Increasing Population Densities Predict Decreasing Fertility Rates over Time: A 174-nation Investigation

Rotella, Amanda M., Michael E. W. Varnum, PhD, Oliver Sng, and Igor Grossmann. 2020. “Increasing Population Densities Predict Decreasing Fertility Rates over Time: A 174-nation Investigation.” PsyArXiv. August 5. doi:10.31234/osf.io/zpc7t

Abstract: Fertility rates have been declining worldwide over the past five centuries, part of a phenomenon known as “the demographic transition”. Prior work suggests that this decline is related to population density, however to date this work has either been largely atheoretical or cross sectional. We draw on life history theory to examine the relationship between population density and fertility over time both within and between countries across 174 countries over 69 years (1950 to 2019) using annual data. Using state-of-the-art methods, we find a robust relationship between density and fertility; increased population densities are associated with lower fertility rates, controlling for a variety of socioeconomic, socioecological, geographic, cultural, population-based, and female empowerment related variables. Importantly, we also generate predictions about the conditions in which this effect should be stronger vs. weaker. Consistent with these predictions, we find that where conditions are harsh and favor engagement in shorter-term strategies (i.e., high homicide rates, low GDP per capita, high economic inequality, and high pathogen prevalence), the effect of increased population density on fertility rates is attenuated. We also find that the density-fertility relationship is moderated by religiousness and strength of social norms. These findings shed new light on why, and under what conditions, rising population densities influence fertility rates.