What is driving the global decline of human fertility? Need for a multidisciplinary approach to the underlying mechanisms

Abstract

An intense period of human population expansion over the past 250 years is about to cease. Total fertility rates are falling dramatically all over the world such that highly industrialized nations, including China and the tiger economies of SE Asia, will see their populations decline significantly in the coming decades. The socioeconomic, geopolitical and environmental ramifications of this change are considerable and invite a multidisciplinary consideration of the underlying mechanisms. In the short-term, socioeconomic factors, particularly urbanization and delayed childbearing are powerful drivers of reduced fertility. In parallel, lifestyle factors such as obesity and the presence of numerous reproductive toxicants in the environment, including air-borne pollutants, nanoplastics and electromagnetic radiation, are seriously compromising reproductive health. In the longer term, it is hypothesized that the reduction in family size that accompanies the demographic transition will decrease selection pressure on high fertility genes leading to a progressive loss of human fecundity. Paradoxically, the uptake of assisted reproductive technologies at scale, may also contribute to such fecundity loss by encouraging the retention of poor fertility genotypes within the population. Since the decline in fertility rate that accompanies the demographic transition appears to be ubiquitous, the public health implications for our species are potentially devastating.

Keywords: demographic transition; female education; human population; infertility; natural selection; reproductive toxicants; total fertility rate; urbanization.

Figure 1

Global changes in total fertility rate and prosperity. (A) Since 1960 the world has witnessed a brief surge in TFR (the average number of children a woman is projected to give birth to in her reproductive lifetime) which peaked in 1963 and then commenced a dramatic decline towards replacement level, which is conventionally defined as 2.1 children per woman. (B) The fall in TFR was associated with a global increase in prosperity measured in terms of Gross Domestic Product (GDP). (C) A plot of TFR against GDP shows that just a modest increase in the latter can precipitate a marked fall in TFR. Source: World Bank Open data retrieved from https://data.worldbank.org/.

Figure 2

Changes in TFR within the tiger economies of SE Asia. Global fertility decline has been particularly marked in the tiger economies of SE Asia including (A) Hong Kong. (B) South Korea (C) Singapore and (D) Taiwan. In all these countries, the dramatic economic growth we have witnessed over the past half century has resulted in a significant decline in fertility rates that have extended to below the replacement level threshold, with no sign of a resurgence. Source: World Bank Open data retrieved from https://data.worldbank.org/.

Figure 3

Total fertility rate and GDP in Sub-saharan Africa. (A) Changes in TFR from 1960 to 2021 in Sub-Saharan (S-SA) reveal an early rise, followed by a steady fall that shows no signs of abating. (B) Over the same period there was a progressive increase in GDP measured in current US$. (C) A plot of TFR against GDP indicating that once the limited phase of population growth was over, TFR values fell rapidly in response to a very small increase in GDP. Source: World Bank Open data retrieved from https://data.worldbank.org/.

Figure 4

Impact of policy on fertility decline in China and India. (A) Fertility rates were falling in China well before the one-child-family policy was introduced in 1979–1980. (B) The one-child-family policy did however lead to an absolute decline in the number of young girls less than 14 years of age in the Chinese population, whereas this cohort increased in size in India, peaking around 2011. (C) When expressed as a function of the total female population, the proportion of girls less than 14 years old declined in both Indian and Chinese populations because the demographic transition led to a change in age structure favouring the older adult. However, the impact of the one-child-family policy is evidenced by an acceleration in the rate at which the proportion of young females in the overall population decreased in China, seriously compromising this country's demographic momentum. As a result, population numbers will decline more rapidly in China than India. Source: World Bank Open data retrieved from https://data.worldbank.org/.

Figure 5

Impact of contraceptive use on fertility decline. (A) There is a clear negative correlation between TFR and contraceptive use; data for 2000–2001. (B) The relationship between contraceptive use and TFR is not necessarily causative. Fertility rates fell in Japan long before the oral contraceptive pill became available in 1999. (C) Similarly, in Albania fertility rates fell dramatically between 1960 and 2020 and were not significantly impacted by the legalization of contraception in 1992. Sources: United Nations Department of Economic and Social Affairs, Population Division (18) and World Bank Open data retrieved from https://data.worldbank.org/.