Summary: Researchers hypothesis the projected decline in global population by 2064 will be a result of social stress.

Source: UMass

A University of Massachusetts Amherst environmental health scientist has developed an “overlooked hypothesis” to help explain the projected global population decline beginning in 2064: social stress.

Stress from social media and other largely empty or overwhelming social interactions may be leading or contributing to changes in reproductive behavior and reproductive physiology, suggests Alexander Suvorov, associate professor in the UMass Amherst School of Public Health and Health Sciences. 

In a review article, published in the journal Endocrinology, he examines various theories surrounding previous human population decline as models predict a “remarkable” decrease from 9.7 billion people in 2064 to 8.8 billion by 2100. Some countries’ populations already have peaked and are projected to decline by 50% by the end of the century.

“A unique feature of the upcoming population drop is that it is almost exclusively caused by decreased reproduction, rather than factors that increase rates of mortality (wars, epidemics, starvation, severe weather conditions, predators, and catastrophic events),” he writes.

Suvorov outlines a hypothesis that connects reproductive trends with population densities, proposing that density reflects the quality and frequency of social interactions. 

“Rising population numbers contribute to less meaningful social interactions, social withdrawal and chronic stress, which subsequently suppresses reproduction,” the manuscript states.

Over the past 50 years, a 50% decrease in sperm counts has occurred. Stress is known to suppress sperm count, ovulation and sexual activity, Suvorov notes. While changes in reproductive physiology are usually attributed to the effects of endocrine-disrupting pollutants, Suvorov believes it is not the only factor.

“Numerous wildlife and laboratory studies demonstrated that population peaks are always followed by increased stress and suppressed reproduction,” Suvorov says. “When a high population density is reached, something is happening in the neuroendocrine system that is suppressing reproduction. The same mechanisms happening in wildlife species may be at work in humans as well.”

Suvorov outlines a hypothesis that connects reproductive trends with population densities, proposing that density reflects the quality and frequency of social interactions. Image is in the public domain

Suvorov points to several changes in reproductive behavior that contribute to the population drop, including people having fewer children and waiting longer to start families or choosing to be child-free. But he says biological changes are likely happening as well. More research is needed, he says, such as studies to determine cortisol levels in human blood, an important measure of stress. 

“A better understanding of the causal chain involved in reproduction suppression by population density-related factors may help develop interventions to treat infertility and other reproductive conditions,” Suvorov writes. 

He hopes his hypothesis offers up an enticing area of research that scientists from different fields will be interested in exploring.

“The goal of this paper is to attract attention to a completely overlooked hypothesis – and this hypothesis is raising more questions than it is giving answers,” Suvorov says. “I hope it will trigger interest of people from very different domains and that after additional studies we will have a much better picture of to what extent population density is connected with social stress and how social stress is connected to reproduction, and what we can do about it.”

A common-sense place to start, he suggests: “Back off social media.”

About this social neuroscience research news

Author: Patty Shillington
Source: UMass
Contact: Patty Shillington – UMass
Image: The image is in the public domain

Original Research: Closed access.
“Population Numbers and Reproductive Health” by Alexander Suvorov. Endocrinology


Abstract

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Population Numbers and Reproductive Healthtion numbers may cause a decline in human fertility

A recent study published in The Lancet predicts a remarkable drop in population numbers following a peak that will be reached by 2064. A unique feature of the upcoming population drop is that it will be almost exclusively caused by decreased reproduction, rather than factors that increase rates of mortality.

The reasons for decreased reproduction are also unique, as, unlike previous centuries, limited reproduction today is hardly due to a shortage in resources. In other words, the predicted population drop is almost exclusively due to changes in reproductive behavior and reproductive physiology.

Today, global changes in reproductive behavior are mostly explained by social sciences in a framework of demographic transition hypotheses, while changes in reproductive physiology are usually attributed to effects of endocrine-disrupting pollutants.

This review outlines a complementary/alternative hypothesis, which connects reproductive trends with population densities.

Numerous wildlife and experimental studies of a broad range of animal species have demonstrated that reproductive behavior and reproductive physiology are negatively controlled via endocrine and neural signaling in response to increasing population densities.

The causal chain of this control system, although not fully understood, includes suppression of every level of hypothalamic-pituitary-gonadal cascade by hypothalamic-pituitary-adrenal axis, activated in response to increasing stress of social interactions.

This paper discusses evidence in support of a hypothesis that current trends in reproductive physiology and behavior may be partly explained by increasing population densities.

Better understanding of the causal chain involved in reproduction suppression by population density–related factors may help in developing interventions to treat infertility and other reproductive conditions.



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