Population numbers across the globe are predicted to peak in about 40 years and then decline by 50% in some countries by the end of the century,1 owing almost exclusively to changes in reproductive behavior and physiology. Differences in population densities may be linked to these reproductive trends, according to Alexander Suvorov, PhD, from the Department of Environmental Health Sciences at University of Massachusetts Amherst School of Public Health and Health Sciences. Dr Suvorov’s analysis of this trend was recently published in Endocrinology.2

In his research article, Dr Suvarov notes that studies of small rodents and other mammals suggest that high population densities can lead to decreased reproduction and inhibited maturation of the young and, when combined with other factors such as suppression of spermatogenesis and a drop in testosterone production, may result in a substantial population decrease,.

Research has addressed molecular cascades involved in “intrinsic” density-dependent mechanisms of population growth regulation, he explained, and a majority of studies link suppression of reproduction with hypothalamic-pituitary-adrenal (HPA) hormonal signaling activated in response to the stress of social interactions. Complex interactions between the HPA axis and hypothalamic-pituitary-gonadal (HPG) axis may be involved in stress-dependent suppression of reproductive function.

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“A large body of literature including animal and clinical studies shows that stress signaling — via multiple endocrine and neural mechanisms — suppresses reproductive behavior and reproductive organ functions at different levels of the HPG axis”, Dr Suvorov wrote.

Studies have also shown that higher human population densities, for example in cities, are associated with increased amygdala activity,3 greater overall stress, and lower happiness levels, Dr Suvarov said, with 1 study concluding that an urban upbringing results in a blunted response to stress.4 “Given that two-thirds of the global population is expected to live in urban areas by 2050,” he explained, “population growth and urbanization may work in concert to slow down reproduction.

:Recent changes in human reproductive behavior,” according to Dr Suvorov, “are reflected by increasing numbers of child-free young couples, a decreasing number of children per family, and an increasing age of first-time parents, as well as downward trends in reproductive physiology indicators (eg, a decline in testosterone levels) not attributable to changes in other factors, including age, lifestyle, and health such as obesity.”5

“Today, humankind for the first time approaches its population density peak,” wrote Dr Suvorov. “I hypothesize that many trends in human reproduction observed today result from ‘intrinsic’ density-dependent mechanisms of population control.” Understanding these mechanisms, he added, may help to develop new interventions for patients with infertility.

Dr Suvarov concluded that experiments with large social mammals that are not prone to regular drastic fluctuations in population density would help researchers to evaluate whether social interactions are a significant determinant of reproduction control in conditions of controlled resources. Epidemiologic studies exploring associations between stress levels and social withdrawal phenotypes in adult humans and their offspring would also be useful, he said.

“Experimental research in human populations is needed to identify specific patterns of social interactions that are responsible for acute and chronic stress-response, including interactions via electronic communication means, and … information exchange (eg, reading updates on Facebook),” Dr Suvorov advised. “Additionally, identification of secular trends in stress and reproductive hormone levels may explain [the] changing biology of reproduction.”


1. Vollset SE, Goren E, Yuan CW, et al. Fertility, mortality, migration, and population scenarios for 195 countries and territories from 2017 to 2100: a forecasting analysis for the Global Burden of Disease Study. Lancet. 2020;396:1285-1306. doi.org/10.1016/S0140-6736(20)30677-2

2. Suvorov A. Population numbers and reproductive health. Endocrinology. 2021;162(11):bqab154. doi:10.1210/endocr/bqab154

3.  Lederbogen F, Kirsch P, Haddad L, et al. City living and urban upbringing affect neural social stress processing in humans. Nature. 2011;474(7352):498-501. doi:10.1038/nature10190

4. Frissen A, Lieverse R, Drukker M, et al. Evidence that childhood urban environment is associated with blunted stress reactivity across groups of patients with psychosis, relatives of patients and controls. Soc Psychiatry Psychiatr Epidemiol. 2014;49(10):1579-1587. doi:10.1007/s00127-014-0859-3

5. Travison TG, Araujo AB, O’Donnell AB, Kupelian V, McKinlay JB. A population-level decline in serum testosterone levels in American Men. J Clin Endocrinol Metab. 2007;92(1):196-202.