BEYOND SIX BILLION

Forecasting the World's Population

Panel on Population Projections

John Bongaarts and Rodolfo A. Bulatao, Editors

Committee on Population

Commission on Behavioral and Social Sciences and Education

National Research Council

NATIONAL ACADEMY PRESS
Washington, D.C.



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BEYOND SIX BILLION: Forecasting the World's Population BEYOND SIX BILLION Forecasting the World's Population Panel on Population Projections John Bongaarts and Rodolfo A. Bulatao, Editors Committee on Population Commission on Behavioral and Social Sciences and Education National Research Council NATIONAL ACADEMY PRESS Washington, D.C.

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BEYOND SIX BILLION: Forecasting the World's Population NATIONAL ACADEMY PRESS 2101 Constitution Ave., N.W. Washington, D.C. 20418 NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the panel responsible for the report were chosen for their special competences and with regard for appropriate balance. This study was supported by grants to the National Academy of Sciences from the Andrew W. Mellon Foundation, the David and Lucile Packard Foundation, the Rockefeller Foundation, the United States Agency for International Development, and the William and Flora Hewlett Foundation. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the organizations or agencies that provided support for the project. Suggested citation: National Research Council (2000) Beyond Six Billion: Forecasting the World's Population. Panel on Population Projections. John Bongaarts and Rodolfo A. Bulatao, eds. Committee on Population, Commission on Behavioral and Social Sciences and Education. Washington, D.C.: National Academy Press. Library of Congress Cataloging-in-Publication Data National Research Council (U.S.) Beyond six billion : forecasting the world's population / Panel on Population Projections, Committee on Population, Commission on Behavioral and Social Sciences and Education, National Research Council; John Bongaarts and Rodolfo A. Bulatao, editors. p. cm. ISBN 0-309-06990-4 (hard) 1. Population forecasting. I. Title: Beyond 6 billion. II. Bongaarts, John, 1945- III. Bulatao, Rodolfo A., 1944- IV. Title. HB849.53 .N385 2000 304.6′2′0112--dc21 00-009983 Additional copies of this report are available fromNational Academy Press , 2101 Constitution Avenue, N.W., Lockbox 285, Washington, D.C. 20055; (800) 624-6242 or (202) 334-3313 (in the Washington metropolitan area); Internet, http://www.nap.edu Printed in the United States of America Copyright 2000 by the National Academy of Sciences. All rights reserved. Cover: Hands, copyright The Stock Market/Don Mason, 2000, and view of the earth from space, NASA.

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BEYOND SIX BILLION: Forecasting the World's Population THE NATIONAL ACADEMIES National Academy of Sciences National Academy of Engineering Institute of Medicine National Research Council The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. Upon the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters. Dr. Bruce M. Alberts is president of the National Academy of Sciences. The National Academy of Engineering was established in 1964, under the charter of the National Academy of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in the selection of its members, sharing with the National Academy of Sciences the responsibility for advising the federal government. The National Academy of Engineering also sponsors engineering programs aimed at meeting national needs, encourages education and research, and recognizes the superior achievements of engineers. Dr. William A. Wulf is president of the National Academy of Engineering. The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the services of eminent members of appropriate professions in the examination of policy matters pertaining to the health of the public. The Institute acts under the responsibility given to the National Academy of Sciences by its congressional charter to be an adviser to the federal government and, upon its own initiative, to identify issues of medical care, research, and education. Dr. Kenneth I. Shine is president of the Institute of Medicine. The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy's purposes of furthering knowledge and advising the federal government. Functioning in accordance with general policies determined by the Academy, the Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is administered jointly by both Academies and the Institute of Medicine. Dr. Bruce M. Alberts and Dr. William A. Wulf are chairman and vice chairman, respectively, of the National Research Council.

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BEYOND SIX BILLION: Forecasting the World's Population PANEL ON POPULATION PROJECTIONS JOHN BONGAARTS (Chair), The Population Council, New York City JUHA M. ALHO, Department of Statistics, University of Joensuu, Finland ALAKA M. BASU, Division of Nutritional Sciences, Cornell University JOHN G. CLELAND, Centre for Population Studies, London School of Hygiene and Tropical Medicine JOEL E. COHEN, Laboratory of Populations, Rockefeller University and Columbia University KENNETH H. HILL, Department of Population and Family Health Services, Johns Hopkins University NICO KEILMAN, Department of Economics, University of Oslo, Norway RONALD D. LEE, Department of Demography, University of California, Berkeley MASSIMO LIVI-BACCI, Department of Statistics, University of Florence, Italy DOUGLAS S. MASSEY, Department of Sociology, University of Pennsylvania S. PHILIP MORGAN, Department of Sociology, Duke University ALBERTO PALLONI, Department of Sociology, University of Wisconsin, Madison ANNE R. PEBLEY, School of Public Health and Department of Sociology, University of California, Los Angeles SHARON STANTON RUSSELL, Center for International Studies, Massachusetts Institute of Technology WARREN C. SANDERSON, Department of Economics, State University of New York, Stony Brook THOMAS SCHELLING, School of Public Affairs, University of Maryland, College Park MICHAEL TEITELBAUM, Alfred P. Sloan Foundation, New York City JAMES W. VAUPEL, Max Planck Institute for Demographic Research, Rostock, Germany RODOLFO A. BULATAO, Study Director HOLLY E. REED, Research Associate ELIZABETH A. WALLACE, Project Assistant

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BEYOND SIX BILLION: Forecasting the World's Population COMMITTEE ON POPULATION JANE MENKEN (Chair), Institute of Behavioral Sciences, University of Colorado, Boulder CAROLINE H. BLEDSOE,* Department of Anthropology, Northwestern University JOHN BONGAARTS, The Population Council, New York City ELLEN BRENNAN-GALVIN, Population Division, United Nations, New York City JOHN N. HOBCRAFT, Population Investigation Committee, London School of Economics F. THOMAS JUSTER, Institute for Social Research, University of Michigan, Ann Arbor CHARLES B. KEELY, Department of Demography, Georgetown University DAVID I. KERTZER, Department of Anthropology, Brown University DAVID A. LAM, Population Studies Center, University of Michigan, Ann Arbor LINDA G. MARTIN,* The Population Council, New York City MARK R. MONTGOMERY,* The Population Council, New York City, and Department of Economics, State University of New York, Stony Brook W. HENRY MOSLEY, Department of Population and Family Health Sciences, Johns Hopkins University ALBERTO PALLONI, Center for Demography and Ecology, University of Wisconsin, Madison JAMES P. SMITH, RAND, Santa Monica, California BETH J. SOLDO,* Population Studies Center, University of Pennsylvania JAMES W. VAUPEL, Max Planck Institute for Demographic Research, Rostock, Germany KENNETH W. WACHTER, Department of Demography, University of California, Berkeley LINDA J. WAITE, Population Research Center, University of Chicago BARNEY COHEN, Director *Through October 1999.

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BEYOND SIX BILLION: Forecasting the World's Population Preface Population projections are the demographic outputs most used by nondemographers and most neglected by population scientists. Nondemographers may be surprised at the lack of a rigorous theoretical or even historical basis for the scenarios underlying the most commonly used projections. The task of making world projections—assessing the plausibility of current demographic estimates and choosing appropriate assumptions about future trends—are left by default to the small projection staffs of the United Nations Population Division, the World Bank, the U.S. Census Bureau, and similar agencies. These staffs, while qualified to do so, have neither the time nor the resources to conduct new research or fully evaluate current scientific work on the whole range of assumptions that undergird projections. The wider public may take notice when projections change, though they may not understand the reason. When the 1996 United Nations projections were released, the medium-variant projection for the year 2050 was smaller by 466 million persons than the projection for the same year made in 1994. This change was widely reported as evidence that population growth was not as big a problem as had been previously thought. In fact, the projected annual rate of growth had been changed only slightly. And both the earlier and the later projections had in any case quite wide and overlapping bands of uncertainty around them, as the producers of the projections understood, but most consumers may not have realized. Recognizing the desirability of both more systematic research atten-

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BEYOND SIX BILLION: Forecasting the World's Population tion to population projections and better public understanding of them, the National Research Council's Committee on Population, at the request of several sponsors, convened a panel of experts in 1998 to look at projections in greater depth. The panel was asked to examine in detail the demographic assumptions, both explicit and implicit, that underlie world population projections. The panel was not asked to produce alternative projections but to firm up the scientific foundation for these continuing efforts through a thoughtful review of projection methods and assumptions and an assessment of recent research on fertility, mortality, and migration that has relevant implications. The panel was also asked to review existing population projections for accuracy and to recommend improvements where appropriate. Finally, the panel was asked to develop a research agenda that would direct attention to areas in which progress might be of benefit to projections. The panel met five times over 18 months to review the relevant knowledge base and formulate its conclusions and recommendations. This report summarizes the panel's work, reflecting its deliberations and presenting its recommendations. We have introduced an innovation, where committee reports are concerned, in publishing this report. For reasons of convenience and cost, the printed volume contains the body of the report but not the supporting technical appendices. Instead, the appendices are available on the National Academy Press web site ( http://www.nap.edu) and can also be printed on demand, at cost. This report represents the collaborative efforts of the members of the Panel on Population Projections, whose names appear at the front. We are grateful to them for their dedication and willingness to review material, prepare drafts, and deliberate long hours on fine points in the report. One deserves special recognition. We were extremely fortunate to have been able to enlist the services of John Bongaarts, who chaired the panel superbly, formulated and promoted a sound framework for the report, and devoted many hours attending to its progress. The panel benefited from essential information, data, clarification, and advice provided by liaison representatives of the agencies that produce world projections: Joseph Chamie and Larry Heligman of the United Nations Population Division, Eduard R. Bos of the World Bank, Peter Way of the U.S. Census Bureau, and Wolfgang Lutz of the International Institute for Applied Systems Analysis. Census Bureau data were also provided by Patricia Rowe and Thomas McDevitt. Background work on long-term fertility was commissioned from John C. Caldwell, Bamikale Feyisetan, Peng Xi Zhe, Ignez Helena Oliva Perpetuo, and Laura Lidia Rodriguez Wong. Assistance in statistical modeling was provided by Anne Ruuskanen at the University of Joensuu and

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BEYOND SIX BILLION: Forecasting the World's Population Gretchen Stockmayer at the University of California, Berkeley. Informal reviews of preliminary statistical work were provided by Michael Stoto and Hania Zlotnik. The panel gratefully acknowledges financial support from several sponsors: the Andrew W. Mellon Foundation, the David and Lucile Packard Foundation, the Rockefeller Foundation, the William and Flora Hewlett Foundation, and the United States Agency for International Development. Partial support for statistical modeling was also provided through a grant from the Academy of Finland. Finally, no project of this magnitude could be undertaken without a well-managed and able staff. In particular, the role of Randy Bulatao as the study director was especially important. He worked closely with panel members in drafting and revising all the chapters, planned the panel meetings and coordinated the interchange among panelists, and contributed an extensive and valuable new analysis of errors in past population projections. In addition, Holly Reed prepared an appendix on projection software, and Elizabeth Wallace organized the panel's various meetings and provided essential administrative support. Christine L. McShane skillfully edited the report and provided other valuable assistance in preparing it for publication. This report has been reviewed in draft form by individuals chosen for their diverse perspectives and technical expertise, in accordance with procedures approved by the Report Review Committee of the National Research Council. The purpose of this independent review is to provide candid and critical comments that will assist the institution in making the published report as sound as possible and to ensure that the report meets institutional standards for objectivity, evidence, and responsiveness to the study charge. The review comments and draft manuscript remain confidential to protect the integrity of the deliberative process. We thank the following individuals for their participation in the review of this report: Brian J.L. Berry, School of Social Science, University of Texas, Dallas; John C. Caldwell, National Centre for Epidemiology and Population Health, Australian National University, Canberra; Jean-Claude Chesnais, Institut National d'Etudes Démographiques, Paris; Henri Leridon, Institut National d'Etudes Démographiques, Paris; John Long, Population Division, U.S. Census Bureau; Geoffrey McNicoll, Population Council, New York, and Australian National University, Canberra; Samuel H. Preston, School of Arts and Sciences, University of Pennsylvania; Andrei Rogers, Institute of Behavioral Science, University of Colorado, Boulder; Michael Stoto, School of Public Health and Health Services, George Washington University; and Shripad Tuljapurkar, Mountain View Research, Inc., Los Altos, California.

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BEYOND SIX BILLION: Forecasting the World's Population Although the individuals listed above provided constructive comments and suggestions, it must be emphasized that responsibility for the final content of this report rests entirely with the authoring panel and the institution. Jane Menken, Chair Committee on Population

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BEYOND SIX BILLION: Forecasting the World's Population Contents     EXECUTIVE SUMMARY   1      Current World Projections,   2      Accuracy of Past Projections,   3      Fertility,   5      Mortality,   7      International Migration,   8      The Uncertainty of Projections,   9      Implications,   11  1   INTRODUCTION   15      Overview of World Projections,   17      Forces Driving Population Growth,   24      How Population Projections Are Made,   29      All Projections Suffer from Uncertainty,   32      Guide to the Report,   34      References,   35  2   THE ACCURACY OF PAST PROJECTIONS   37      Projected Population Size,   38      Correlates of Projection Errors,   40      Projected Age Structures,   45      Projected Component Rates,   46      Conclusions,   50      References,   52

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BEYOND SIX BILLION: Forecasting the World's Population  3   TRANSITIONAL FERTILITY   53      Fertility Change in Developing Regions,   53      Reasons for Fertility Decline,   56      Current Methods of Projecting Fertility,   63      Fertility Transition in the 21st Century,   68      Conclusions,   75      References,   78  4   POSTTRANSITION FERTILITY   83      Fertility Levels and Past Trends,   84      Projected Fertility Trends,   87      Interpreting Fertility Trends,   91      Explaining Fertility Trends,   97      Possible Policy Responses,   101      Future Technological Developments,   104      Conclusions,   106      References,   108  5   MORTALITY   114      Current Levels of Life Expectancy,   115      Mortality Transition,   117      Mortality Projections,   127      Future Trends in Life Expectancy,   135      Conclusions,   146      References,   150  6   INTERNATIONAL MIGRATION   156      Current Levels and Trends,   157      Future Migration Trends,   168      Projecting Migration,   174      Improving Migration Projections,   177      Conclusions,   182      References,   185  7   THE UNCERTAINTY OF POPULATION FORECASTS   188      The Scenario Approach and Its Problems,   190      Thinking About Forecast Errors,   194      Three Approaches to Constructing Predictive Distributions,   200      New Estimates of Uncertainty Based on Ex Post Analysis,   206      Conclusions,   214      References,   216     BIOGRAPHICAL SKETCHES   218

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BEYOND SIX BILLION: Forecasting the World's Population     INDEX   225     APPENDICES*    A   Computer Software Packages for Projecting Population,   237  B   Accuracy of Population Projections from the 1970s to the 1990s,   254  C   Predicting the Pace of Fertility Decline,   303  D   The Effect of Projection Error in Life Expectancy,   315  E   Simulating Migration Projections,   318  F   Estimating Expected Errors from Past Errors,   326 *Appendices are not printed in this volume but are available online. Go to http://www.nap.edu and search for Beyond Six Billion.

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BEYOND SIX BILLION: Forecasting the World's Population Tables and Figures TABLES  1-1   Population projections by four international agencies for the world and major regions,   21  1-2   Population of the world's 10 largest countries, 1995 and 2050,   22  1-3   Percent of population aged 65 and over for the world and major regions,   23  3-1   Difference between projected total fertility and 1998 estimate,   66  4-1   Low-fertility countries by region, their populations, and their shares of world population,   85  4-2   Classification of countries by total fertility in 1950-1955 and 1990-1995,   89  4-3   Change in average total fertility by birth order, and the amount of change due to tempo effects: Italy and the United States,   97  5-1   Factors expected to affect life expectancy trends in developing countries,   140  5-2   Number of cases of life expectancy decline in the period 1950-1995,   142  6-1   Foreign-born population by world region, 1965-1990,   158  6-2   Net migration per thousand by world region, 1985-1995,   161

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BEYOND SIX BILLION: Forecasting the World's Population  B-1   Error in projected population: Means across countries, by forecast and projection length,   260  B-2   Error in projected population by target year,   262  B-3   Alternative proportional-error measures adjusted for erroneous base estimates,   269  B-4   Error in projected component rates,   270  B-5   Error in net migration rate by target period,   273  B-6   Analyses of covariance for absolute proportional error in projected population,   276  B-7   Deviations from the grand mean for absolute percentage error in projected population,   278  B-8   Deviations from the grand mean for absolute percentage error in base population,   280  B-9   Regressions to account for proportional error in projected population,   281  B-10   Summary of analysis of covariance for absolute error in projected component rates,   284  B-11   Summary of analysis of variance for absolute error in base component rates,   285  B-12   Adjusted deviations from mean absolute error in component rates,   286  B-13   Proportional error in projected world population,   291  B-14   Absolute proportional error in projected world population,   292  C-1   Cases available for analysis of fertility transition,   305  C-2   Means and standard deviations,   306  C-3   Correlations between rates of fertility change by period,   308  C-4   Regressions for change in total fertility between 5-year periods during midtransition,   310  C-5   Regressions for change in total fertility between 5-year periods during pre- and early transition,   312  C-6   Regressions for change in total fertility between 5-year periods during late transition,   313  D-1   Selected population outcomes in simulated projections with varying gains in life expectancy,   317  E-1   Mean error in migration projected in different ways,   320  E-2   Mean absolute error in projected net migration rate,   321  F-1   Regional classification used in estimating uncertainty,   334  F-2   Estimates of within-region correlations of errors,   336

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BEYOND SIX BILLION: Forecasting the World's Population  F-3   Upper endpoints of 95-percent prediction intervals for relative error and multipliers to obtain the average regional scale,   339  F-4   Ratio of the upper endpoint of the 95-percent prediction interval to the median world forecast,   342  F-5   U.N. forecast for 2050 and ratios of upper and lower bounds to 50-year medium or median forecast,   343  F-6   Quantiles of the predictive distribution for world population in 2010, 2030, and 2050,   344  F-7   Quantiles of the predictive distribution for world population, adjusted for possible errors and possible interregional correlation,   346 FIGURES  1-1   World population size: Historical estimates and alternative projections,   18  1-2   World population growth rates and annual increments, 1950-2050,   20  1-3   Percentage change in population between 2000 and 2050 in standard projections and change due to population momentum alone,   28  2-1   U.N. forecasts of world population in the year 2000 and their percentage error,   39  2-2   Mean bias and mean absolute error in country population projections for the year 2000,   40  2-3   Mean absolute error in country population projections,   43  2-4   Mean percentage error by age group and projection length: Europe and Northern America combined,   46  2-5   Mean percentage error by age group and projection length: Asia, Africa, and Latin America combined,   47  2-6   Variance of the error in projected population accounted for by different factors,   49  3-1   Percentage of countries that have started fertility decline by a given date,   54  3-2   Estimated and projected total fertility rates by region: 1950-2050,   56  3-3   Percentage of population literate in countries starting fertility transition,   62  3-4   Trend in total fertility for developing regions and earlier projected trends, 1960-2000,   67

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BEYOND SIX BILLION: Forecasting the World's Population  4-1   Past and projected total fertility in low-fertility countries, 1950-2050,   86  4-2   Error in projected total fertility averaged across countries and forecasts,   91  4-3   Observed total fertility and total fertility adjusted for the tempo effect: Italy and the United States, 1950-1995,   95  5-1   Estimated and projected life expectancy by region, 1950-2050,   116  5-2   Age patterns of female mortality and life expectancies, Sweden, 1900-1996,   116  5-3   Percentage declines in mortality among females by age, Sweden, 1900-1996,   121  5-4   Historical trends in life expectancy, England and Wales and Sweden, 1540-1996,   123  5-5   Mean annual gains in life expectancy for industrial countries and three groups of developing countries, 1955-1995,   126  5-6   Currently estimated trend in world life expectancy and various projections,   131  5-7   Mean error in projected life expectancy, across countries and forecasts,   132  5-8   Observed U.S. life expectancies and various projections by the U.S. Social Security Administration, 1930-2030,   134  5-9   Impact of HIV/AIDS: Estimated and projected life expectancy in South Africa and Zimbabwe,   144  6-1   Percentage of regional populations who are migrants, and regional shares of world migrant stock, 1990,   160  6-2   Net migration rate, 1990-1995, by percentage of labor force in agriculture,   168  6-3   Natural increase and net migration rate per thousand by region, 1990-1995,   173  6-4   Absolute error from projecting net migration rate as zero, constant, or differentiated,   176  7-1   Percentage of times the projected U.N. high-low interval encloses the actual subsequent population,   193  7-2   Absolute proportional error in world and country projections, by projection length,   197  7-3   Correlations between regional errors in projected population over the period 1970-1995,   199  7-4   Four stochastic sample paths for industrial-region total fertility and 95-percent prediction interval,   205

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BEYOND SIX BILLION: Forecasting the World's Population  7-5   Estimated 95-percent prediction interval for population projected 10 years: 13 large countries,   209  7-6   Estimated 95-percent prediction interval for population projected 50 years: 13 large countries,   210  7-7   Estimated 95-percent prediction interval for population projected 50 years: 10 regions and the world,   211  7-8   Projected world population: U.N. projections and estimated 95-percent prediction interval,   213  B-1   Proportional error and absolute proportional error,   263  B-2   Proportional error by projection length: Percentiles for all countries,   264  B-3   Population trends in five countries experiencing demographic quakes,   265  B-4   Proportional error and absolute proportional error by the occurrence of demographic quakes,   266  B-5   Proportional error by region,   267  B-6   Absolute proportional error by region,   268  B-7   Error in projected total fertility,   271  B-8   Error in projected life expectancy,   272  B-9   Error in projected net migration rate,   274  B-10   Percent of variance in proportional error in projected population explained by proportional error in base population and error in component rates,   282  B-11   Deviations from mean absolute error in projected and base total fertility,   288  B-12   Deviations from mean absolute error in projected and base life expectancy,   289  B-13   Deviations from mean absolute error in projected and base net migration rate,   290  B-14   Mean country proportional error and world proportional error,   293  B-15   Proportion of country error unoffset in world projections,   294  B-16   World total fertility rate in various forecasts,   295  B-17   World life expectancy in various forecasts,   297  C-1   Five-year change in total fertility as a function of initial level,   307  C-2   Simulated trends in midtransition total fertility,   311  E-1   Net migrants and absolute error assuming constant net migrants, by country,   322  E-2   Mean absolute error from projecting net migrants under zero, constant, or differentiated assumptions,   324

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BEYOND SIX BILLION: Forecasting the World's Population BEYOND SIX BILLION

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