Population Bulletin
Vol. 53 No. 3
September 1998

Table of Contents

Introduction

Fertility

Mortality

Migration

Population Size and Growth

Population Composition

Population Distribution

Population Growth Issues

Concern About Population

Conclusion

Suggested Resources

References

Related Publications

Population: A Lively Introduction

Population Size and Growth

Whether a population grows or wanes, the changes can be traced to the net effects of the three demographic processes already discussed: fertility, mortality, and migration.

Fertility adds members to the population and mortality removes them. Thus, the annual number of deaths in a population can be subtracted from the annual number of births to find the net number of persons added through natural increase as summarized by the population balancing equation (see Box 4). In 1997, there were 1.6 million more births than deaths in the United States. Worldwide, there were 83.7 million more births than deaths.

The death rate can be subtracted from the birth rate to find the rate of natural increase. The crude death rate for the United States in 1997 was 8.6 deaths per 1,000 inhabitants. Subtracting this from the 1997 crude birth rate of 14.6 yields a rate of natural increase of 6.0 additional persons per 1,000 inhabitants, or as it is more commonly expressed, 0.6 percent. The birth rate and the death rate for the world were 23.1 and 9.0, respectively, which produced a rate of natural increase of 1.4 percent, more than twice the U.S. rate. The rate of natural increase is added to the rate of net migration to yield the overall population growth rate.

Populations increase through migration and natural increase in most places; but populations may also decline, as they have on some Caribbean islands and in many central cities in the United States. Births, deaths, and in- and out-migrants sometimes cancel each other out and produce neither growth nor decline, as they did in the Czech Republic in 1994.

The rate of growth can be used to estimate a population's hypothetical doubling time, which is the number of years until the population will double if the rate of growth remains constant. Doubling time can be estimated by dividing the number 70 by the growth rate expressed as a percent. A population growing at 2 percent annually, for example, would double in 35 years; one growing at 1 percent would double in 70 years. When the rate of population growth is negative or zero, of course, the population will never double.

World and National Populations

World population in 1998 was 5.9 billion and was growing at a rate of about 1.4 percent annually. World population is growing today because births exceed deaths by a wide margin—by more than 80 million in 1997. Net migration, of course, is not a factor in world population growth, and it never will be unless colonizing other planets becomes a reality. Natural increase is likely to push the world population total above 9 billion by 2050.

The actual world population in 2050 or thereafter is unknown. But demographers can project the future population of the world or a country. Beginning with current estimates of population size and growth rates (see Box 6), they make assumptions—really educated guesses—about how much fertility, mortality, and migration rates will change. A country's projected population in 2050, for example, equals its current size plus the total births and immigrants expected from now until 2050 (under the assumed rates), minus the expected deaths and emigrants.

Using these basic principles, the United Nations recently projected that world population will be about 9.4 billion by 2050, assuming continued declines in fertility and mortality rates.50

China is the world's most populous nation with a 1998 population of 1.2 billion. Its population is increasing by 1.0 percent each year assuming minimal net migration. India has fewer inhabitants (989 million) but a higher annual growth rate (about 1.9 percent). India is likely to surpass China as the world's most populous country by the middle of the 21st century.

In the 1990s, most of the world's fastest growing countries are in the Middle East and Africa. Kuwait's 1998 population of 1.9 million is growing by about 3.7 percent per year including net immigration. At that rate, the population will double in 19 years unless there is a significant decline in fertility or increase in emigration. The population of the African continent is growing at 2.5 percent, yielding a doubling time of only 27 years.

In contrast, many countries are experiencing extremely slow growth and even natural decrease because death rates have risen above birth rates. Deaths exceeded births in 13 European countries (including Russia, Germany, and the Czech Republic) in the late 1990s. In some countries, net immigration provides the only population growth.

U.S. Population

The United States is the third most populous nation in the world, trailing only China and India. The U.S. population increased by an estimated 2.5 million people during 1997 because the number of births and immigrants exceeded the number of deaths and emigrants. Note the relative contribution made to growth by natural increase and net migration in Box 4. Net migration of legal and illegal migrants accounted for over one-fourth of population growth during the 1980s and about one-third of the growth during the 1990s. Because fertility and mortality are expected to remain at relatively low levels in the United States, the most volatile demographic variable driving future growth is immigration.

According to U.S. Census Bureau projections, the U.S. population could reach 394 million by 2050.51 But we should remember that these projections are based on educated guesses about future trends in fertility, mortality, and migration.

BOX 4

The Demographic Balancing Equation

Populations grow or decline as the result of three processes: birth, death, and migration. The role of these processes in population change can be depicted in what is termed the balancing equation. To show population change between 1997 and 1998, the equation would look like this:

Population in 1997

+ Births
-Deaths During
+ Immigrants 1997
- Emigrants
= Population in 1998

The births minus the deaths can also be shown as natural increase. The immigrants minus the emigrants can be shown as net migration:

             Births                                    Immigrants
          -  Deaths             &n bsp;                 - Emigrants
          = Natural increase                 = Net migration

The balancing equation is shown below for the world and the United States:

                                                       World       United States

Starting Population
          Jan. 1, 1997             5,800,202,000      266,487,000

         +          Births         + 136,967,000        + 3,882,000
         -         Deaths          -   53,282,000        - 2,294,000   
= Natural Increase         +     83,685,000     +   1,588,000

 

+ Immigrants                             NA                   + 1,124,000
- Emigrants                                                             - 277,000
= Net Migration                                                   +    847,000

Ending Population
Jan. 1, 1998                   = 5,883,887,000    = 268,922,000

Note: The migration figures do not show the total flow of migrants into and out of the United States, which is unknown.  The immigration total includes an estimated net immigration of 225,000 illegal immigrants.

Sources: Population Reference Bureau estimates based on Carl Haub and Diana Cornelius, 1998 World Population Data Sheet, and on estimates and provisional data from the Immigration and Naturalization Service, National Center for Health Statistics, and U.S. Census Bureau.

 

BOX 6

Where Do Demographic Data Come From?

Demographers use a variety of rates, ratios, and other measures to study population.1 But these measures are only as accurate as the data from which they are calculated. Where do these demographic data come from? How accurate are they?

Most demographic measures are based on counts of people or demographic events (for example, births) in a specific area during a specific time period. There were 248,709,873 residents in the United States during 1990, for example, and 1,730,000 births in Iran. Counts come from population censuses, vital registration systems, national registers, and surveys. Their accuracy varies greatly by country and even by regions within countries.

The population census forms the cornerstone of demographic analysis. In many countries the census -- an enumeration of all households—is the main source of national population data. During the 1980s, about 94 percent of the world's population was covered in a national census. But censuses usually miss a small percentage of the population, especially in hard-to-enumerate areas such as the mountains of Turkey or poor areas within some U.S. cities.

The population characteristics that censuses record also are subject to error. Residents may lie about their income or forget the exact ages of some household members.

Vital events—births, deaths, marriages, and divorces—are usually recorded in national vital registration systems. These are the source of the counts used to calculate fertility, mortality, marriage, and divorce rates. But in countries in which mothers give birth at home, or where many residents are illiterate, a large proportion of vital events are never recorded. Less than half of the world's population lives in countries that have "complete" vital registration systems. Even "complete" systems may miss up to 10 percent of a country's vital events.

A few countries have comprehensive registration systems, or national population registers, that track individuals from birth to death and record changes in their residence or marital status.

Surveys often provide estimates of demographic events where registration systems are inadequate. They also aid in developing estimates of population size during the long interval between censuses. Surveys usually collect data for a sample group within a specific geographic area. In the United States, a monthly national survey is used to track the unemployment rate as well as many demographic indicators. But surveys suffer from many of the same accuracy problems as censuses and registration systems, and their data are subject to varying degrees of error.

Demographers have developed statistical techniques to overcome some of the shortcomings of the basic data with which they work. They apply these techniques to the best data available to compute estimates of the actual population counts and measures. Although estimates based on good data can be quite accurate, users of these estimates should not forget that estimates are only approximations of the true number.

Likewise, users of demographic data always should question the source and quality of the data that underlie the rates, ratios, and proportions they cite. Judging the quality of data is one of the most important skills demographers must learn. Indeed, everyone would benefit from taking a hard look at the myriad of statistics we encounter daily.

Reference

1.  Arthur Haupt and Thomas T. Kane, Population Handbook, 4th ed. (Washington, DC: Population Reference Bureau, 1997).


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