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.

Fertility rates in high-income countries have been declining for decades. Few children have been born for thirty years and, as a result, few women are of childbearing age today. As the article points out, this means that we are facing increasingly ageing societies. This may have economic and social consequences, such as the sustainability of public pension systems and the financing of welfare states. However, given the current scenario, I would not go so far as to say that the welfare state is in danger in the sense that it will disappear, but there will certainly be a change and the system of the future will be very different from the one we know today. There is in fact already a silent transformation towards models in which there is a basic figure, everyone receives the same pension and the extras depend on private pension plans or those established by collective agreements.  

For the Spanish case in particular, the problem is not so much the lack of workers as the low productivity of the Spanish economic system. Spain has a labour market that is very biased towards sectors with low productivity and little added value, as in the case of construction, and there is very little investment in R+D. We currently have the most educated young generations in history, but they are generations that have suffered several economic crises. Moreover, these young people have a very temporary labour market and very low salaries, so their contributions are also very low. So, it is true that demography is not very good for sustaining the current pension system, but it is not the culprit, but rather the lack of political action and the structural conditions of the Spanish labour market aggravate the problem.