Scientists have long speculated that the “dirtier” the environment we grow up in—with lots of germs from different people and even animals—the better off our immune system and physical health ultimately will be. A new study published Monday in Proceedings of the National Academy of Science provides early evidence that a dirty world might even be better for our mental health, too.
The hygiene hypothesis, as it’s called, says that our immune system needs to spar with relatively harmless germs and foreign substances (including foods like peanuts) in its earliest years so it can calibrate itself. Without this training, it can become too sensitive and overreact to things it shouldn’t, like house dust and pollen, leading to allergies and asthma. Plenty of research has shown that growing up in a rural environment, or with pets, is associated with lower rates of autoimmune disorders, while rates of allergies and autoimmune disorders have steadily climbed in urban areas.
Several of the new study’s authors, in particular Christopher Lowry, a professor of integrative physiology at Colorado University Boulder, had theorized a decade ago that a too-hygienic world could also influence our risk of certain psychiatric illnesses, like depression and post-traumatic stress disorder. If true, it would help explain why rates of psychiatric illnesses are more common among people living in urban areas.
But it wasn’t until Lowry collaborated with researchers at University of Ulm in Germany that they tested this theory directly.
In this latest study, they recruited 40 young, healthy men from Germany to take part in their experiment. Half of the men said they had been raised (up to the age of 15) on a farm with lots of animals, while the other half had been raised in a city with no pets. Both groups had their blood and saliva taken before and at various points during the experiment.
The volunteers were asked to complete a series of tasks meant to stress most anyone out: First, they gave a speech in front of people in lab coats about why they deserved their dream job. Then, they had to count backwards in increments of 17 from 3,079.
Following the tests, the men who were raised in cities were found on average to have higher levels of certain blood cells and proteins in their blood. These specific components of the immune system, known as peripheral blood mononuclear cells (PBMCs) and interleukin 6, indicate a strong inflammatory immune response. The city men also had elevated levels of these markers for longer—at least up to two hours, when they stopped having their blood drawn.
While inflammation is a crucial part of a healthy immune system, previous research has suggested that people who have chronic, low levels of inflammation are at higher risk of developing mental health disorders like major depression. The study’s findings suggest that city-raised people might be more likely to fit this bill, though more research would be needed to definitively make that connection.
“In our field of research, this social stress test is the most effective way to induce a neuroendocrine stress response. And we also know it induces a pro-inflammatory response in humans that is exaggerated in people who have major depressive disorder, for example,” Lowry told Gizmodo.
However, the study also had some counterintuitive results. Farm-raised men, on average, reported that they felt more stressed by the task than city men did, and they actually had higher levels of cortisol, a hormone indicative of stress, in their saliva. This seemingly contrary finding could be explained by the fact that our reactions to stress can manifest through interconnected bodily systems that work independently of one another. Cortisol is a key component of the hypothalamic–pituitary–adrenal axis, a pathway that runs along our brain and hormonal glands that regulates everything from digestion to our flight-and-fight reaction. The presence of PBMCs and interleukins brought on by an overactive immune response to stress might not affect cortisol levels, and vice versa.
There are important limitations to the study, such as a small sample size and lack of women subjects, who are more likely than men to report mental illness. The connection between where we grow up and our later mental health might also have more to do with the well-documented psychological benefits of living near nature than its direct effect on our immune system, Lowry said.
Lowry and his team are quick to caution their work hasn’t confirmed any link between hygiene and mental health, at least not yet. But given that more and more families are moving into cities, they believe their research could have some pretty huge implications.
“It’s potentially alarming, for several reasons,” he said. “There’s a massive increase worldwide in urbanization, and by 2050, two-thirds of the world’s population is expected to live in cities. So we’re gradually shifting towards an existence that creates a mismatch between our evolutionary past and co-evolved relationship with things like microorganisms.”
“One thing we can anticipate is that we will somehow have to compensate for that lack of exposure, especially during development, to these microorganisms,” Lowry added. “And the strategy for doing that is not quite clear... hopefully we can resolve that over time.”
The authors next plan to study larger groups of people, including women, who live in different areas of the world. And they hope to better single out whether it’s exposure to animals (which would include pets, not just farm animals) or other aspects of rural life that explain the link.