Human population density and reproductive health: a changing world needs endocrinology
Many animal species, from insects to elephants, show reduced rates of reproduction when population density is high, an effect that is often mediated by the stress axis. A recent review in the journal Endocrinology, titled “Population numbers and reproductive health,” suggests this density-dependent reproduction could also be occuring in humans, contributing to the observed global decline in fertility. In this invited commentary, we explore this hypothesis further and suggest ways in which it could be tested. For example, defining population density as social interactions per unit time, rather than by the number of individuals per unit area, could allow many confounding factors to be untangled. In addition, we suggest that a further examination of how and where density is encoded in the brain could advance our understanding of the interaction between population density, social interactions, and reproductive physiology. Adequately testing the hypothesis put forward is important. The outcome has the potential to yield novel insight into causes of infertility, as well as implications for modeling and planning for changes in population growth.