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