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Population

In this section, we review population and fertility trends, factors that govern fertility, and the consequences of population trends. We discuss population in the context of energy and greenhouse gas emissions in the context of energy.

Population Outlook

Today's world population is approximately 7.7 billion people and growing to a projected 9.6-10.9 billion in 2100 1. Uncertainty in the distant future depends on how fertility trends evolve.

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Recent trends suggest, however, that the UN may be underestimating future population growth in Africa 2.

Unforeseen social trends could significantly alter population forecasts 3. The advent of life extension 4 and advancement in assistive reproductive technology such as in vitro fertilization 5 might modestly increase forecasts in the 21st century. It is also unclear the extent to which the current low fertility regime will induce natural selection pressures toward large families, causing an eventual rebound 3.

The main variable governing long-term population trends is the total fertility rate: the number of children born to women on average. TFR tends to be higher in lower income countries. Sustained over a long time, a TFR above about 2.1 in countries with strong health care systems and no net immigration means that a country's population will grow, and a TFR below 2.1 means that population will shrink 6.

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Source: World Bank 7.

Since 1964, world TFR has been decreasing.

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Source: World Bank 7.

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Sources: 8, 9.

Determinants of Fertility

Broadly speaking, there are three classes of explanations for falling fertility. The cultural evolution hypothesis that modernity cultural attitudes tend to encourage fewer births 1011. The quality/quantity tradeoff holds that, as children's educational needs grow, parents make a rational decision to have few children while improving the education (quality) of those they do have 12. Modernity may reduce the impulse to have children by allowing the pursuit of competing desires, such as noncommittal romance or women's career fulfillment 13.

The following correlates with fertility rates have been observed.

Birth Rate Influencing Factors
FactorEffect
Education10% increase in primary education causes 3-4% fertility decrease in OECD countries 14
ReligionAcross countries, Christians have higher fertility than nonbelievers 15
Baby BonusOne induced birth for every $30,000 to $300,000 spent in Australia 16
Subsidized Childcare10% increase in public childcare coverage increases fertility 3.2% 17
PensionsBoth pensions such as Social Security and access to capital markets for retirement savings reduce fertility 18
Family PlanningAccess to contraception reduces births in high fertility countries, reduces abortion in all countries 19
Legalized AbortionFertility in the U.S. is 11% lower that it would be with completely outlawed abortion 20
ZoningRestrictive zoning correlated with lower fertility in the U.S. 21
UrbanizationFertility lower in urban than rural areas in developing countries 22
SuburbanizationFertility higher in suburbs than urban cores in Northern Europe 23
City SizeFertility higher in smaller cities relative to big cities 24

Select factors that correlated with fertility and observed effects. Note that many studies establish correlation but not necessarily causation.

It is debated how many births were prevented by China's One-Child Policy, the most notable coercive population control policy in history, though the policy is responsible for severe human rights abuses and long term social damage 25.

Sperm Count

Recent meta-studies have document a severe decrease in male sperm count. Although the methodology of these studies has been disputed 26, evidence favors the view that sperm counts are declining.

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Meta-studies of sperm count were conducted by Levine et al. worldwide 27, Sengupta et al. for Europe 28, Sengupta et al. for Africa 29, and Sengupta et al. for Asia 30.

Increase rates of male infertility have also been observed 31.

A cause of sperm count decline has not been definitively identified. Suggested causes of sperm count decline include bad diet 32, pesticide exposure 33, and microplastic pollution 34.

Economists generally argue that, in industrialized countries but not necessarily in poorer countries, a higher population should lead to higher per capita economic growth due to there being more researchers, more opportunity for specialization, and economies of scale 35, 36, 37, 38, 39, 40, though see Coleman and Rowthorn 41 for an alternate view.

If coupled with investments in human capital, especially education, a transition from a high birth and death rate to a lower birth and death rate can free up resources from childcare and catalyze rapid economic development 42. Following a protracted period of low birth rates, countries experience an increase in the share of the retired population, which depresses growth 43.

See also our analysis of the role of population growth in human environmental impacts and philosophical concerns.

References

  1. United Nations, Department of Economic and Social Affairs, Population Division, Population Estimates and Projections Section. "World Population Prospects: The 2017 Revision". Accessed April 18, 2019. ↩

  2. Gerland, P. et al. "World population stabilization unlikely this century". Science 10 Vol. 346 no. 6206 pp. 234-237. October 2014. ↩

  3. Burger, O., DeLong, J. "What if fertility decline is not permanent? The need for an evolutionarily informed approach to understanding low fertility". Philosophical Transactions of the Royal Society B 371(1692): 20150157. April 2016. ↩ ↩2

  4. Gavrilov, L., Gavrilova, N. "Demographic Consequences of Defeating Aging". Rejuvenation Research 13(2-3), pp. 329-334. April 2010. ↩

  5. Faddy, M., Gosden, M., Gosden, R. "A demographic projection of the contribution of assisted reproductive technologies to world population growth". Reproductive Biomedicine Online 36(4), pp. 455-458. April 2018. ↩

  6. World Health Organization. "Total Fertility Rate". Accessed June 27, 2020. ↩

  7. The World Bank. "Fertility rate, total (births per woman)". Accessed June 27, 2020. ↩ ↩2

  8. Espenshade, T., Guzman, J., Westoff, C. "The Surprising Global Variation in Replacement Fertility". Population Research and Policy Review 22, pp. 575-583. December 2003. ↩

  9. Gietel-Basten, S., Scherbov, S. "Is half the world’s population really below 'replacement-rate'?". PLoS One 14(12): e0224985. December 2019. ↩

  10. Colleran, H. "The cultural evolution of fertility decline". Philosophical Transactions of the Royal Society B 371:20150152. April 2016. ↩

  11. Newson, L., Postmes, T., Lea, S. E. G., Webley, P. "Why Are Modern Families Small? Toward an Evolutionary and Cultural Explanation for the Demographic Transition". Personality and Social Psychology Review 9(4). November 2005. ↩

  12. Galor, O. "The demographic transition: causes and consequences". Cliometrica 6, pp. 1-28. 2012. ↩

  13. Zaidi, B., Morgan, S. "The Second Demographic Transition Theory: A Review and Appraisal". Annual Review of Sociology 43, pp. 473-492. July 2017. ↩

  14. Madsen, J., Strulik, H. "Testing unified growth theory: Technological progress and the child quantity-quality tradeoff". Center for European, Governance and Economic Development Research Discussion Papers 393, University of Goettingen, Department of Economics. May 2020. ↩

  15. Herzer, D. "A Note on the Effect of Religiosity on Fertility". Demography 56, pp. 991-998. April 2019. ↩

  16. Stone, L. "Cash for Kids? Assessing the American Family Act". Institute for Family Studies. March 2019. ↩

  17. Bauernschuster, S., Hener, T., Rainer, H. "Does the Expansion of Public Child Care Increase Birth Rates? Evidence from a Low-Fertility Country". Annual Conference 2013 (Duesseldorf): Competition Policy and Regulation in a Global Economic Order 79909, Verein für Socialpolitik / German Economic Association. 2013. ↩

  18. Boldrin, M., De Nardi, M., Jones, L. E. "Fertility and Social Security". Journal of Demographic Economics 81(3), pp. 261-299. September 2015 ↩

  19. Stover, J., Winfrey, W. "The effects of family planning and other factors on fertility, abortion, miscarriage, and stillbirths in the Spectrum model". BMC Public Health 17(775). November 2017. ↩

  20. Levine, P. B., Staiger, D., Kane, T. J., Zimmerman, D. J. "Roe v Wade and American fertility". American Journal of Public Health 89(2), pp. 199-203. February 1999. ↩

  21. Shoag, D., Russell, L. Land Use Regulations and Fertility Rates. In: One Hundred Years of Zoning and the Future of Cities, Amnon Lehavi (ed.). Springer. 2018. ↩

  22. Lerch, M. "Fertility Decline in Urban and Rural Areas of Developing Countries". Population and Development Review 45(2), pp. 1-20. December 2018. ↩

  23. Kulu, H., Boyle, P., Andersson, G. "High suburban fertility: Evidence from four Northern European countries". Demographic Research 21(31), pp. 915-944. December 2009. ↩

  24. Kulu, H., Washbrook, E. "Residential context, migration and fertility in a modern urban society". Advances in Life Course Research 21, pp. 168-182. September 2014. ↩

  25. Feng, W., Gu, B., Cai, Y. "The End of China’s One-Child Policy". Studies in Family Planning 47(1), pp. 83-86. March 2016. ↩

  26. Boulicault M., Perret M., Galka J., Borsa A., Gompers A., Reiches M., Richardson S. "The future of sperm: a biovariability framework for understanding global sperm count trends". Human Fertility, pp. 1-5. April 2021. ↩

  27. Levine H., Jørgensen N., Martino-Andrade A., Mendiola J., Weksler-Derri D., Mindlis I., Pinotti R., Swan S. H. "Temporal trends in sperm count: a systematic review and meta-regression analysis". Human Reproduction Update 23(6), pp. 646-59. November 2017. ↩

  28. Sengupta P., Borges Jr E., Dutta S., Krajewska-Kulak E. "Decline in sperm count in European men during the past 50 years". Human & Experimental Toxicology 37(3), pp. 247-255. March 2018. ↩

  29. Sengupta P., Nwagha U., Dutta S., Krajewska-Kulak E., Izuka E. "Evidence for decreasing sperm count in African population from 1965 to 2015". African Health Sciences 17(2), pp. 418-427. June 2017. ↩

  30. Sengupta P., Dutta S., Tusimin M. B., İrez T., Krajewska-Kulak E. "Sperm counts in Asian men: Reviewing the trend of past 50 years". Asian Pacific Journal of Reproduction, pp. 87-92. 2018. ↩

  31. Ravitsky V., Kimmins S. "The forgotten men: rising rates of male infertility urgently require new approaches for its prevention, diagnosis and treatment". Biology of Reproduction 101(5), pp. 872-874. November 2019. ↩

  32. Nassan F. L., Jensen T. K., Priskorn L., Halldorsson T. I., Chavarro J. E., Jørgensen N. "Association of dietary patterns with testicular function in young Danish men". JAMA Network Open 3(2), e1921610. February 2020. ↩

  33. Swan S. H., Kruse R. L., Liu F., Barr D. B., Drobnis E. Z., Redmon J. B., Wang C., Brazil C., Overstreet J. W., "Semen quality in relation to biomarkers of pesticide exposure". Environmental Health Perspectives 111(12), pp. 1478-1484. September 2003. ↩

  34. Swan, S. H. "Environmental phthalate exposure in relation to reproductive outcomes and other health endpoints in humans". Environmental Research 108(2), pp. 177-184. October 2008. ↩

  35. Garza-Rodriguez, J., Andrade-Velasco, C., Martinez-Silva, K., Renteria-Rodriguez, F., Vallejo-Castillo, P. "The relationship between population growth and economic growth in Mexico". Economics Bulletin, AccessEcon, 36(1), pp. 97-107. 2016. ↩

  36. Jones, C. "The End of Economic Growth? Unintended Consequences of a Declining Population". National Bureau of Economic Research, NBER Working Paper No. 26651. January 2020. ↩

  37. Kremer, M. "Population Growth and Technological Change: One Million B.C. to 1990". The Quarterly Journal of Economics 108(3), pp. 681-716. August 1993. ↩

  38. Pegou Sibe, J., Chiatchoua, C., Noel Megne, M. "The Long Run Relationship between Population Growth and Economic Growth: a Panel Data Analysis of 30 of the most Populated Countries of the World". Análisis Económico, XXXI(77), pp. 205-218. May-August 2016. ↩

  39. Simon, J. The Ultimate Resource. Princeton University Press, ISBN 0-691-00381-5. 1981, rev. 1996. ↩

  40. Wesley, E., Peterson, F. "The Role of Population in Economic Growth". SAGE Open 7(4): 2158244017736094. 2017. ↩

  41. Coleman, D., Rowthorn, R. "Who’s Afraid of Population Decline? A Critical Examination of Its Consequences". Demographic Transition and Its Consequences 37, pp. 217-248. 2011. ↩

  42. Lutz, W., Cuaresma, J., Kebede, E., Prskawetz, A., Sanderson, W., Striessnig, E. "Education rather than age structure brings demographic dividend". Proceedings of the National Academy of Sciences of the United States of America 116(26), pp. 12798-12803. June 2019. ↩

  43. Lee, H., Shin, K. "Nonlinear effects of population aging on economic growth". Japan and the World Economy 51: 100963. September 2019. ↩