1952 India took the lead among developing nations in adopting an official policy to slow its population growth. India's stated purpose was to facilitate social and economic development by reducing the burden of a young and rapidly growing population. Surveys to ascertain contraceptive knowledge, attitude, and practice showed a high proportion of couples wishing no more children. Few, however, practiced efficient contraception.
Family-planning programs were seen as a way to satisfy a desire for contraception by a large segment of the population and also to confer health benefits from spacing and limiting births. Asia's lowered growth rate can be attributed mainly to the stringent population policies of China. Although it has a huge population, China has successfully reduced both fertility and mortality. The government has recently been advocating one-child families to lower the nation's growth rate from a current estimate of 14 per 1000 annually to close to zero by the year 2000. By 1979 more than 90 percent of the population in developing countries lived under governments that, in principle at least, supported access to contraceptives by their citizens, based on considerations of health and the right to choose to have children and to space them at desired intervals. Recent evidence indicates that progress toward the objectives of lowered fertility and national growth is being achieved in many nations, in part by government support for family-planning programs..