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expert reaction to study using modelling to predict decline in global birth rate

A study published in The Lancet looks at a decline in global birth rates.


Prof Melinda Mills, Professor of Demography and Population Health, Oxford Population Health, and Director, Leverhulme Centre for Demographic Science & Demographic Science Unit, University of Oxford, said:

“This work synthesises a vast amount of global data sources and applies advanced modelling to provide a comprehensive summary of global fertility from 1950 to present and forecasting up to 2010. The approach and results are similar to recent United Nations and similar estimates that highlight recent and future shifts in fertility and with that, dynamics of growing and shrinking populations across the globe.

“As with previous estimates, they forecast future fertility trends while also making different assumptions to adjust for changes such as if there were shifts in contraceptive use, child mortality, women’s education and urbanisation. With that, they face similar limitations of previous studies in the certainty of their assumptions. This includes certainty about whether women may increasingly gain access to effective contraception or education in sub-Saharan Africa which could lower fertility, or the assumption that there will be continued difficulties to reconcile childbearing with employment in low-fertility settings in East Asia and Western Europe. There is also uncertainty surrounding the impact of external factors such as climate change, war or pandemics on populations.

“The implications the authors focus on for high-fertility; low-income countries are threats of food security, health and geopolitics. For low-fertility, high-income countries with shrinking populations, they discuss maintaining population size through parental support and more open immigration.

“The study implicitly takes the position that shrinking populations or maintaining an equilibrium is a given, whereas it is debatable if that is the key issue. The highest consumption occurs within some of the wealthiest and shrinking populations, which is more pressing than absolute population numbers. The study’s focus on fertility and emphasizing comparative births in sub-Saharan Africa in 2050 and beyond needs a broader context of global population numbers, to avoid the tendency to create a demographic divide. Put into a broader context, by 2050 the UN predicts that India, China and the US will still be the three most populated countries, with others such as Indonesia, Pakistan and Brazil following. 

“They do not look in detail about biological aspects or changes in infertility related to birth postponement. A key is not only about equilibrium but, about whether women and couples are able to realise their fertility intentions. In low-fertility societies, such as Western Europe and East Asia, the known barriers to having children and reasons behind shrinking fertility are inability to combine employment with family, lack of affordable childcare, economic uncertainty, problems with finding a partner, but also housing. Whereas in high fertility societies, the issues are around the ability to obtain access to effective contraception, education and reproductive and women’s rights.

“Shrinking and aging populations demand preparedness and reorganisation of societies. From impacted food security and migration patterns to the very infrastructures of countries. Population composition affects infrastructure such as schools, housing, transport, housing and health care and pensions but also cultural and voting changes.”


The following comment was provided by our friends at the Spanish SMC:


Prof Teresa Castro Martín, Research Professor at the Institute of Economics, Geography and Demography (IEGD) of the Spanish National Research Council (CSIC), said:

“In 2020, the same research team at the Institute for Health Metrics and Evaluation (IHME) also published highly publicised global population projections in the Lancet. That study received some criticism from demographers for methodological inconsistencies.

“The most widely used population projections globally are still those produced biannually by the United Nations Population Division (latest: World Population Prospects 2022) and the Wittgenstein Centre for Demography and Global Human Capital (Austria). Those produced by the IHME are more publicised, but it is noticeable that the main authors (out of a team of more than 500 contributing authors) are not fertility experts.

“Nevertheless, the study illustrates well the expected trends in the near future (2050) and the more distant future (2100): a sustained decline in fertility rates globally and in almost all countries. This study estimates a decline in fertility worldwide, and especially in sub-Saharan Africa, faster than the United Nations. The Lancet study predicts that the global fertility rate will fall below replacement level (2.1 children per woman) around 2030, whereas the UN predicts this to occur around 2050.

“An important contribution of the study is to highlight the demographic contrast between the richest countries (with very low fertility) and the poorest countries (with still high fertility). Globally, births will be increasingly concentrated in the areas of the world most vulnerable to climate change, resource scarcity, political instability, poverty and infant mortality.

“Another important contribution is an estimate of the impact that family policies – such as extending parental leave, making nursery schools universal, providing childcare support or facilitating access to assisted reproductive treatment – could have in countries with very low fertility. The impact is estimated to be modest, but could prevent further fertility decline in these societies.

“The limitations of the study are those of all population projections. The margin of error depends on the data, assumptions and models used and increases progressively with the time horizon of the projection. For this reason, it is standard practice to revise them periodically.”



Global fertility in 204 countries and territories, 1950-2021, with forecast to 2100: a comprehensive demographic analysis for the Global Burden of Disease Study 2021’ by GBD 2021 Fertility and Forecasting Collaborators was published in The Lancet at 23:30 UK TIME on Wednesday 20 March 2024.


DOI: 10.1016/s140-6736(24)00490-2



Declared interests

Prof Teresa Castro Martín: No response as to whether or not she has a conflict of interest.

Prof Melinda Mills: no conflicts to declare.

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