According to research in press at American Psychologist, across 174 countries and over a span of 69 years, increases in population density were associated with decreasing fertility rates. This pattern was observed for 116 of the 174 countries studied and remained even when controlling for alternative factors that could explain shifts in fertility, such as gender inequality or strength of social norms.
“Broadly, I’m interested in how people compete with one another, and how environmental factors influence competitive strategies,” explained study author Amanda Rotella (@AMRotella), a lecturer at Kingston University London. “Population density has been previously found to be related to fertility rates in human populations, however, little research has been done on why population density and fertility rates are related.”
Rotella and colleagues obtained population density and fertility rate data for the years 1950-2019 from Macrotrends, which collects and synthesizes data from a variety of reliable sources, such as the United Nations. The mid-year population divided by land area provided the population density metric per country, while number of children that would be born if the mother were to live until the end of her childbearing years (15-49 years) provided the fertility rate metric.
Numerous variables were controlled for, including socioeconomic, geographic, cultural, and female empowerment variables – for example, GDP, pathogen prevalence, religiosity, access to contraception – as well as population characteristics, such as sex ratio.
The researchers theorized that in increasingly competitive (dense) environments, people would opt to have fewer children and invest more resources per child. Contrarily, they hypothesized this effect would be weaker or eliminated in harsh environments with high risk of mortality.
They derived these predictions from life history theory which suggests that in environments with low mortality risk, whereby ability-based competition is heightened, people can enhance their competitive edge by investing more resources into long-term strategies – such as, pursuing education. This slower strategy involves fewer children, but more parental investment per child, and enhances the long-term competitiveness of both adults and their offspring.
However, harsher environments that are less stable and more unpredictable (high mortality risk, limited resources), provide less incentive to invest in long-term strategies, thus, favouring quicker strategies that involve having more children and investing less resources per child. The results of this research were consistent with life history theory.
“In harsh environments where there is higher risk of illness and death, population density does not affect fertility rates as much (compared to safer environments). We think this is because in dense and harsh environments people have children at an earlier age and have more children because it is riskier if they wait,” Rotella explained. “On the flipside, in safe environments delaying reproduction has greater advantages. In high density safe places, people delay having to compete more to get a job by achieving higher education (compared to less dense environments), so they delay reproduction and invest more time and resources in each individual child because they have to compete in a highly competitive (dense) environment.”
Given this is a correlational study, strong causation cannot be inferred from it. A second limitation the researchers note is that the current findings speak to why the relationship between population density and fertility rates exists, but not how. “We still need to understand the underlying mechanism behind this relationship,” said Rotella.
“I think this is an important question because fertility rates are declining worldwide, and our research suggests that part of the reason why is that people are leaving rural areas to live in more densely populated cities,” she added. “Often population density is left out of conversations on declining fertility rates, and we hope that this work might help policy makers, institutions, or any interested parties when they plan for changing population structures.”
The study, “Increasing population densities predict decreasing fertility rates over time: A 174-nation investigation”, was authored by Amanda Rotella, Michael E. W. Varnum, Oliver Sng, and Igor Grossmann.