"Does Overcrowding Make It Harder to Be Born? A New Study Shows How Overcrowding Affects Reproduction"

"When the population increases too much, fertility somehow decreases"—this phenomenon has been known in animal studies for some time. Chickens kept in confined spaces lay fewer eggs, and densely packed mice have fewer offspring at a time. In humans, there are studies showing the relationship between high population density and declining birth rates.

However, many factors are involved. Rising housing costs, childcare expenses, competitive society, stress, noise, pollution, career development, and changing values toward marriage and childbirth. The reasons for the difficulty of having children in cities can be explained both socially and economically.

This time, however, the research team at the University of Colorado Boulder focused on a deeper biological mechanism. Overcrowding itself may increase specific chemical messengers in the body, damage the DNA of reproductive cells, and affect the development of eggs, embryos, and offspring.

At the center of the research is not humans but nematodes. The small organism, C. elegans, about 1 millimeter in length, has long been used as a model in life sciences experiments because of its fast generational turnover and the ease of tracking gene and cell behavior.

The research team raised nematodes in groups of different densities. They found that a protein called CPR-4, which is rarely secreted in normal environments, increased when the groups became extremely dense. Particularly in overcrowded conditions with colonies exceeding 3,000 nematodes, the secretion of this protein became noticeable.

CPR-4 is related to an enzyme called cathepsin B, which has similar functions in humans and mice. According to the research team, this protein was not just a marker of stress response but was related to DNA damage in reproductive cells. In nematodes placed in overcrowded conditions, mutations in reproductive cells increased by an average of 87%, the number of offspring decreased, and developmental abnormalities were observed in the surviving progeny.

More importantly, the impact did not end in one generation. Genome analysis suggested that some mutations that occurred in overcrowded conditions could be passed on to offspring. In other words, overcrowding might not only "reduce the fertility of current individuals" but also be involved in genetic changes in the population.

What makes this finding intriguing is that the effects of overcrowding resembled the "bystander effect" of radiation. The bystander effect refers to the phenomenon where cells not directly exposed to radiation are damaged by signals emitted from surrounding cells. The research team had previously investigated this phenomenon in nematodes, reporting that cells stressed by radiation released CPR-4, affecting the DNA of distant cells.

In this study, it was shown that similar molecular reactions could occur with just the environment of overcrowding, without using radiation. Borrowing the researchers' expression, animals in overcrowded conditions were approaching a state at the molecular level akin to "being irradiated."

Of course, there are points to be cautious about here. This research was primarily an experiment on nematodes, and although similar results were confirmed in mice, it cannot be directly applied to humans. It is not a story that people living in crowded areas in cities become infertile due to the same mechanism.

Human infertility is extremely complex. Many factors are involved, including aging, hormones, genetics, infections, lifestyle, environmental pollution, medical access, economic conditions, and psychological stress. The WHO reports that about 1 in 6 adults worldwide experiences infertility at some point in their lives, but the cause cannot be narrowed down to one.

Nevertheless, this study adds a new perspective when considering the impact of "overcrowding" on reproduction. Until now, the effects of overcrowding have often been explained by a lack of food and space, stress hormones, social hierarchy, and the spread of infections. However, this study suggested that when animals are densely packed, they secrete specific proteins that could lead to DNA damage and mutations in reproductive cells.

Furthermore, the research team demonstrated that suppressing the function of this protein could prevent the adverse effects of overcrowding. This could potentially be applied in fields such as livestock and aquaculture in the future. For example, it might lead to technologies that suppress the negative impact of overcrowding stress on reproduction to increase egg production in chickens or breeding efficiency in fish.

On the other hand, caution should be exercised regarding applications to humans. Even if compounds that inhibit cathepsin B are developed, whether they directly relate to human infertility treatment is a completely separate issue. Reproductive medicine is an area that must be carefully examined for safety, ethics, and long-term effects. Particularly, topics such as DNA damage, mutations, and effects on the next generation should not be discussed with mere optimism.

Looking at reactions on social media, this study, being newly published, seems to be quietly shared through science news feeds and article aggregation sites rather than spreading explosively. Indicators from Nature Communications also did not show a large number of reactions at the time of confirmation, and there were hardly any comments on the Phys.org article.

However, the direction of reactions seems to be divided into four main categories.

The first is the reaction of wanting to connect urban life stress with declining birth rates. High-density cities, crowded trains, small housing, high living costs. Those familiar with these realities are more likely to feel an intuitive persuasiveness in the headline "overcrowding affects reproduction."

The second is the cautious argument that "it's too early to apply to humans." This is very important. The results in nematodes and mice are valuable for understanding biological mechanisms. However, explaining the decline in birth rates in human society with just this enzyme is unrealistic. Declining birth rates are intricately related to education, employment, housing, gender norms, healthcare systems, and childcare support.

The third is the reaction focusing on applications to livestock and aquaculture. Overcrowded breeding is a productivity issue and an animal welfare issue. If the molecular mechanism of reproductive decline due to overcrowding is clarified, it could lead to not only improved production efficiency but also a review of breeding environments.

The fourth is the caution towards the word "overcrowding" itself. Population issues have historically been discussed in connection with discrimination, eugenics, and forced population policies. Therefore, when introducing this type of research to society, it is necessary to avoid misunderstandings such as "regions with many people are bad" or "people living in cities are inferior."

The essence of this study is not to simply problematize the number of humans. Rather, it is the possibility that organisms can sense changes in density and change molecular signals in the body accordingly. Overcrowded environments can become biological information that reaches cells and genes inside the body, not just physical congestion.

This perspective is suggestive even when considering modern society. The world population has increased about threefold since 1950, now approaching about 8.3 billion. Meanwhile, the global birth rate has decreased from about 5 children per woman in 1950 to about 2.3 in 2021. Although the population continues to grow, it is becoming harder to have children in many regions. This seemingly contradictory situation is due to a combination of societal maturity, education, urbanization, healthcare, economy, and biological factors.

Overcrowding cannot necessarily be measured by the single word "large population." Even in large countries, overcrowding can occur if the population is concentrated in urban areas. Conversely, even with high population density, if housing, transportation, healthcare, green spaces, work styles, and childcare support are well-developed, stress can change significantly. What is important for human society is not the simple number of people but the environment in which they live.

This study showed part of the molecular mechanism by which overcrowding affects reproduction. However, it is not a simplistic story that "living in a city leads to infertility." Rather, it is research exploring what signals a crowded environment sends to organisms and how those signals can affect the next generation.

There are many challenges ahead. To what extent does a similar mechanism work in human cells and tissues? Which elements of urban life affect reproductive health? Is it safe and effective to suppress cathepsin B enzymes? How much of the stress response due to overcrowding is an evolutionary adaptation, and where does it become a health risk?

Nevertheless, the questions raised by this study are strong. Organisms may not just be enduring the surrounding congestion. They may sense changes in density, react at the cellular level, and sometimes leave an impact on the next generation. Overcrowding is both a social and a biological issue.

Declining birth rates, infertility, urbanization, population growth. These are often discussed as separate news. However, this study illuminates the invisible connections between them. How densely do people, animals, and cells live? This question will become increasingly important for future urban design, healthcare, agriculture, and life sciences.

Source URL

Phys.org. Introduction to the potential of overcrowding to impair fertility and research on CPR-4/cathepsin B.
https://phys.org/news/2026-05-overpopulation-impair-fertility.html

Official article from the University of Colorado Boulder. Researcher comments, effects of overcrowding on reproductive cells, and an overview of nematode and mouse experiments.
https://www.colorado.edu/today/2026/05/21/overpopulation-can-impair-fertility-new-study-explains-why

Original paper published in Nature Communications. Primary research on CPR-4/cathepsin B, overcrowded environments, DNA damage in reproductive cells, increased mutation rates, and transgenerational effects.
https://www.nature.com/articles/s41467-026-72521-6

Research announcement page on EurekAlert! Confirmation of research methods, the fact that the subject is animal experiments, DOI, announcement information, and key points of the research.
https://www.eurekalert.org/news-releases/1129213

WHO announcement on infertility. Background information confirming that about 1 in 6 adults globally experiences infertility.
https://www.who.int/news/item/04-04-2023-1-in-6-people-globally-affected-by-infertility