Uncertain population futures: Critical reflections on the IHME Scenarios of future fertility, mortality, migration and population trends from 2017 to 2100

Abstract

In July 2020 The Lancet published global scenarios of fertility, mortality, migration and population trends from 2017 to 2100 produced by the research team from the Institute for Health Metrics and Evaluation (IHME) (Vollset et al. 2020). These projections, based on a vast amount of data, complex estimates and models, have gained high visibility, also in subsequent media coverage and interviews. Yet, IHME highly publicised population data and scenarios suffer from numerous issues with the underlying data, models and scenarios as well as over-simplistic interpretations of their results. This study aims to substantiate our concerns, spelled out earlier in a letter to the Lancet (Basten, Sobotka et al. 2020), and review major issues and weaknesses with IHME projections and their interpretation. First, we critically examine the data issues, models and their underlying assumptions, including implausible uncertainty intervals and contrasts in predicted trends among countries from broader regions, which often share similar economic and cultural characteristics. Second, we discuss the interpretations of the main findings presented in the discussion part of the paper by Vollset et al. and reflect on the media representation of the projection results. In conclusion, we highlight internal inconsistencies in the IHME modelling approach, which lead to questionable projections for many countries and scenarios.

Probabilistic population forecasting: Short to very long-term

“…A different set of statistically-based probabilistic population projections for all countries to 2100 has recently been published by Vollset, et al (2020) . These have been criticized by Gietel-Basten and Sobotka (2020) and Gietel-Basten et al (2020) (the latter a letter signed by over 100 population scientists) because they suffer from numerous issues with the underlying data, models and scenarios as well as over-simplistic interpretations of their results. The most significant differences between the results of Vollset et al and those of the UN ( United Nations, 2019a ) lie in their different forecasts of fertility in high-fertility countries.…”

Growing Pains: Can Family Policies Revert the Decline of Fertility in Spain?

“…In the specific case of Spain, social research has analyzed the explanatory variables for this extremely low fertility, identifying, alongside the transformation of traditional family values common to other Western societies, specific elements in Spanish society such as job insecurity, housing problems, and unsatisfactory work-life balance mechanisms (Bueno & García Román, 2020;Castro-Martín et al, 2020;Gietel-Basten & Sobotka, 2020;Matsyak et al, 2021), as well as insufficient public aid and an erratic and incoherent architecture of family policies (Castro-Martín & Martín García, 2013;Castro-Martín et al, 2018;Esteve & Treviño, 2019;Moreno, 2008;Moreno Mínguez, 2013).…”

Impact of urbanization on mental health and well being

“…The UN world population prospect 2022 [3 && ] demonstrates that changes in methodology and protocols might substantially impact future predictions [17]. The need for open-minded uncertainty of predictions is also shared by other groups [18,19].…”