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Updated 11 November 2004

Consequences Vol. 1, No. 2, Summer 1995








Global and U.S. National Population Trends

by Carl Haub

A passing century is sometimes remembered in terms of a single event of lasting significance. For ours--the 20th--that singular happening may well be the sudden and unprecedented expansion of the world's population.

In the year 1900 the Earth was home to about 1.6 billion people. The total had grown by 600 million in the 100 years since 1800, the year that the first billion was reached; but the change in the 19th century gave no hint of things to come. By the middle of the present century another billion had been added, in the remarkably short span of only 50 years. Moreover, and significantly, 80 percent of the growth had taken place in the world's poorer, or "developing," nations. In 1995, but 45 years later, world population had risen by an additional three billion (Fig. 1), with most of the increase, as before, in Asia, Africa, and Latin America (Table 1).

The dramatic fashion in which new numbers have been added to the world's population since 1950 is shown in Table 2. While it took the several million years of human history to reach the first billion, and 130 years to reach the second, today each new billion is added in but 11 years.

It came as something of a surprise. The fact that world population would grow by billions in but a few decades was not anticipated by demographers in the initial postwar period. The earliest United Nations projections showed more concern with the possibility of devastating mortality striking the developing countries. As a result, U.N. projections made in 1951 predicted the 1980 population at anywhere from 3 to 3.6 billion, but the higher limit was considered optimistic and unlikely. The actual figure, as best we now know, proved to be about 4.4 billion: a large difference in a less than 30 year projection.

Counting The World's People

How are these numbers obtained? In the past, demographic data were often lacking or severely deficient in many countries of the developing world, and particularly in Africa and parts of Asia. Since the 1970s, however, there has been a veritable population information explosion. Nearly every one of the 220-odd nations of the world has taken a usable census or maintains a national population register. Gaps in birth rate information caused by the general lack of vital statistics in developing countries have been filled in by national surveys. These efforts began with the World Fertility Survey program in the mid-1970s and continue today with Demographic and Health Surveys funded in part by the U.S. Agency for International Development.

How closely do we know the real population of the world?

There has never been an organized global census per se ; nor is there any standard method of estimating errors. Many national census counts are subsequently adjusted, independently, by the U.N. or others who use them in the name of attempted uniformity, although the corrections made are generally small. Still, based upon the national estimates that are now available, it is likely that the world total is not in error by more than about three percent. Assuming that most censuses undercount rather than overcount-- that is, that the probable error is -3% and not +/- 3%--we can assume that the total number of people in the world today is known within about 200 million.

What caused so great a change in world population-- and why did it happen so fast?

Put in simplest terms, the recent population growth in all regions of the world was the consequence not of increased birth rates but of unprecedented, worldwide decreases in the death rate. In the 20th century the human race began at last to declare victory over both famine-related and infant mortality, at the same time that significant advances in public health and medicine were applied. But these advances, although felt around the world, did not happen in the same way in today's richer and poorer nations. Death rates--already lower in Europe and North America--declined more gradually in these more developed areas. This slower pace of change allowed time for trends of urbanization and industrialization to work their almost inevitable effect on family size: a change in preference from large numbers of children to the much smaller family sizes prevalent today. The process of change through which a nation's previously high levels of fertility and mortality shift to lower values is commonly known as the demographic transition .

Behind the post-1950 world population explosion was the very different way in which the demographic transition took place and is still taking place in the countries of the developing world. Benefits of advances in public health and medicine were felt immediately and life expectancy rose very rapidly (Fig. 2). But these advances came at a different stage of development from what applied in earlier decades in Western Europe and North America. With the majority of population still agrarian in nature, children were still deemed an economic asset, and as a result, birth rates in these countries did not typically fall in concert with death rates. Population growth rates climbed to unheard-of heights: at three or four percent per year--a pace sufficient to double a country's population in about 20 years. The very different way in which the demographic transition has progressed in more and in less developed countries is clearly evident in the two examples shown in Figure 3.

In the years since 1950, 89 percent of global population growth has occurred in the developing countries. During 1995 itself, about 97 percent of world population increase will occur there. Some, perhaps several million, will migrate to industrialized countries but most of the growth will remain in the country of origin, taxing national resources and burdening local environments.

Fertility Rates

One key measure of a country's population growth, either now or in the future, is the total fertility rate or TFR, defined as the average number of children women will bear in their lifetime, based on the current age- specific rates for the specific country or region. The TFR for several representative developing countries is shown in Table 3, along with life expectancy at birth, for three periods of the past forty years. The patterns are roughly similar. All of the countries had very high fertility in the 1950s, exhibiting great potential for growth should life expectancy rise (and infant mortality fall). In the postwar period, that is precisely what happened.

All developing countries had very high TFRs as the era of population explosion began. By the early 1970s, life expectancy had begun to climb, but the reaction shown by the TFR varied considerably. In some cases, fertility remained at the 1950s level while in others it dropped somewhat, but to levels that were still very high. No simple, rigid pattern can be seen: the course of events in any country resulted from a complex mix of the level of development and of cultural mores and governmental policies regarding fertility.

In matters of population, the TFR is the key to the future. Today, women in developing countries bear an average of about 3.6 children, or 4.2 when numbers for the much-lower fertility of giant China (with over a billion people and more than 21% of the world's population) are removed. Either figure is well down from the 6.1 of the early 1950s. Does this change remove concerns about a population explosion? The answer is found in simple mathematics. If the TFR remained constant in all countries at its present level, world population would rise from 5.7 billion today to 22 billion by 2050 and to 694 billion by 2150. At that point, it would still be growing, at over four percent per year! This purely illustrative projection ignores the likely possibility of associated calamities such as famine; still, it suffices to show the purely mathematical consequences of sustained high fertility. The graph in Figure 4 includes this rather fanciful ("constant" fertility) scenario--already rising off the chart by the middle of the now approaching century. Such is the nature of population growth rates, which behave exactly like compound interest.

Global Projections

That the current state of the world birth rate will result in population growth of such magnitude places considerable importance on the art of projecting the population of the future, which can be done with mathematical tools that are not particularly complex. The most widely used projections are those of the United Nations Population Division.

Assumptions regarding fertility

The critical ingredient in population projections is the assumptions that are made in preparing them. By far the most important is that of fertility: what trend will the birth rate follow in the future of any country? For example, the TFR of Mali in West Africa is today a very high 7.3 children per mother and it shows little sign of decreasing. In India (the second most populous nation on Earth, with about 900 million people) the TFR is today down to 3.4 after many decades of government family planning efforts. Italy has the world's lowest TFR at 1.3, a level that will lead to a population decrease in that country since couples there are currently not "replacing" themselves with two children. Population projections are so dependent upon the fertility assumption that those who use them should always look first at the TFR assumption (and for countries such as the U.S., Canada and Germany, at the assumptions regarding migration) and only then at the projected results.

Partly to avoid any impression that the future can be seen in advance, the U.N. prepares a menu of long-range projections from which the user can choose (Fig. 4). Their "medium" projection assumes that all countries will ultimately settle on a replacement-level TFR of 2.1--an assumption that is, clearly, highly stylized but still quite useful. The U.N. "above replacement" and "below replacement" scenarios assume that all countries will stabilize at a TFR of 2.5 children per woman (what might be called the "Argentine" model) or at 1.7 (the "European" model).

The difference made in the long run by this seemingly small spread in TFR is striking, to say the least. The results of the lower and higher projections range from a world population of 4.3 billion to fully 28 billion, i.e. from a total less than today's 5.7 to one that is five times larger. Moreover, in 2150--the year to which projections were made--the total population in the lower of the two projections would be slowly declining (since couples have less than two children) while that in the higher projection would be slowly rising (since they have more than two children).

Unanswered questions

The key issue is replacement level fertility, or the "two child family." Is the world approaching that long-term, stable level of constant population? Without doubt, the birth rate has dropped in most countries which have experienced a shift to a more urban, or as some would say, a more "modern" society. But future birth rates are not just difficult to predict, they can be next to impossible. If we make the rather tidy assumption that in time all countries will converge on the replacement level--perhaps through forces of economic security or environmental concerns--the people on the Earth will achieve zero population growth and a stationary, global population. But the assumption is over generalized, and may prove unreal. Couples in Europe, who on average have only 1.6 children may not be motivated to increase their family size beyond this present preference. At the other end of the spectrum, when will Mali's TFR begin to decline, and what path will it then follow? Demographers have no experience of any kind with long-term fertility trends in Africa. Can we expect African countries to reach European fertility levels in the next two or three generations? Based on what has happened before, these key questions will probably all remain unanswered until the date of projection has arrived.

Finally, what can we say about countries which have achieved modest levels of fertility, such as India? Will the fertility of that populous country continue to decline to replacement level, as have those of regional neighbors such as Thailand, or will it stabilize at some higher level? Other "developing" countries, such as Argentina, have had a TFR at three or below since the 1950s and have not dropped to the approximate replacement level of 2.1. In the early 1990's, Argentina's TFR was still 2.9.

What will almost surely ensue is a mix of outcomes worldwide. Some countries may see their birth rate drop to very low levels and remain there while others find that somewhat higher fertility becomes their national norm. To date, nearly every pattern has been observed. Sweden surprised many demographers when its TFR rose to 2.1 in the early 1990s from 1.6 in previous years. It has since begun to inch back down, however. The protracted U.S. "baby boom" was also a surprise, albeit a temporary one. Cultural factors can keep fertility high in the presence of wealth and affluence, as happens today in parts of the Middle East.

Other factors

The truth is that we simply do not know in more than general terms what the future holds: which regions of the world will grow far beyond the expectations of the medium projections and which may fall below them. But a sense of urgency remains due to the compound interest nature of the problem. Population will not stop growing when couples have only two children. Most developing nations are characterized by a very young age structure that ensures that large numbers of young people will enter the parenting ages for the next several generations. For that reason, births will continue to outnumber deaths long after the replacement level has been reached. If, for example, Ethiopia (present TFR 6.9) were to reach replacement level in 2050, its population would continue to rise for decades. Under this scenario, Ethiopia's present population of 57 million would rise to 225 million (about as large as the U.S. today) by 2050 when the TFR reaches 2.1, and continue growing until it levels off, 100 years later, at 370 million.

Given the vast economic inequalities that separate richer and poorer nations, the path of future world population growth carries an enormous potential for problems and far-reaching changes: environmentally, economically, and geopolitically. There is little doubt that pressure for immigration from poor rich areas will increase, both within countries and across national borders. Migration is even now emerging as one of the world's major demographic issues, in the face of mounting resistance to continued immigration in the industrialized countries.

Finally, the effect of AIDS on mortality rates is now sufficiently defined to be taken into account in population projections by the United Nations for the highest HIV-prevalent countries in Africa. Projections that include AIDS are now made for 15 countries, based upon known levels of infection and best estimates of the population at risk. In terms of total numbers, the projected impacts are not overwhelming; nor are they inconsiderable. For four of the most seriously-afflicted countries--Mali, Rwanda, Uganda and Zambia-- population projections are reduced by about eight percent until at least the year 2005.

The United States

The U.S.--the most populous of today's developed countries-- has also one of the highest population growth rates of the industrialized nations: about one percent annually. This adds some 2.5 million people every year-- equivalent to a new city the size of San Diego-- which is by far the largest numerical increase of any developed country. By the end of 1994, the U.S. population had climbed to 262 million, up from 203 million in 1970. The change stands in sharp contrast with Europe which today adds less than a million people per year to a population of about 728 million and Japan's addition of 0.3 million to a total of 125 million.

Immigration now contributes roughly a third of the annual U.S. increase, although natural increase, or births minus deaths, remains substantial at about 1.7 million per year. This is in part the consequence of a younger age distribution resulting from the postwar baby boom (1946-1964), but U.S. fertility has also been among the highest of the industrialized countries. The U.S. TFR remained at about 1.8 from the mid-1970s to the latter 1980s, when it began rising (much on the order of Sweden's increase) to 2.1 in 1991. As in Sweden, the U.S. TFR has since receded slightly, dropping to about 2.0 in 1993.

The relatively high U.S. fertility is one of this country's most striking demographic features. Should the present rate persist, the country will remain at or near the replacement level, avoiding population decline as in Europe. Although fertility levels do vary among ethnic groups, the TFR for U.S. non-Hispanic white women in 1992 was nonetheless 1.8; for blacks, it was 2.4; Asian and Pacific Islanders, 1.9; American Indians, 2.2; and Hispanics 3.0.

The U.S. National Census

Americans have one of the longest records of continuous population censuses, beginning with the first count in 1790--required, then as now, by the U.S. Constitution for the purpose of allocating seats in the House of Representatives. The first census was undertaken by U.S. marshals who were not even provided paper for their task. They turned in a total count of 3.9 million, a number that Thomas Jefferson, then Secretary of State, decried as too low. Thus, the census began in controversy. The U.S. Census Bureau--an arm of the Department of Commerce--has always conducted the census in years ending in 0. The Bureau has earned world renown for its history of innovation in statistical techniques, including the punch card designed by Herman Hollerith in the late 1880s, and involvement in the development of the computer itself.

The census itself is a huge undertaking, as one might imagine. Counting each and every resident at his or her "place of usual residence" is no simple task. It is also obvious that any national canvass will almost certainly fail to find and enumerate all residents. The inherent obstacles change with time, but, at present, the Census Bureau must contend with the following difficulties: the fact that many people do not live in predictable housing arrangements, particularly in larger cities; the growing number of dual residences; the unwillingness of some persons to cooperate with government efforts; and the fear of contact with the government by certain groups, particularly illegal aliens. The census process begins with a comprehensive listing of residences for the mail-out, mail- back census, with follow-up by traditional, temporary enumerators for non -responsive households.

The cost of the census has been steadily rising. The price of the 1960 Census was about $500 million in constant 1990 dollars; for the 1990 Census the tab was $2.6 billion. Per housing unit, the 1960 Census cost $10, rising to $25 in constant dollars per household in 1990. A great deal of attention is now paid to each new census, in part because of the increased dependence upon census counts by cities and states for federal funds that are linked by law to census numbers.

The U.S. census process is at this time under an intensive review whose intent is to reduce undercounts and cut costs. A variety of new methods are under consideration including a method based upon administrative records and the use of a population "register" as is done in several European countries; a continuous or "rolling" census; and a census that combines traditional techniques with more limited sampling. Regardless of criticisms and in spite of the obstacles encountered, censuses remain as the one irreplaceable source for the data that are needed to both know and govern any country intelligently.

National trends

The U.S. population is undergoing at least two major shifts: a significant change in its geographic distribution and a similarly important change in ethnic composition. Immigration--accounting for a third of the present annual growth--is also an undeniable factor. The stream of immigrants into the U.S. is highly directed toward but six of the 50 states (California, with 261,000 legal immigrants in 1993; New York 151,000; Texas 67,000; Florida 61,000; New Jersey 50,000; and Illinois 47,000.) Within these, moreover, the flow is concentrated within but a few metropolitan areas.

Shifts in geographic distribution

As can be seen in Table 4, the balance of population is shifting rapidly to the South and West. The Northeast and Midwest have experienced a continuous loss of population to the South and West by migration from other areas of the U.S. These two "sun-belt" regions have also received a higher influx of immigrants.

From 1990 to 1993, the Northeast and Midwest experienced a net loss of 367,000 residents through migration to other parts of the country while gaining 260,000 immigrants from abroad. The South gained 471,000 migrants from the balance of the U.S. and another 180,000 from outside the country. The West actually lost population because of migration to other regions, because of the numerical dominance of the state of California in the region. It is likely that the loss of jobs in California in defense-related employment was responsible for this unexpected turn of events. The South and West received no fewer than 500,000 net immigration from abroad, just over half of which was to California alone.

Changes in ethnic composition

One-fourth of the present population of the U.S. is now composed of racial minorities, which are defined as anyone who is not white and non- Hispanic. In the next 50 years that fraction will grow to nearly half the total, according to recent projections. The Hispanic population is projected by the Census Bureau to rise from 22.5 million in 1990 to just under 90 million by 2050, due to the combination of immigration and higher fertility. If that projection proves true, Hispanics would increase their share of the total population to about 22% from 9% in 1990. Asian and Pacific Islanders, although a lower fertility group, would rise from 7.6 million in the 1990 Census to 41 million by 2050. Much slower growth is projected for the Black population: from 30.6 million in 1990 to 62 million by mid-century. The slower pace of growth in this group is based upon present, relatively low levels of immigration--a situation which could well change in time.

Population prospects differ regionally in yet another, often- overlooked way: the birth rate. Due to both higher birth rates and younger populations, births outnumber deaths by a wider margin in the "sun-belt" states of the South and West than in the North and East. From 1990 to 1993, for example, there were only 124 births per 100 deaths in Pennsylvania while there were 260 in California and 336 in Utah. The majority of "frost-belt" states have TFRs below the replacement level; the opposite is true in the South and West, in part because immigrants to the U.S. have generally higher fertility rates. This means, of course, that the northern states could ultimately experience population decline due to low fertility which may or may not be offset by migration. While migration trends can change rapidly, fertility rates tend to be comparatively stable.


Not only does the U.S. have a high growth rate for a developed country at present, but significant population growth likely lies ahead, with or without immigration. Census Bureau "middle series" projections, which assume that the present TFR will remain roughly stable and that net immigration will also remain at the present level of 880,000 per year, call for a total U.S. population of 392 million by 2050: an increase of about 50% over the present number. Furthermore, the population at that time will still be growing by 0.5 percent per year, which is about half the present rate (Fig. 5). Were there no immigration, however, the projected 2050 population would be 310 million (an 18% increase) and the growth rate near zero. In using U.S. projections, it is important to keep in mind that the Bureau's basic assumption is that fertility will remain roughly constant at today's approximately replacement level.

Conclusions and Consequences

What are the consequences of population growth both for the U.S. and the for the world as a whole? In the recent past, the primary concern for developing countries--where virtually all the growth is found--dealt with matters of adequate food supply. Thus far, food production has generally kept pace with population growth in all regions except Africa. But what about the future, when world population will inexorably increase?

A detailed study of world food production potential conducted by the U.N. Food and Agriculture Organization in the 1980s concluded that there is now sufficient capacity to provide for the world population that is projected by the U.N. in its middle series (TFR = 2.1) projection. Meeting this goal, however, depended upon increasing land under cultivation and also raising production per acre or hectare to about the level of a development farm in Asia--i.e., expanding the scope of the agricultural "Green Revolution." To achieve this level of production will require significant inputs of capital and fertilizers, which opens the larger issue of "sustainable development." Can developing countries modernize agriculture, industrialize and raise Gross National Product without excessive harm to the environment? There are other obvious concerns that go beyond simple food production, including food distribution and storage and associated costs; local and regional availability of freshwater; energy demands; land use; and land, air, and water quality. The ramifications of these issues in the face of continued, rapid growth in world population will pose profound policy questions for those who govern the world of tomorrow--as they do indeed today.

For the U.S., the key issues are these: the "middle series" Census Bureau projections call for an addition of about 130 million people by 2050 if current trends continue, with modest growth after that. We may also reasonably assume that immigration will continue to contribute significantly to the projected increase. The U.S. has a long history of immigration and many groups, including business interests, would lobby strenuously to maintain it as a element of expanding markets. In fact, the most recent revision to U.S. immigration law raised the number of allowed immigrants from about 550,000 to about 800,000 per year, including refugees. It is often not fully appreciated that roughly half of present immigration into the U.S. is made up of immediate relatives of those who are now residents of the country, who are entitled to come, and there is a large backlog of immigrant applications in this category.

The primary impact of U.S. population growth will be felt in the areas of the country where in recent years growth has tended to concentrate. For the most part, this can be divided into two categories: (1) high-growth states such as Florida, California, and Texas and (2) smaller metropolitan areas which have experienced accelerated growth in recent years. Cities such as Charlotte, North Carolina, have emerged as significant centers of growth with expanding employment markets. Lastly, growth around large metropolitan areas, such as New York and Washington, has deconcentrated with shifts to more distant suburbs which are seen as more attractive alternatives to city centers and which are also the focus of relocating commerce and industry. Continued growth into these regions can, in turn, reach resort areas once thought too remote, such as rural New England.

Although predicting the future is always risky, we can be fairly certain of the general trends now expected in global population in the next few decades. The world will add billions to its population, through additions made almost exclusively in the world's poorer nations. This can only be expected, since developing countries already represent most of the world's population. The United States, as well, faces the new century as the only remaining industrialized country with high-impact growth in its future. And, the United States has never lived in isolation from the rest of the world: over half of its anticipated growth is expected to come in the future from immigration, following a well-established, historical pattern. These anticipated changes, that now seem almost inevitable, loom large as the backdrop against which today's policy choices are to be made, both in and out of government.

Reviewed by Frank Hobbs and William Frey

Dr. Frank Hobbs is a Demographic Statistician in the Population Division of the U.S. Bureau of the Census. He has worked in the demographic area of the Census Bureau for the past 17 years, primarily concentrating in international population studies.

Dr. William H. Frey is a demographer and Associate Director for Training at the Population Studies Center of the University of Michigan in Ann Arbor. He has published widely on issues related to U.S. migration and urban population change.

For Further Reading

1994 World Population Data Sheet of the Population Reference Bureau. The Population Reference Bureau, Inc. Washington, D.C., 1994.

Beyond the Numbers. A Reader on Population, Consumption, and the Environment , edited by L. A. Mazur. 444 pp. Island Press, Washington, D.C., 1994.

"New Perspectives on Population: Lessons from Cairo" by Lori S. Ashford. Population Bulletin vol. 50, No. 1, 44pp, Population Reference Bureau, Washington, D.C. 1995

Population, An Introduction to Concepts and Issues , 5th Edition, by John R. Weeks. Wadsworth Publishing Company, Belmont, Calif., 1992.

"The UN Long-Range Population Projections: What They Tell Us" by Carl Haub. The Population Reference Bureau, Inc., Washington, D.C., 1992.

U.S. Bureau of the Census. Current Population Reports. Various series. Series P-25 covers U.S. population estimates and projections; Series P-20 covers socioeconomic issues such as marital status, fertility, and ethnic populations. U.S. Government Printing Office, Washington, D.C.

World Population Prospects, the 1994 Revision. United Nations Population Division, New York, 1995 (forthcoming).

Carl Haub, a demographer, is Director of Information and Education at the Population Resource Bureau, Inc., of Washington, D.C. The PRB is a private, non-profit educational organization, founded in 1929, that gathers, interprets, and disseminates information on the facts and implications of national and world population trends. He is the author of numerous articles and studies on population issues.

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