A population is a grouping of individuals subject to the processes of birth, death, and migration. A population may consist of insects, fish, or land vertebrates, as studied by ecologists, but this article deals with human populations. These are usually defined territorially as the population living within specified boundaries, such as those of the United States or of the Maldive Islands, but such nonterritorial groups as the female or black inhabitants of the United States may also be described as populations. POPULATION GROWTH During the first 2 million or so years of its history the human population was a minor element in the world ecosystem, with at most 10 million members. In the New Stone Age, less than 10,000 years ago, the number of humans began to increase more rapidly. The rough equilibrium maintained before Neolithic times gave way when the human population developed agriculture and animal husbandry and no longer had to spread out in search of game. With the abandonment of a hunting-gathering way of life and the rise of permanent settlements and eventually cities, the human population underwent dramatic growth. By the beginning of the Christian era it had reached 250 million, and by 1650, half a billion. In various parts of the world, plagues and epidemics appeared and reappeared, usually spreading from populations habituated and at least partly immune--especially in East Asia--to hitherto untouched populations in Europe and the Americas. The Black Death in the 14th century wiped out a third of the population of Europe. In the 17th century the plagues stopped in Europe, but the emigration of Europeans to the Americas gave old diseases a new life--they decreased the Indian populations, who were not biologically immune. Triumph over disease on a worldwide scale came during the 20th century with antibiotics and \TDDT\t; all continents showed a striking fall in death rates. But birthrates did not go down in proportion. Populations with birthrates continuing at the primitive, or base, level of 45 per 1,000 people per year, and whose death rates have fallen well below 20 per 1,000 people per year, are growing by as much as 3.5% per year and thus doubling over 20 years. People and Subsistence The vast population increases of the 20th century have been accompanied by advances in agriculture. Domestic animals supply about 145 million metric tons (1980-87 average) of meat each year; this, in addition to about 495 million metric tons of wheat and a somewhat smaller quantity of rice, provides the main sustenance of the human population. Lesser elements are an average annual 97 million metric tons of sugar and that part of 441 million metric tons of corn which is not fed to animals. About 82 million metric tons of fish are caught in the oceans each year, but much of this is processed into animal feed and fertilizer (1980-87 average). Populations tend to grow rapidly to the highest level at which the available technology can provide sustenance, and then remain constant. As technology becomes more sophisticated, population increases. Thomas \TMalthus\t, the British economist and pioneer in the modern study of population, failed to predict technological change and consequently underestimated future growth in both populations and their incomes. The 38 million British counted in the census of 1901 were far better off than the 12 million counted in 1801 soon after Malthus wrote An Essay on the Principle of Population, As It Affects the Future Improvement of Society. It does seem to be true, as the influential German social theorist Karl Marx insisted, that each society has its own law of population growth. Rome discovered how to organize its human resources into a fighting force that for several centuries was able to overcome any other along its borders; given this military superiority, a local population surplus could be sent out to conquer and settle foreign lands; thus overpopulation in the Roman Empire was nonexistent. During the 19th century the expansion of industry required people, as the noted Scottish economist Adam \TSmith\t discerned, and population grew in accord with this need. By contrast, in the 20th century population growth has tended to be autonomous--that is, independent of political or economic factors. Much contemporary discussion of population is framed in terms of a demographic model in which deaths decline from a level of about 30 per 1,000 each year to about 10 per 1,000; after a longer or shorter lag the decline in deaths is followed by a decline in births from about 45 per 1,000 or higher to about 20 per 1,000 or lower. No one knows whether the fall in the death rate directly causes the fall in the birthrate that follows it, or how long the lag will be in those countries where a fall in the birthrate has not yet occurred. The time lag separating the fall in births from the fall in deaths is of crucial importance; with a delay of 45 years the population can multiply fourfold, and with a delay of 75 years it can multiply ninefold. What is certain is that if no fall in birthrate occurs, the death rate will have to rise in those countries which are technologically backward; only a certain number of people will be able to support themselves within a given territory through subsistence agriculture. Current Population Figures In 1990 the total world population was estimated at about 5.3 billion people. The most significant world trend is that death rates are currently falling in poor and rich countries alike, while birthrates remain high in most poor countries and low in most rich ones. Exceptions are the generally higher death rates of Africa and the high birthrates of the rich oil-producing countries. The four most populous countries, in descending order, are China, India, the \TUSSR\t, and the United States. The 1990 census of the United States population totaled 249,632,692. Each year about 3.8 million children are born in the United States, and 2 million persons die. The greater number of births is due to the high proportion of young couples--themselves the product of the high birthrates of the 1950s, which in a number of years exceeded 4 million. Once these couples have passed through their childbearing years, births will be fewer than deaths, unless a considerable rise in the average number of children born per couple takes place. International immigration, both legal and illegal, is another major element in U.S. population growth. Legal immigration has recently amounted to more than 600,000 per year; illegal immigration is thought to be even greater. Uncertainty exists at the margins of population accounting in the United States. For the world's most populous country, China, only the roughest indications of even the most basic facts--number of people and rate of increase--are available. The 1953 census counted 584 million in mainland China; the population in 1990 was 1,133,682,501. China's annual increase in 1990 was 1.47%. India's population of 853,400,000 (1990 est.) is increasing faster than that of China, and if present trends continue it will catch up with or surpass China in the early decades of the 21st century. The \TUSSR\t has about 289 million people (1990 est.). Its overall rate of increase corresponds with that of other industrialized societies, although differentials in its birthrate are large: the birthrate in European \TUSSR\t is comparable to the rates in the United States and Western Europe, whereas birthrates in parts of Soviet Central Asia are higher. Indonesia, Brazil, and Japan rank 5th, 6th, and 7th, respectively, in order of total population. For these and other population figures, see tables at the end of this article as well as individual articles on continents, countries, states, provinces, and cities. Population Movement Internal migration continues to be important everywhere. The United States has seen a movement outward from the cities, first to the \Tsuburbs\t and more recently to the countryside beyond. Since the 1950s the main interregional current has been toward the west and the south--the Sun Belt. An increasing segment of the population, and some industry as well, is footloose--people on pensions and other incomes that do not require their presence in any particular locality and industries whose raw materials and products are easily transported. Given this liberation from place, one can expect continued population movement toward regions of agreeable climate. Around the world the classical internal migration pattern is from poor to better-off areas. Thus in Italy the main movement is from the south (including Sicily) to the north; in Britain, from Scotland and the now-depressed towns of early industrialization to London and the south of England generally; in Brazil, from the northeast to the south and especially to Sao Paulo. This pattern is paralleled in Europe as a whole, with workers from poorer countries migrating to the richer, more highly industrialized nations: France, Germany, Sweden, the United Kingdom. By far the most important migration pattern in the world as a whole is that from countryside to city. In the past this trend has usually accompanied industrialization. Today rural-urban migration is most rapid in poor countries, and by the end of the century it is expected that cities in these poorer countries will have passed the 2 billion mark and will contain twice the population found in cities of rich countries. Metropolitan Mexico City is said to contain more than 20 million (1990 est.) people, making it even larger than such other major cities in the less-developed countries of the world as Shanghai, Calcutta, Bombay, Cairo, Jakarta, and Manila. Many other such cities are not only large but continue to increase at greater annual rates of growth than those in the world's developed countries. As countries become urbanized the natural increase in the size of their cities eventually surpasses migration as the source of further urban growth. New Distribution Patterns Crowding is not so much a matter of too many people in general as of their unequal distribution. For instance, if the United States had a stable population of about 300 million people it would not suffer from standing room only; in fact much of its territory would still remain empty. In other words, it is not the size of a population per se but the tendency of people to cluster in the largest cities that creates the problem of congestion. This concentration is now diminishing, but it is being supplanted by a trend toward the creation of vast, continuously expanding urbanized areas. The metropolitan area of New York, plus that of Hartford, plus that of Philadelphia, plus that of Providence, and so on, may no longer be the best description of the East Coast agglomeration; in the 1960s geographers adopted the term \TMegalopolis\t to describe the continuous built-up area from Boston to Washington, D.C., and similarly for the area south of Lake Michigan, for southern California, and for a dozen other areas. Such spreading out will relieve population pressures but result in an increased use of energy for transportation, and energy shortages could change residential patterns greatly. POPULATION CONTROL What are the prospects for world population? It seems to be approaching an upper limit, to be attained toward the end of the next century. With good fortune population stabilization will be brought about through a falling birthrate. In some parts of the world, however, malnutrition and such diseases as malaria are actually on the increase, and population control may occur through a rise in the death rate. Projections for Future Growth Presuming that the birth rates and death rates coincide in all parts of the world by the end of the century, demographers estimate that the world population will level off at between 8 and 9 billion about the year 2075. This figure includes China at 1.5 billion and India at 1.6 billion. Whether Brazil's population will increase to the 296 million the projection indicates, or Mexico to 180 million, no one can now say. The mean ages of the populations of those countries are young enough to bring about such an increase even with a drop to bare replacement in the next 20 years, provided the death rates do not increase. For the developed countries the same assumptions show a much smaller change from present levels. Japan would increase from 123 million (1989) to 143 million in 2075; the United Kingdom, from 57.4 to 69 million; the United States, from 249.6 to 292 million; and the \TUSSR\t, from 289 to 354 million. A 1990 United Nations report on population presents a bleaker view, predicting that world population will stabilize at 11.3 billion by the end of the 21st century or, if growth is not better controlled, at an alarmingly higher 14 billion. Certain features generally accompany the stabilization or slowdown of population growth. Slowing means proportionately more old people and fewer young people in the society; less-rapid promotion of individuals within organizations; possibly a slower rate of innovation resulting from the higher average age; and with all these factors, probably a less optimistic culture. The euphoria of the years of rapid population growth in the United States--for instance, the 1920s and the 1960s--has now been replaced by the less-bright outlook of the 1970s and 1980s. Yet demographers are cautious about attributing economic or psychological slowdown to the slowing of population growth; equally persuasive arguments can be presented against any such imputation. Population as an Object of Policy Population has been a perennial preoccupation of government policy. Rome rewarded mothers and taxed bachelors, as the \TUSSR\t began to do in Stalin's time; abortion and even contraception were made illegal in 19th-century France and in many other countries; Sweden provided government housing at low rents; many countries, including Canada, have directly subsidized children by providing family allowances. The effectiveness of such pronatalist measures in raising the birthrate has, however, never been unambiguously demonstrated. French citizens deprived of contraceptive equipment outwitted their government by turning to coitus interruptus and illegal abortions. Countries with subsidized housing have birthrates no higher than those with free-market housing. When Romania made abortion illegal the birthrate went up immediately, but this temporary response was succeeded by better use of contraception and a subsequently lowered birthrate. Any effect that such measures may have is largely swamped by contemporary social changes that governments cannot or do not wish to control. These include the liberation of women from subservient roles, the increasing involvement of women with work and careers, and rising divorce rates. A government like that of Austria--one of more than a half-dozen countries where the actual number of births is about equal or has fallen below the number of deaths--wants to see its birthrate rise at least to replacement level, but it cannot wholeheartedly press its women to abandon the labor market and stay home. By funding family allowances and day-care centers Austria tries to encourage the birth of children without pushing women out of the labor force, but reconciling such mutually contradictory objectives will not be easy. Birth-Control Programs Lowering births is the unambiguous object of government policy in more and more of the developing countries. Increasing populations press on the environment and its resources, take away from the capital available for new investment, crowd cities with people for whom jobs are not available, upset the balance of payments, and cause political turmoil. After a period of uncertainty, during which the ancient idea that population is a source of national power prevailed in many high places, governments have become aware that national power--not to mention individual welfare--is more likely to be attained with fewer people. Contraception is freely available in most countries and is actively promoted in many. Such promotion has little effect when people want large families, but when the other forces of modernization have come into play and the birthrate has started to fall spontaneously, promotion of birth control speeds the decline. Typically, the better-off take up family limitation first; governmental sponsorship of birth control and foreign aid that makes contraceptives available to the poor apparently help to spread the practice of family limitation among ever-wider strata of the population. Recent surveys in Colombia, Indonesia, and other places with active programs point to a more rapid decline in birthrates than had previously been thought possible. (See \Tbirth control\t.) Such government action is more likely to be effective when it goes along with social and economic changes, such as the equalizing of incomes. Those countries where income inequality is slight and declining have shown the greatest fall in the birthrate; it is as though people will only limit the number of their children when they see some chance of rising out of poverty. It seems to be for such reasons that Sri Lanka's birthrate, for instance, has fallen faster than those of India and Brazil. Measures that penalize large families have been tried, but no measure that serves to disadvantage children already born is likely to prove acceptable. One would like to urge people to have fewer children and at the same time provide food subsidies so that children already born will grow up healthy. These are both worthy aims, but subsidized food tends to cancel out birth-control programs, this being one of many dilemmas of population policy. A realistic sense of the limits of action to influence historical events, and of the way action in one field has repercussions in others, could well reduce government intervention while making such intervention as is exercised more effective. Nathan Keyfitz Bibliography: Alonso, William, ed., Population in an Interacting World (1987); Coale, A. J., "The History of Human Populations," Scientific American, September 1974; Ehrlich, Paul, The Population Bomb, new rev. ed. (1986); Ehrlich, Paul and Anne, The Population Explosion (1990); Fraser, Dean, The People Problem (1971); Freedman, Ronald, and Berelson, Bernard, "The Human Population," Scientific American, September 1974; Glass, D. V., and Eversley, D. E. C., eds., Population in History (1965); Gupte, Pranay, The Crowded Earth (1984); Hauser, Philip M., et al., Population and the Urban Future (1983); Johnson, D. Gale, and Lee, Ronald D., eds., Population Growth and Economic Development (1987); Matras, Judah, Introduction to Population: A Sociological Approach (1977); Menard, Scott, and Moen, Elizabeth, Perspectives on Population (1987); Myrdal, Gunnar, Population: A Problem for Democracy (1962); Petersen, William, Population, 3d ed. (1975), and, as ed., Readings in Population (1972); Roberts, Godfrey, ed., Population Policy: Contemporary Issues (1990); Spengler, J. J., Facing Zero Population Growth (1988); Teitelbaum, Michael S., and Winter, Jay M., The Fear of Population Decline (1985); Wattenberg, Ben, The Birth Dearth (1987). See also: \Tagriculture and the food supply\t; \Tdemography\t.